4 Strot-e Perfoi’man‹e Tuning

Having determincd where the cam is, you may decide tochange its phasing. to
tune ii to your engine. The next problem is just wow todc› it. With some motors this is easy
as they have vernier iinning whee1s (eg. .laguar and AIfa-Romeo) or taper fit wheels (Ford
BDA). The majority, however. use either dowels or keys to positi vet y locate the cam or
cran ksprc›cket. Dowelled drives will need offset dowels fitted and if inore than a centr¡il
hot t l’ixes the spree ket t‹› the cam, the bolt h tiles will requirc elongation with a round
l’ile. The amonnIct offset requ ired is minute. For example, wit h the pushrod English Fords
and Lotus Twin Cam, 0.006 in offset (at Ihe cam) represents I 'at the crank, so tochange the
cam posit ion by 6"(at the crank) requires a dowel with 0.036 in offset. Some cam
manufacturers, instead of supplying offset dti we1s, have a range of’ ofl’set bushes which
are fitted to the cam sproc ket. Others are ablc to suppJy vernier crank sprockets (Pinto)
and manv America n V8s have ;i range ofcrank drive sprockets available with three key
ways cui tochange the cam ph:ising.
If there is nothing available for your particular engine then you will have to make
up ihe appropriate hardware. Kcep in mind that the dowel or key does not require
hardening in most instances as it does not actually transmit the driving force. The
driving torce is taken by the friction between the sproc kci and shaft (kev located) or by
pressure between the sprocket and cam created by the fixing bolts (down l‹›ca icd).
Incidentty, these fixing bolts sht›uld be locked using Ltictitc.
With the cam phasing sorted out. the next thing we have to think about is the
valx’c clearance (lash). The cam grinder wit I supply a cleara rice f’or the intake and exha
ust va I ve. Sometimes he will specify a different cleararice for difterent types of valve
materia 1. If he gives a ‘cold’ cleara rice he means just that: t he inotor should not have been
run in the previous five hours. The ‘ hot’ cleara rice is for a motor at normal operat ing
ternperature: usually 70 — 88'C water tempcrature, 90 — 100' oil temperature. It is verv
importani to mainta in sufficient valve clearance ti›allow the valve to xeat firm I y rind to
remain on the seal long enough to transfer heai to the cooling medium. Running the
valves ti;;ht to quieten them ix a surc wa y to get val ve burning in a performance engine.
5omctimcs we can benefit by changing the valve clearance within reason and
according to the type of competition involved. Note we are talking about a competition
motor here, not a road or riilly motor. We havc a1rea‹Jy pointed out that for scrious
competition the mobor must be set up to suit the track or st rip, but our best laid plans can
go astray. Say we haveset our cam for a particular dirt speedway but as the night
progresses the surface begins to break up or thc weat her Iurns foul. Obviously you are noi
going to have time to advance t he cam so t he only way to pick up some bottom end power
in a hurry is to increase the valve lash. This will have the effect o(shortening t he t inning by
3 - 4' for each 0.00J in the clearance is increased. There is a very definite limit to how
far we can go however, as this has the effect ofs hortcning the openingand closing ramps.
Thiscould lead to serious shock loads in the valve train. Theretore theclcaf ance shouId not
be increased by morc th¡in 0.005 in unless the cam grinder says ct hcrwise. Keep in mind
thai ihe valve float limit wit I now be lowered, so be careful.
If, on the either hand, you find you need more top end power on Ihe night, you can
decrease the lash, but this is only feasable for short distance events such asdrags,
hillclimbs or speedway sprints. Remember, decreesing the lash will increase valve
tcmperat ures, so always maintain 75fi› of the recommended clearance. Also. il’you know
the valves and seats atc getting a bit rough or the guides are shot, do not decrease the
clearance at all; you could easily wreck the motor. An increase in valve tern peratures,
undcr competition condit ions,
142 could cause pre-ignit ion or detonation. Bot h :ire pist on and engine breakers.
 Camshaft and Valve Train

You must maintain a valve to piston clearance of 0.080 in and 0.100 in

respectively for twe inlet and ez liaust valves. Therefore it will be necessary to set the
pistons up with adequate valve cut-outs before you atlempt tochange the valve clearance
or change the carn phasing.
I t is surprising how’ many enthusiasts have little idea of sctting valve clearances
accurately. Firstl v, let me point out that it is impossible to do much of a i b if the contact
face of the rocker or valve stem tip has a dip worn into it. The feeler blade will be too
wide to
fit into the dip, so we will not be able to take this wear into account. When we measure
the clearance we want to have t he lifter fairly closc to the centre of the base circle,
With most ohc engines it is possible tosee when the lifter ison ihe base circle, but with
other types we have to use another method.
Thc simplest method for in-line engines is visually to divide the engine in half. A
11 the rockers in the front ha If we number from I to 4(or ) to 6 if it’s a6 cylinder) and
we do likcwise for the rear half but t his lime we start No I from theextreme rearof the
engine. In this way we have boih No 1 rockers a1 opposite ends of the head, and both
No 4(or No 6) rockers in the middle, alongside each olher. As we turn the motor over
we always adjust the valve partner to the one that is tin Jy open. Therefore if No 1 at
the rearof the motor is fully open, weadjust its partner No I at the front of the motor. If
No 3 at the front is fully open, weadjust No 3 at the rear, and so on.
With flat and vee inotors, the safest way is to turn the motor over and adjust the
inlet and exhaust rocker for each cylinder together, midway between the inlet closing
point and the exhaust opening point. ie. with thepiston at TDC at the end of
thecompression stroke. If you adjust each cylinder in turn according to t he firing
order, it will save time turning the motor over unnecessarily.
When adjusting clearances on ohc motors, I have found it simpler and more
accurate to adjust the cleara rice by inserting the feeler st rip bet ween the cam and rocker
rathert han at the valve stem. However, when using this method it is necessary to
reduce the clearance because of lhe effect of the rocker arm ratio. Therefore if t he
clearance at the val ve is norma I ty 0.013 in and the rocker arm ratio is 1.45: 1, then
the clearance at the cam lobe will be 0.013 = 1.45 = 0.009 in.

