TECH BULLETIN

REASONS AND CAUSES

FOR CAM FAILURE
Cam failure is rarely caused by the cam itself. The only The engine should
things we can control during manufacture pertaining to cam start quickly and run
lobe wear are lobe taper, lobe hardness and surface finish. between 1500 and
Of all the damaged cams we have checked over the years, 3000 rpm.
more than 99.99 percent have been manufactured If the engine will not
correctly. start, don’t continue
Some people have the misconception that it is common for to crank for long
a cast iron flat tappet cam to occasionally have a soft lobe. periods, as that is
We have yet to see a cast iron flat tappet cam that had very detrimental to
a soft lobe! Likewise, we have not seen a reputable flat the life of the cam. Check for the cause and correct. Vary
tappet lifter that is “soft”. the RPM up and down in this rpm range during the first 15
When the cast core is made at the casting foundry, all the to 20 minutes, and avoid idling the motor. (Do not run the
lobes are flame hardened. That process hardens all the engine at a steady rpm). During this break-in time, verify
lobes to a depth below the barrel of the core. That depth of that the pushrods are rotating, as this will show that the
hardness allows the finish cam grinder to finish grind the lifters are also rotating. If the lifters don’t rotate, the cam
cam lobes with a Rockwell hardness above 50Rc. lobe and lifter will fail. Sometimes you may need to help
The generally accepted hardness on a finished cast cam spin the pushrod to start the rotation process during this
should be between 48Rc to 58Rc. break-in procedure.
All of the finished cams that we have checked are always
above 50Rc hardness on the lobes. Many outside factors, (i) Note: Lifter rotation is created by a taper ground on
or a combination of factors, can cause cam failures. the cam lobe and the convex shape of the face of the flat
We will list some of the factors we have determined that tappet lifter. Also in some cases, the lobe is slightly offset
may cause camshaft failure. from the center of the lifter bore in the block. If the linear
spacing of the lifter bores in the block is not to the correct
1. Lobe wear factory specifications, or the angle of the lifter bore is not
A) Incorrect break-in lubricants. 90 degrees to the centerline of the cam, the lifter may not
Use only the Moly Paste, Part Number 99002-1 that is rotate.
included with the cam. This Moly Paste must be applied to Even if the engine you’re rebuilding had 100,000 miles on
every cam lobe surface, and to the bottom of every lifter it and the cam you removed had no badly worn lobes, this
face, of all flat tappet cams. Roller tappet cams only require still doesn’t mean that your block is made correctly. It just
engine oil to be applied to the lifters and cam. Also, apply means that the break in procedure caused everything to
the Moly Paste to the distributor gears on the cam and work correctly. Be careful to watch the pushrods during
distributor for all camshafts. For extra protection, an anti- break in to verify lifter rotation. Don’t assume everything
wear additive containing the correct concentration of Zinc will work correctly the second time.
dithiophosphate MUST be added, such as Crane Super
Lube, Part Number 99003-1. (ii) Note: Always use new lifters on a new flat tappet
cam. If you are removing a good used flat tappet cam and
NOTE: Do not use synthetic oil during the break-in period. lifters and are planning to use them again in the same (or
It is not recommended to use any type of oil restrictors to another) engine, you must keep the lifters in order as to
the lifter galley, or use windage trays, baffles, or plug any what lobe of the cam they were on. The lifter breaks-in to
oil return holes in the valley. Oil has a two-fold purpose, not the specific lobe it is mated with and it can’t be changed. If
only to lubricate, but to draw the heat away from whatever the used lifters get mixed up, you should discard them and
it comes in contact with. The cam needs oil splash from the install a new set of lifters and break the cam in again as you
crankcase, and oil run-back from the top of the engine to would on a new cam and lifters. You can use new lifters on
help draw the heat away. Without this oil flow, all the heat a good used cam, but never try to use used lifters on a new
generated at the cam is transferred to the lifter, which cam.
will contribute to it’s early demise.
(iii) Note: Roller tappet cams don’t require any break-in. You
B) Correct break-in procedure. can use roller lifters over again on a new cam if they are in
After the correct break-in lubricant is applied to the cam good condition. There will be, of course, no lifter or pushrod
and lifters, fill the crankcase with fresh non-synthetic oil. rotation with the use of a roller tappet cam.
Prime the oil system with a priming tool and an electric Roller Tappet cams and lifters will fail in street use if used
drill so that all oil passages and the oil filter are full of oil. with excessive spring pressure, or if low RPM is used for
Do NOT turn the engine over on the starter motor to get oil extended periods in conjunction with excessive spring
pressure up. This just wipes the lube from the camshaft! pressure.
Pre-set the ignition timing and prime the fuel system.
Fill the cooling system. Start the engine.