143
 Chapter
The Bottom End
ENTHUSIASTS associate high performance with cams, carburettors and exhaust
modifications. However, manv hidden horsepower can be found by proper attention
to blueprintinq the bottom end ofa motor. An additional consideration is the added ref
iabitit y we can expect when things are done properly to an exacting specification.
Logically, the place to start is the cylinder block, becausc it is the homc f‹ r
everything elsc in or on the engine. First, inspect the water passages l’or rust or scale.
If’any is present it will be necessary to have the block boiled in a chemical bath for
cleaning. Th is should be done with all the welch plugs (freeze or core plugs) and oil
gal Jery plugs removed.
A mildly dirty block can be steam cleaned or washed with solvent. At the same
time the oilways should be carefully cleaned, using brushes designed for this task. St
nd holes, and particularly head arid main bearing capstud holes, de.serve thesame
treatment. Finally high pressure air must be used to clean and dry oft‘ evervt hing. Even
brand new (green) blocks should be cleaned as outlined, with particular attention to
ensure no casting sand has been left in the water passages.
With the block perfectly dry, it can be visually inspected for cracks. A competition
block should be crack tested. Next take a pt ug tap and clean the head stud arid main
bearing cap threads of din or burrs. This is important to get a true stud tension
reading. Any stud holes that have nut heen chamfered should be, to prevent threads
pulling up. A putted head stud thread ca n cause head gasket sealing problems. A
pulled main bearing cap thread can allow a bearing to turn. Finally, check the depth
of’each hole to be certain the stud will not bottom.
To avoid cuts to yourself and possible enyine damage, carefully grind away any
casting slap. The main area for concern is around the main bearing webs, thesump pan
deck, and in vee motors, around the oit drain back holes and the rest of the valley area.
The next opcruiions involve ‘squaring’ the block. A check must be made toensure
that the main bearing hores are perfectly aligned. Any misalignment will wrcck the
bearings and possibly even the crank. 11 will also soak upa lot o1’poweras the fractional
losses areprettier, I have tound new hJocks with up to 11.007 in misalignment, so do
not assume that the
1 44 alignment is correct. Anv mis—alignment can be corrected by line-boring. When main
 The Bottom End

bearing caps are replaced bY heavy duty items, line-boring will also be necessary.
Toensure ihat alignment is maintained. the main bearing caps should bc num bered
and have the front posit ion marked. This will assist in fitting each cap in its correct
location each time the motor is rebuilt.
With the main bearing bores true, the block should be checked at each corner to
guarantee that the distances betwecn the top of the block (the deck) and the
crankshaft ccntrcline are ident ical. If the hlock ts out oftrue, millinbwill bc required.
Even if thedeck is
true, we may decide to have it decked to reduce the disk ance between thc top of the piston
and the top of the block, to increase the compression ratio.
The cylinder bores must be true at 90" to the crank centreline (ie. across the
motor) to keep the frictit›nal losses low and maintain good ring sealing. If the bore
iscan ted slightly to the front or the rear, the pision pin will hammer out, soacheck
must also be made in line (ie. at 150') with the crank. There must not be any steps or
taper f’rom the top to the bottom of the bore. (FI GURE 6. 1)
8oring and honing should bc carried out by a firm with precision equipment able
to maintain a tolerance of 0.0003 in. Forget about garages using small boring machines
that bolt onto the block as the tolerance can be as bad as 0.002 in.
There are definite limits as to how faracylinder can beover-bored. If the cylinder
wal 1s become too thin, they wi II warp due to the pressure and hen. Asituaiion like this
will allow bJow-by past the rings. resulting in Iost power. Another problem not often
recognised is also duc to thin cylinder walls. In a racing motor, or when a long.st roke
crank is used, the main bearing webs can actua11y brea k away from the bottom of the
cylinders.
Genera11) . 0.060 in oversize is about the limit. Some of the English motors will go to
0. 150 in oversize, but care must be exercised. Many Americiin motors are limited to plus
0.030 in due to thin wall casting techniqucs. It is possible to obtain specia I thick wall r;icin
g blocks for some engines. As a bonus these arc genera fly stronger and less prone to
freeing, duc to t he use of a special grade of cast iron.
Motorcycle cylinders a re restricted to 0. 120 - U. 160 in oversize, depend ing on the
thic k ness of’ t he stee l liners. By boring out thcoriginal liners and pressing in new sleex’es
w’e can at times go to 0.400 in. This. however , is I inn ited by t he bore centres and bloc k
rigid ity. Remember there is no benefit lobe gained if an increase in bore size will induce
cylinder warping or block flexing. Bot h condit ions will reb us of power and lead to
reliability t rou bles. Also to be avoided is the common practice o1 dry slewing cylinders
tocompensate for thin watls or to eliminate water seepage. What generally happens ix that
the sleex’e shil’ts due to the verv thin wa11s. This may aIlow water into the suinp or resutt
in a blown trend gasket, Many tuners feel thai slewing is a good thing as ihe sleeve at wa
ys has enough wa11
th ick ness not to warp. They forget that a sleeve inust be adequate I y supported.
Even when an engine is n‹it excessive I y over-bored there is no guar‹intee you wilt
cc h ieve 'a m'assivc increase in power. I thin k inost peop Ie understand t hu1 a I OC increase in
cylinder displacement wil I noi vield a corresponding rise of 107c in maximum
horsepower. H clever, t’ez' tuners realise just how ill-advised targe capacity increases can
be in some situai ions. The point to be remembered is this;it is of no usegiving the engine big
1ungs if the induction system (ie. carburettor, manifold, ports and valves) docs not have
the capability, or the potential capability, to flow sufficient air to fill thosc big lungs.
A situation likc this is not really a problem ifthc engine is running in a street car.
but in a road race vehicle a nd particularly in a rally car, this type of over—
modification must be
avoided because of the disastersus cl’1ect it has of compressing the power band. True, the 145
increase in mid-range power is a very important plus in favour of a displacement
increase, but if the engine does not have the breathing ability to be able to rev as hard
as previously, the increascd number ct’ gears changes now necessary may easily reduce
the performance lev’el of the carZengineZdriver combination to what it was before the
capacity jump.
In TABLE 6. I yort can see how the power range was affected on an ohc Vauxhall
rally motor when thc capacity was raised from 2279 cc to 249G cc. abou' a IOT, increase.
The smaller motor makes good power between 4500 rpm and 7500 rpm, and it need be
it can be lugged down to 40(i0 rpm which gives a verv nice 3500 rpm operating range.
The 2,5 litre engine is working vcrv well at 4000 rpm (15 hp increase over the 2.3 litre
engine) but it runs out of steam at around 6500 rpm, which means thal the power
band has becn narrowed down by 1000 rpm. You will note too that there is onlv a
dit’ference of I hp in iuiiximum power between both engines.
Iu its present form the 2.5 litre motor would not beany better than thc smaller engine
in a i alty car, However, with a lot of development t he big motor conId provc to be margins
11 v superior. A wiIder camshafl raised maximum power to almost 209 hp and enabled
the motor to run to 7000 rpm ( 192 hp) but the mid-range was so t›adly affected that
the power band was narrowed to even worse than before.
After this a l6 valve Lotus head was tried to determine what effect beher breath
ing would hnve (2’ABLE 6.2). The small engine has a 3250 rpm power range extending
from 5000 rpm to 8250 rpm, while the 2.5 litre motor is best between 4750 and 7500
rpm, a 2750 rpm spread. W hen a wilder in let camshaft was fitted(noot herc harries) this
motor ran inuch better at 7500 rpm (239 hp) and ii ma‹Je230 hp at 7750 rpm wit hjust a
small decrease in mid— range ( 190 hp at 5000 rpm). Maximum power weni up to 253
hp.
Obviously in this instanCe the 2496 cc motor should be superior, bui since that
time the Lot us head has been cm lawed and the 16 valve Van xhall head will not, at this
stage of development at any raie, pertorm any where near as well (ie. 20 hp less for the
2.3 lit re

Of course not everv type ofcompetition machinc requires an engine with a good w
ide spreud ol’power, so large increases in displacement can be made without ads’ersely
affecting performance. Therelore ifyou are involved in rat lvcross or /, mile dirt speedway
the 2.5 litre Vauxhall in either 5 valve or 16 valve form should prove supcrior.
However, as you can see t’rom both tables, there is no significant increase in maximum
power, so to gain any benefit from the big motor the suspension wilt have to be
capable of getting t hoseextra l5- 20 mid-

TABLE 6.1 Dyno test of Vauxhall ohc rally engine

2279 cc engine 2496 cc engine
hp torque (lbfft) ftp torque(i«rr‹›
J‹JU 16 I .6 I0? [bf›.4
108 162.1 lls [77.[
4000 127 lf›fi,7 142 186.4
4500 136 15$.7 tf›3 190.2
5( (I 170 178.5 ISO 192.2
5500 185 17b.6 194 165,2
6l1Ut I99 i74.2 20f› 180.3
6500 202 163.2 2‹J0 16 1,a
7fx)0 205 I 53.8 178 I 12.6
14G 197 137,9
7500
 The Bottom End

range horsepower down onto the Itack to accelerate the car faster.
During the boring and honing operations, a boring plate should be fitted, along
with the head gasket, and tensioned to normal head stud tension. This operation is
necessary on racing engines to guarantee perfect bores. The pull of head studs can cause
acert ain degree of cylinder distortion. W hen a boring plate is fitted at the specified head
tensitin, the block is distorted during the boring and honing procedure. 7’herefore the
bores wit I be true when the hcad is fitted. The main bearing caps, when tensioned, also
distort the cylinders, so these should be fitted as wcll.
I prefer to here the cylinders to within 0.005 in of the finished size and then hone
each cylinder to fit a particular piston. In this way I can keep piston clearances to a
tolerance of about 0.0003 in.
The cross-hatch pattern the hone leaves on the cy[ inder walls is crit ica1. I t innet
bejust right i1‘ the piston rings are to bed-in quickly and have a long life. I prefer a 45'
cross-hatch wit h a tinish of 10- 12 inicro-inches. This type of finish makes it necessary to
run the rings in, but they last for a long time and seldom leak. A finish any smoot her
will not hold enough oil and will altow a glaze to form on the ring lace and bore wall.
Oil consumption will be a problem and power will be lost, due to blow-by. A rougher
finish will usually eliminate the need to bed in t he rings; however ring lil’e is greatly
reduced. Keep in mind too that glazing can again be a real problem but not due to lack of
lubrication. A rough tinish acts like a tile on the rings; the added trict ion increases iheir
temperature and allows glazing to form.
The upper and Iower lip o1’ each cylinder should be chamfered light ly to remove the
sharp edge produced by boring. A smooth cut half-round r:l‹ will do thejob but take
care noi to let it slip and nick t he cylinder wall. While you have the file at hand, lightly
dress the sharp edges of the main bearing caps and webs.
Finally, check the mating l’ace on the back and front of the block. They should be
squarc to the crankshati centre-line.
Two more inspcctions are necessary for racing engines. "the camshaft bearing
bores must be in line and each tappei bore must be gif theconect diameter and
perpendicular to the centre-line of the camshaft. A tappet tipped off centre may easi Iy
dig into the lobe of a racing cam, causing premature wear andZor breakage.
About the only other parts of the block we have to give consideration to are the
main bearing caps and studs. I use aircraft quality high tensile studs when these are
available. Main bearing st uds should not be reused on racing engines. and the same
applies to big end