ph. 08 8363 5566 • www.cranecams.com.au

 TECH BULLETIN
C) Spring pressure (v) Camshaft end play. Some engines have a thrust plate
Normal recommended spring seat pressure for most mild to control the forward and backward movement of the cam.
street-type flat tappet cams is between 85 to 105 lbs. More The recommended end play on these types of engines is
radical street and race applications may use valve spring between .003” to .008”. Many factors may cause this end
seat pressure between 105 to 130 lbs. play to be changed. When installing a new cam, timing
Flat Faced lifter open pressures should not typically gears, or thrust plates, be sure to verify end play after the
exceed 330 lbs open pressure for acceptable cam and cam bolts are torqued to factory specs. If the end play is
lifter life. (AFTER run in) Open pressures higher than this excessive, i.e loose cam gear attaching bolt(s), it will cause
can be sustained in some applications for limited use, but the cam to move back in the block, causing the side of the
are not generally recommended. lobe to contact an adjacent lifter.
For street hydraulic roller cams, seat pressure should range
from 105 to 140 lb. Spring seat pressure for mechanical 2) Distributor gear wear.
street roller cams should not exceed 150 lb. Roller lifter The main cause for distributor gear wear is the use of high
life, for street use, will decrease in proportion to the open volume or high-pressure oil pumps. We don’t recommend
spring pressures. Race roller cams with high lift and spring the use of these types of oil pumps. If you do run these
pressure are not recommended for street use, because types of oil pumps, you can expect short life of the cam and
of a lack of oil splash onto the cam at low speed running distributor gears, especially for low speed running, in street
to help cool the cam and lubricate the lifters. This high type applications. If you must run these types of oil pumps,
spring pressure causes the heat created at the cam to you can increase the life of the gears by adding more oil
be transferred to the roller wheel, resulting in its early flow over the gear area to help cool off the point of contact.
failure. Any springs that may be used must be assembled NOTE: distributors that have end play adjustment (up and
to the manufacturer’s recommended height. Never install down movement of distributor shaft and gear), Maintain
springs without verifying the correct assembled height and a maximum of .010” end play, to help prevent premature
pressures. wear.
NOTE: Increased spring pressure from a spring change and/
or increased valve lift can hinder lifter rotation during cam 3) Broken dowel pins or keys.
break-in. We have found that decreasing spring pressure The dowel pin or Woodruff key does not drive the cam; the
during the break-in period will be a great help. This can be torque of the timing gear bolt, or bolts, against the front of
accomplished by using a shorter ratio rocker arm to lower the cam drives the cam.
the valve lift; and/ or removing the inner spring, during the Some reasons for the dowel pin or key failing are: Bolts not
cam break-in time, if dual springs are being used. Open being torqued to correct specs; Incorrect bolts of a lower
spring pressures should be BELOW 300lb during run in. grade being used; Stretching and losing torque; Not using
the correct hardened washer that may distort and cause
D) Mechanical interference. torque of the bolt to change; LocTite not being used; Or
There are many factors that can cause mechanical some interference with the cam and lifters or connecting
interference. rods causing the cam to stop rotation.
(i) Spring coil bind: This is when all of the coils of the spring
(outside, inside or flat damper) contact each other before 4) Broken cam
the full lift of the valve. We recommend that the spring you A broken cam is usually caused by the cam being hit by a
are using be capable of traveling at least .060” more than connecting rod, or other rotating parts of the engine coming
the valve lift of the cam from its assembled height. loose and hitting the cam. When this happens, the cam will
(ii) Retainer to seal/ valve guide boss interference. You usually break in more than two-pieces. Sometimes the cam
need at least .060” clearance between the bottom of the will break in two pieces after a short time of use because of
retainer and the seal or the top of the valve guide when the a crack or fracture in the cam due to rough handling during
valve is at full lift. shipping, or some time before installation.
(iii) Valve to piston interference: this occurs when a change Breakage can also be caused by bad harmonics being
in cam specs. (i.e.; lift, duration or centerline) is enough transmitted from the crank rotating assembly, especially if
to cause this mechanical interference. Also: increased gear drives are used, or if the timing chain is too tight.
valve size, surfacing the block and/or cylinder head may Most people will not have any means of checking cam
cause this problem. If you have any doubt, piston to valve straightness. As a general rule, if you can install the cam
clearance should be checked. Minimum recommended in the engine and install the timing gear, the cam should
clearance: .080” intake and .100” exhaust. turn freely with just your finger pressure. There should not
(iv) Rocker arm slot to stud interference: As you increase be any drag or resistance in turning the cam. This free
valve lift, the rocker arm swings farther on its axis. turning of the cam is assuming that if new cam bearings
Therefore the slot in the bottom of the rocker arm may run were installed, they were the correct parts and they were
out of travel, and the end of the slot will contact the stud installed correctly.
and stop the movement of the rocker arm. NOTE: When removing a used cam that may be worn,
The slot in the rocker arm must be able to travel at least you may have difficulty turning or removing it. This may
.060” more than the full lift of the valve. Some engine not mean that the cam is cracked or fractured. The heat
families, like small block Chevrolet, have stamped steel generated at the cam during the failure of the cam lobe,
rocker arms available in long and extra long slot versions for and/or lifter, will distort the cam and cause it not to be
this purpose. straight any more.

ph. 08 8363 5566 • www.cranecams.com.au