TABLE 6.2 Dyno test of Vauxhall twin cam l6 valve rally engine
2279 cc engine
249f› cc engine
hp torque
ftp torque (lbfft)
(MffT)
139 1 7 .8
156.4 i 54 l 79.7
181, 7 I95 2fJ4.H
5500 b8 i7q.1 272 202.4
189. I 216 205,x
I 56.6 25/ 202.8
1SG. I ?44 ]X?.I
1 7tl.5 2?4 If›4.2
8000 21B I 5fi. 2
S25¢ 2?2 ! 47.7 147
4 Strok e Performance Tunlng

bolts. Usually, the standard cast iron main bearing caps are acceptable for medium
performance but engines in semi-race tune and hotter requiresteel caps for reliability. In
the verv hottest motors I use a main bearing support saddle. This is a one-piece device
that supports all the main bearings, taking the place of the individual main bearing
caps. It extends out to the oii pan deck and transfers the beating load, so thai it is
shared by the outside of’ the block as wet I as the main bearing webs.
After all the machining work has been completed, the block shou Id be thoroughly
washed with hot. soapv water. Be sure to gei all traces of honing grit scrubbed out of
the cylinders. using a bristle scrubbing brush. Blow the block dry, usiny compressed
air, and spray all the cylinders, tappet bores and bearing bores with awater dispcrsant
such asWD-
40. 7’o assist oiI flow back into the oi1 pan I recommend that the inside of the block be
painted v/ith a suitable oil resistant engine paint and sealer.
With the block prepared, we now turn our attention to the crankshaft. The
majority of engines use cast modular iron crankshafts which are suitable for use up to
semi-race iune, when balanced. Engines with a higher output wit I require a Tuftrided cast
iron. forged steel, forged nitrided steel, or a billet steel crank, Tuftrided or nitrided in
thai order of ascending merit.
Assuming the standard crank is suit able, we must have it crack tested bel’ore doing
any work on it. If it is free from flaws, a check should be made to determine its
straightness. Cranks can be straightened but generally it is a waste ot’timeas the
pressures ofcombustion and inertia loads will reverse thestraightening process. Because a
bent crankshaft increases bearing loads, it must be discarded if bent by more than
0.002 in.
Next measure each main bearing and crank pin journal. Rememberiiig that crank
journals wear oval, measure the diameter at several angles around the journal. Also
me¡isure at each end and in the middle ol’every journal , as wear can vary along
thejournal. Ovality and taper should be less than 0.(I(J03 in in a h igh pcrformance
orracing crank. Ift he wear is greater than this, grinding the journals underside will be
necessary. However. grinding weakens the crankshaft. so a crank replacement may be
necessary in high horsepower and high rpm engines. When the crankshaf’t is machined,
instruct the grinder to pay close attention to maintaining the fillet radii specified by the
manufacturer (FIGURE 6.2). Any reduction in the fillet radius will weaken the
crankshaft. On the other h¡ind an increased radius serves tostrcngt hen thcshaft. This
should not beoverdone or we can arrive at a sit nation where the big end or main
bearings are locking on to the fillet.
Any casting slag should be ground off’the crank and the area dressed. These slag
spots can become a stress raiser or future failure point.
To assist in the smooth operation of the motor and reduce bearing inertia loading,
the crankshaft should be dynamically balanced. Balancing will increase the file of the
crankshaft as a result of a reduction in the shock loading and vibriition to which any
imbalance would give rise.
Some manufacturers list Tuftrided replacement crankshafts. They are excellent
for high performance road and i ally engines. These can be used in racing motors but I
would suggest a strict eye be kept on rpm I inn its and thiit they be crack tested each
300 miles.
litandard cast iron cranks can be Tuft rided but there is quite a deal of risk
involved. Any ferrous metal object can be Tuft rided, but as the process requires
immersion in a chemical bath at a temperature of I06v°r for 180 minutes, internal
stress can lead to deformat ion. Cast iron crankshafts arc susceptible to bending and
changes in the journal
148 diameters. Ford and British Leviand scrap 40°? of their Tuftridcd cranks so I feel it a better
 The Bottom F.nd

Cylinders must not be offset

S
i

Cy!inder5 must intersect

cronksr!aft centre at 9O .

" -i— ") "

+ /
'”
J
Cylinders must not be offset
from crankshaft cef›tre.

Fig . 6, 1 Cylinders must oe occurote y bored.

proposition to buy a prcireated crank as in general they are not very much more
expensive than a standard cast iron shaft.
To check whether a crank has been Tuftrided, use on ordinary medium cut file
and attempt to file metal off one ot the webs. I l’this presents nodifficulty. the metal
has noi been especially treated. If, on the other hand, the file does not easily bite into
thecrank, but instead tends to skate across the sunace. then it is probable ihat the
crankshaft has been Tuft rided. A treated crank will not be harmed. soshould
asalesman refuse to let you carry out this test, shop cl.sewhere.
There is a good dea I ol’misunderstandingabout the Tuftriding processso I will
explain the benefits that it will impart to a treatcd object. Contrary to popular opinion,
it does not increase the core strength of’ the component. The Tuftride bath, composed
primarily of cyanide and cyanate compounds, releases specific quantities of carbon
and nitrogen in the presencc of’ f’errous maiei ials such as cast iron and alloy steel.
Nitrogen is moresoluble than carbon in ihese metals and diffuses into the component while
thecarbon forms iron-carbide particles at or near the sui face. These particles act as a
nuclei, precipitating some of the Perfused nitrogen to torm a tough compound zone of
carbon-bearingepsilon iron niiride. The compound zone, (0.0003 - 0.0005 in deep in
treated cast-iron) is tough and very resistant to wear, galling, seizing and corrosion. The
nitrogen diffusion zone(0.0008 -0.014
in deep in treated casl iron) underlying the tough compound zone is responsible for the 149
 4 Sirok r Perkorman‹’‹• Tuning

improved fatigue properties. This fatigue resistance is the most important value of the
Tul’tridc process in the racing engine. The nitrogen in stolid .solution prevents
incipient cracks from becoming fatigue failures, so ihe endurance limit of a cast iron
crank shaft can be increases by 20 - 60.
Forged cranks are more suitable for higher output rally and racing engines, but this will
depend on the type of steel in them. Forging increases the density of the component as
the meta I is litera fly squeezed into Ihe requ ired shape and compatted. This resu! ts in a
stronger core and better fatigue resistance. Many American manufacturers have used
r‹›rged crankx in their hiyh performance models and petro I truck engines over the years,
and these are availahle at moderate cosl.
Forged nitrided steel and Tuftrided billet steel cranks are the best money can buy. In
England, forged nitrided EN40B cr‹inkshafis are available for racing applications. In
America, bil Jet stee1 4340 crankshafts are prc1‘crred.
The nitriding process can only be applied to special mtriding alloy steels. For
crankshafts, a steel wit h a high core strength, such as EN40B, is used. Nitriding adds a
tough near resistant sk in. The crankshaft is mainta ined at a ternperalureof I 0tI0"F for
between 40 and I t)0 hours in a gas-t ighi chain ber t hrough which aminoniagas
iscirculated. Nitrogen is absorbed into t he surface, forming cxtrcmcly hard iron nitride.
and nit rides of ihe chruminm, molybdenuin and aluminium present in the steel.
4340 steel has similar core properties to EN40B and when iuftrided exhibits go‹xJ
wear resislance. My personal preference is for a crankshaft fully machined l’rom a
hammer- forged billet of EN25 steel, followed by Tuftriding. This steel machines freely.
lcaving a smcoth surfacc. Core properties are margin:illy supcrior to 4540.
Machining a crankshaft from a bar of steel is ii time consuming and costly operation.
H owevcr, the finished componen 1 is superior to the forged alternative. The taligue st
rength (endura rice linn it) of a heat-t reated a11oy steel bar is improved about 60%r by
machining its surt’acc clean. A ground or polished surt’ace will raise the l’atigue strength a
fun her 40fi.. Fat igue strength is a very importani cc nsiderat ion in a racing engjne as it
has a large bearing on component life and failure resistan ce. Forged crarikshafts can be
polished (very expensive) but this must be followed by shot peening to restore the
tough skin f’ormed as a by-product of forging.
Full counterbalancing is necessary on the racing crank, to reduce bearing loads. All
cmnks are connterba laneed overall, but not on each crank pin. As a result, the individual
throws are subject to forces which, in combination, tend to make the crank whip and
flex. Thc way to overcome this is by connterbalancin g each throw. This does not change
the overall crank balance, bui achieves an internal balance for each throw.
2’o prevent main bearing failure or case stretching in the VW enginc, fit a lully
countcrbalanced crank. Even in a very mild state of tune this is good insurancc against
engine damage. The standard crank will whip and pound the case, allowing the centre
main to turn and wreck the engine. If you can’t afford a fully counterweightedcrank for
your VW then havc wcigh ts weldcd onto the standard crank.
In the high rpm engine, crankshaft are in order to prevent main bearing
failurc due to lack of lubricant. Looking at FIGURE 6.3 you will scc how the straight
oilways in the crankshaft act as acentrifuge, pumping oil away from the main bearings.
The scheme to overcome this is to cross-drill or I holes to break out at the sidesof the
pins. Great care is necessary to ensure that the new hole breads fully into the existing
one, otherwisc oil
150 flow will be reduced. The original hole must be tapped and have a threaded plug Loctited in
 The Bottom End

Counterweights

Wcb
Fi[tet

*ig. 6. 2 B.M.C. cronkshaft.

io block ii off. In engines whereone main bearing supplies twocrankpins.

thismodification is necessary in the very early stages of tune if it is intended to run the
engine at high rpm.
The crankshaft journal oil feed holes should be chamfered, to improve oil flow and
disperse fatigue stress loads.
Thus far we have only considered the crankshaft from the aspect of engine reliability,
but it can also affect overall engine performance. Theoretically, crank throws should be
of equal length and correctly indexed (phased). Slight differences in the stroke from
cylinder to cylinder will not havea profound effect on performance. However,
incorrect phasing will knock power. A fourcylinderengineshould havea piston at
topdead centreevery 180', a six every 120', an eight every 90° ( 180' with single plane
crank.) If any cylinder has a crank throw out of phase by 5° this will have rhe same
effect on performance asan ignition timing error of 5° or a camsha(t timing error of 5'.
An expensive racing crank should have perfect phasing, but make a check to be sure.
Welding to increase the stroke of a crankshaft is not a practice to be recommended.
The welding of the crankpins will cause a structural change in the crysialine structure
of the underlying steel. The affected area then becomes astress point, likely to fail.
When thepin is ground on the new centre, much of the original core material is
destroyed. further weakening the beam. Additional to these considerations, give some
thought to the higher inertia loads generated by the longer throw.
At times, mariutacturers increase the crankpin diameter in later model engines
and in these instances it is possible togain asmall increase in stroke by grinding the
crankpin tothe original small diameter on a different centre (FIGURE 6.4), This
modification reduces the crank beam strength and increases bearing loads, but on the
plus side a useful increase in engine capacity is gained along with reduced frictional
losses due to the reduced crankpin diameter. In lower stressed engines for sprint type
speedway and hillclimb events, and moderate duty road engines, Ihis modification is
useful, but there are definite limitations.
 4 Stroke Performance Tuning

Pbtotion
Threaded plug.

Section X - X
C Ang ie ”B’ must equol original oilWQ
Clng[e”A B

Fig. 6. 3 Crankshaft oilway modif ication

Specially-manufactured billet steel or forged cranks can be obtained with a

stroke up to /. in more than stock. The strength of the crankshaft is not of primary
concern here: it is block rigidity ihat matters. II’ the motor is to bc tiscci in sprint type
competition, a standard, crack—free block with heavy duty main bearing caps and rods
wit I be adequate. H owever, if the motor is expected to live for a minimum of 300 miles
and more between rebut Ids, then a special heavy-duty racing block should be
conside i ed. At times, a stronger, more suitable block, may be ava ilabie from anot her
inode l vehicle by ihe same man ufact u rer.
W hen a Iorig stroke crank is fitted, somegrinding may be necessary inside the block
l’or connecting rod clearance. The clearance slots at the boitom of the bores wil I
probably require enlaning. The camshaft also may need cleara rice slois but do not
machine the cam at I the way aiound as this wit I wcak cii t lie sliat’t excessively and
lead re he xing und inaccurate valve timing. Do not forget to check the sump pan
clearance.
The connecting rod provides the mechanical link between piston and crankshaft. I t is
»• b.i• ctcd to alternating compression and tensile loads. I t is irtie to sav t hat the connect ing
rods in an engine have a tougher job to do than any ot her component. So it is not
surprising that a large number of l’a ilures in high performance and racing enqines
are causecl by t he
152 rods lett ing go.
 The Bottom End

The highest load is reached when the piston is at TDC on the exhaust stroke. This
tension load can range from a few thousand pourids in a small low rev engine lo well over
15,000 lb in larger, high rev engines. Interestingly, this maximum load occurs on the
non-firing stroke and is caused by i he inertia of ihe reciprocating assembly comprising
the smal I end, piston and pin, At TDC the piston is suddenl y stopped and then
reversed. I t is t his sudden change that produces tlle high tensiIe !oad.
On the compression stroke the load is noi as high (up to I 1,500 lb) as
compression bui!ns up slowlv, and soon after TDC, when combustion has finished, the
road changes from tension to a light compression load.
It is noi only the fact that stresses are high in a rod, but more important thai they
are applied and reversed each time the engine completes a cycle. This pulsing is much
worse than if the loads were constant and applied continually, and it is this that leads
to eventual fatigue and failure. A rod has to survive millions of stresscyclesduring a
lifetime so it has to be tough and thoroughly prepared.
Most rods are forged from carbon stee1, but they may also be made of cast iron (eg.
American Motors), aluminium or iitanium.
Cast iron rods have no place in a modified engine and should not be used.
Titanium is prohi bitivel y expensive and is used only i n small, high speed ( 15,000 -
20,000 rpm) motorcycle engines. Alumin in in rods are Iigh t and reasonably strong,
but fatigue quic k ly, so thev should be used out y in drag race and sprint-type speedway
engines where frequent replacement is possible. Chevrolet has recently released some
information on connecting rod fat igue test ing that they has'e carried out, Some
aluininiuin rods t’ailed n1’ter 150,000 cycles. The heavy duty small block rod, polished
and shot peened, survived 1 to 2 million cycles at t we equiva lent of’ 7500 rpm. The
Chevy K rod .survived a ininimu in of 10 milIion cycles at 8000 rpm. When it is
considered that the average race engine is required to stay iogether l’or 300 to 500
miles between rebuilds, the equis'alent ot’ l to 1 /. million cycles, it is evident ihat care is
needed, firstly in selecting thecorrect rod, and secondly in regular crack test ing and
rcplacemcnt.
Connecting rods designed for the mosi stressl’ul conditions are usually forged or
machined from 4340 steel. I prefer rods 1’ully machined from a hammcr-forged billet
or EN25 steel, dressed a ton g the beams and shot peened.
Man y heavy duty rods are in tire massive lhan the standard component. so t here are
a few things that in ust be consideteD out ing insta Hatton. As they are physica lty heavier, t
he erankshaft counterwe i#h ting will have to be reba tanccd. 5ince the bid cnd of Ihe rod
is larger, carc in ust be ta ken to verify t laat it, and also t he rod boIis, clear the block
and camshaft.
R nd cap hotts have to be designeci to withstand a considera ble force, to stop t
he cap t’rom brea k ing away from the rod. A t limes it is necessary to fit boIts of a
larger diameter tha ia standard, Whatever the diameter, the tensile st rength of t he bolts
should be 185,fl00 psi minimum. Big end bolts should never be reused.
I t you intend to retain t he standard con rods, ihey musl be carefully, and
individual ly, hand picked. Rods wit h an y forging irregularities or indentations shout
d be avoided. Buinps standing proud of ihe rod can be removed. but indentiit ions are st
ress points, likely to cause failure. The smal I end eye should be in the centre with an
even thickness of material amou n‹J the eye. Again the big end should be well formed
and symmet rical. Select rods Ihat are reasonably equal in weigh t. A heavy rod may he
excess ivel y wea kencd when materia! is
ground ot’f to bala nce it wit h the ligh ter rods. II they arc not at ready numbered, nuinher 153
 4 Stroke Performance Tiinin,q

each rod and cap, using a numbcr punch, to errs ure that they are never inismatcticd.
Standard connect ing rod preparation inns I always include a check to cueure
that t he rod is not twisted or ben i. Eit her condition will wreck I he piston andfor big
end bearings, Also necessary is resin.ing to bring the rods to an equal tength. At ihe
same time the bigend inside diamcter should be chec ked forsize and concent ricity wit h
the bearings fitled and the bolts torqued to specification. The small end bush should be
honcd to give the correct piston pin fit.
The failure point for many rods is at the corner formed by the flat machined fort
he rod bolt and nut seai. These corners must be radiused toavoid stress concentration
in this arca. Every con rod will require some modification here (FIGURE 6.5).
The tough skin formed on the con rod bjforginggives the rod much of its strength
and fatigue resistance. Therefore the rod should never be polished unless it is
folrowed by shot peening, to create another work toughened and compressed skin. It
is a waste of time pmish ing the entire rod and then having ii shot pecned. If you have
a look at the shank ofa rod you will see, along its ed#cs, a rough band where
metsIappears to have becn sa wn away. That is where the excess meta I, called flash, was
squeezed out Irom betwecn the forging dies when the rod was icing made. Later, most
of t he t)asti is trimmed off but a bead is lel’t, as vou can see. Of course, there is no
hard sk in :ilorig this ridge, in fact its rough ness is a stress raiser, so this ridge should
be removed on a sanding belt. Give theentire beam a polish with fine emerv cloth and
ihen follow up with buf’fing and shot peening.
The shot pecning process has a very useful place in connect ing rod preparai ion.
Fat igue f’a ilure aImost at ways stnrts at t he surface due to a combination of’ two
things. Firstly, under most forms of l‹›ading, maximum stress is at the surface, and
sccondly. str rface imperfect ions are st ress ra isers. By bombarding the surface with
steel shot the surface layer is compressed and unified. This mea us that when the
component is under toad, any tensile slresses ai the surface will be reduced and any
cr›mpressive siresses will be increased. Because peening can sea Ioversurface cracks, a11
crack test ing in ust be done first. Also any stra igh tening should be carried out before
pecnin g, or t his will remove the effeci ot’ peening. After peening, check the
componeni dimensionally, as there will be some growt h and if the peening has been
done incorrcct Jv, bending may cccur.
The relationship betwen con rod lengt h and ihe erankshaft st roke. called t he
con rod ratio (con rod length - crank stroke = con rod ratio), is of importance in the
high performance and racing engine. Most rod ratios range from 1.5 to 2:1; 1.65 to 1.
75 is average.

Stock c ronkpin

5tOC\

154 Fig. 6. A Crankshaft stroking fry cran kpir d ia. reduction

 The Bottom F.nd

The length of the rod directly affects the perl’ormancc of on engine. Basical Jv. a
tong rod will improve top end perrormance and also reduce piston and cylinder wear.
A short rod picks up bottom cnd pcrl’ormance but increases pislon and cylinder wear.
’I he long rod will cause the piston todwell at TDC longer and to move away frrim
TDC more sl‹iwly. This is at a time when piston velocity is critical for good cylinder
filling. If the inlet valve is opening too soon or too quickly, or il’ the ports are trio large,
mixture velocity and power will be lost at lower engine speeds. Therelore to pick up
bottom end power with a long rod it will be nccessar v to change the valve t›pcnin
gcharactcrisi icsofthe cam and also reduce the valve and port sizes.
A short rud will accelerate the pision quickly from T’DC, fi fling the cylinder
completely at lower engine speeds, at the expense of high speed pcrformancc. This
mcans that intake valve opening can be more rapid and larger valves and ports can
beused to take advantage of the higher piston velocity to aid cy linder filling at higher
speeds. With this understanding it is possible to compensate and make anv engine
perform equally, regardless of whether it has a long or a short rod (forgetting ab‹›ul
wear for the moment). On the other hand if we want top power for a long, l’ast circuit,
we can usea long rod, or if we need low end power for a short slow circuit. we use a
short rod.
Wilh shorter rods. engine wear is a problem, and for this reason I do not like
rod ratios below 1.65: 1. This is worth th inking about bcfore you fit a long stroke
crank as the tcndciicy for the piston to rock in the bore is increased as the rod length
to stroke ratio is rcduccd. Shorier pistons with a reduced comprcssion height must be
used when a stroker crank is fitted. otherwise they would pop out the top of the
cylinder. Short pistons rock more than standard pistons, accelerating the wear problem.
Seatings in a high power enginc, as web I as providing a low friction wearing
surf’ace, also have to absorb tremenclous shock loads. Therefore only top quality
trimeial bearings should be used for the marine and big ends. White metal bearings
are satisfactory for carrving ihe camshaft.
I use only Vandcrvell lead-indium and TRW Clevite CL-77 bearings because of
their special l’eat ures. Boih types are able to stand up to the worst competition loads
when correci ly installed. These bearings are steel backed, with acopper-lead intermediate
iaver. This layer gives the bearing good fatigue strength and load carrying capacity,
also resistance to h vdraulic break out. The running surface for the Vandervell bearing
is a precisit›n overlay of lead-indium while the CL-77 overlay is lead-tin.
Correct bearing clearance is an obvious nccessity. Excessive clearance will
promote knock ing and pounding and allow excessive oiI throw off into the cylinder
bores. This will cause h igher frictional losses in the cylinder and increase oil
consumption. Excessive clearance at the big ends will lcad to oil starvation at the main
bea rings, with rcsultant bcaring fai lure, Insufficient clearance will cause rapid
bearing deterioration as a result of the incfcased temperatures that come about due to
insufficient or i flow or a thin oil film (2’A b ICE b.3). Con rod side clearance (end-float)
also affects bearing lubrication, so this should be ctieckcd on cvery big end to ensure
proper oil control.
Therc are jusi a few simple rules to follow for correct bearing installation.
Accurately measurc and record the inside diameter ofevery main bearing and bigeud
housing (without bearings titted). Now do the same with the crankshaft main and
crankpi .iournals. Unwrap all thc bcarings and carel’ully wash thcm in ciean solvent,
to remove the protective film, Meilsure the bearing sheil ihickness. The dill”erencc
between the shafi and housing
diumei ers is the sp‹ice left for the beaming shell, plus the running clearance. When you sii 155
 4 Stroke Performance Tuniny

down to work out the clearances, don’t forget to double the bearing shell thickness as
there is a shell on each side of the shaft. If the clearances are too right, the
offendingjournalswill need a light lapping.

TABLE 6.3 Bearing clearances for trimetal copper lead bearings

Clearance between shaft
Diameter of shaft { in) and bearing 5 ide clearance
1.5 0.0012-0.001 7 0.004-0.00f›
2.0 0.0015-0.002 0.005-11.007
2.25 0.00 15-0.0025 0.005H.007
2.5 0.0022-II.0027 0,005-0.fI07
2,75 0.0024-0.0025 0.006-II.0fI8
3.0 0.0025—11.0028 0.007-0.009
3.25 0.0025-0.003 0.007-€i,009

Note.- In engine.s z herr twin con rods .vhare a rommon gran I:yin (eg n V8) multiply sit:ie clearaii r bv
.1 for steel rods, ond bv 4 for aluminium reds.

After ensuring that all the bearing hriusings and bearing shells are perfectly clean
and dry, l’it ihe shells and check oil hole alignment. Any misalignment should be
corrected, using a small round ke; r Ie. After r Iin z. carefully drcss the steel back of the
sheII to remove any metal haze. Next coat all the bearings with engine oil and fit the
crankshat’t. Do not use a 50 - 50 mixture ol’oil andany substance like STP. Fit the main
bearing caps in their correct order with arrows lacing the front of the block and
gradually tighten them down. Before final tightening, the crank should be tapped to
each side with a soft hammer in order to line up the bearing caps. Now check the
crankshaf’t end float. It should be within 0.004-0.006 in with a cast iron block. If the
clearance is more than this, fit thicker thrust washers.
When it comes to fitting the big end bearings follow the same procedure as for
the fitting of’ the main bearings. There are still a few engines oTou nd using lock tabs
on the big end bolts, Throw these away, as they crush and givea false bolt tension
reading. Use Loctite on the threads and you won’t have anv problems with loose bolts.
Moving further up in the block. the next components forconsiderai ion are the
pisttans, pistofl rings and piston pin. Firstly, von must decide whether the standard
piston is up to thejob. Il the motor is in sports tune, it is quite probable that the
standard cast and sJottcd pistons will be sntisfaci ory. higher states of tune will
demand unslotted cast pistons, and in racing tune unslotled forged pistons will be
necessary.
Most production engines use cast pistons. because this type is easy to produce
and shape as required. Some high performance engines will be fitted with high quality
forged or cast pistons right from the factory eg. Cosworth Vega, Lotus/ Ford Twin
Cam. By looking
inside the piston you will be able to see whether it is cast or torged. Cast Fistc›ns have
quite intricate under-crown shapes around the gudgeon pin boss. Forged pistons are
machined smooth inside and lack intricate struts and braces.
The cast piston has a relatively low material density. Forged pistons are much
denser and consequently have a higher tensile strength. They are capable of
withstanding higher pressure and heat loads than the cast piston. Dun to their h
igh density, tdermal
156 conductivity is improved such that a piston crown temperature reduction of’ l00'F is usual,
 The Bollam h.nd

Additional to these advantages, the forgcd piston can be machined to liavc a much thinner
crown and skirt, and can therefore be much lighter and still stronger than a cast item.
The worst feature of production pistons is the not for oil drainage behind the oil cont
ro1 ring. This usually extends almost from one boss around to the other boss, on both sides of
the piston. To rna ke the sit nation worse, expansion c‹introl slot.s cut into the pist on skin
usuallv break into the oi! drain slot. Slots weaken the piston considerably, allowing the sk irt
to brca k uwav from the top of the piston . In standard production engines this can be a
problem. so you can imagine what you are up against i1’slotted pistons are retained in a
high pcrformancc unit. Pistous suitable for high perl’ormance usc(sit hercast orforgecl) do not
have anv slots for expansion control or oil drain back. Instead, small holes are dril1ecl right
t he way around the oil cont rod ring groove, for oil drainagc. Ext ra piston cleara rice and
special design takc care of expansion.
Racing pistons may be of either the full skirt or the slipper variety. Many American
pistons are a cross between both designs, having a skin extending to belowt he pin boss, but
cut back io the thrust faces bel‹›w this point. Generally, I prefer the full skirt or the
American design for best expansion control and scuff’ resistance.
We tend to think of pistons as being round, but actually iheskin iscam ground an oval
shape. The piston also tapers from bottom to top. Both ovality and taper are necessary to
prevent seizure. The top of ttie piston is almost twice as hot as the bottom of the skirt, so it
expands more. Due to the exti»a metal around the pin bosses, more heat is directed to th is
area, elongating the piston across the piston pin axis. ’Fo compensate, the piston skirt is
ground oval. Most pistons are from 0,005 - 0.012 in less in diameter across the pin axis than
across the thrust l’aces. Be careful to measure piston clearance only on the thrust faccs, and
at the bottom of the skirt.
Accidcntally dropping a pistt›n may damage the skirt and lead toevent ual seizure, due tc›
skirt distortion. Never bang a piston pin out with a hammer and drift. This is a sure way to
push the skirt out of shape. If the pin will not push out easily, heat the piston in boiling
water or oil and then gently' tap it out with the con rtid secured in a vice to prevent any
pressure at all on the piston.
For better lubrication, and to allow l‘or extru piston expansion, high perl’ormance
engines musi have more piston to cylinder cle‹irance than models oll’thc showroom floor. I
prefer 11.001 3 - 0.0015 in clearance per inch of bore for road and rally engines. Race engines
require 0,00 IS - 0.0025 in clear:ince per inch of bore. There should be a maximum o(0.0005 in
dil”terence in cleara rice between the ligh test and sloppiest cylinders. If the tolerance is greater
than this. t ry swapping the pistons around in difterent cy I inders. W hen it is finafly decided
which piston does where, number each piston inside the skirt, using a suitabie mark ing pen. Do
not mark ihe crown as this may be machined Iuter to give lhe correct deck heighi.
As the majority of motors have heads with inclined or canted val ves, some though t niust
be given to provid ing adequate valve to piston clear;Ince. There should be a ininimum of U.t)60
in vertical clearance, although I prefer o.og0 in for the inlets and 0. 100 in for the exha ust.
The cut out diameter for saf’ety is 0. 120 in greater than the valve head diametcr.
There are two basic ways to go about checking t he safe valve working clearance, but both
methods involvc quite a deal of’ work. Either technique requires that the motor be at inost
totally assembled. The cam must be installed and accurately timcd, nnd the head and head
gasket need to be fiited.
The first way involves the use of a piece of modelline clay pressed down ten thc top of a 157