Cookie Head Modification 2011
Introduction
These are the head modifications as carried out by me to my mk3 Spitfire head. The most significant
modifications came from Neil Collingwood’s DIY Heads thread on Sideways Technologies forum. Which
included a simple procedure and short side radii profiles. A condensed version can be found here.
I am writing this to share my experiences on what steps and tools worked and which didn’t. This
document meant to supplement Neil’s guide. The information found here is a compilation of the
Competition Preparation Manual 5th Edition and Gareth Thomas’s Tuning Manual for Standard Triumph
Car Produced between 1959 & 1980. Both are available online with enough digging.
The head I used was a 515973, which is a high compression head (9:1) from a 1300 with small intake
valves (1.303”). The more idea head would be the 218142 from a late mk4 which has large intake valves
(1.438) and still retains the 9:1 compression ratio.
Tools
I used an electric die grinder from Princess Auto. It was
very cheap and did a great job. I would recommend this 4
route if you don’t have a compressor, or you have a small
2 3
one. It was very easy to hold much like a pencil and take
very small light cuts.
1
For cutting, grinding, finishing I used a combination on 4
bits.
1. ¼” flame cut bur which was quite fine which I used
on the manifold side of the intake and exhaust
ports for gasket matching. It cut at a nice workable
speed and left a respectable finish.
Figure 1 Bits Used
2. ½” x 1-1/4” Rough flame cut which was used to
make the initial short side radius cut as per Neil’s instructions.
3. 1” Spherical stone was used to expand the initial cut made by 2, as per Neil’s instructions.
4. 60 & 80 Grit flappers were used to smooth out the final port
shape. They do a great job and relieving small irregularities in
the surface and leave a nice finish. They also wear into shapes
that are quite useful for reaching up into the ports to finish the
whole port.
Figure 2 Useful Flapper Shape
1 Triumph Spitfire Head Modifications by Alexander Christie
Cookie Head Modification 2011
Manifold Side
I found it useful to do this section first as you can work perfect down 1-1/4” with the valve and have the
cross section almost perfectly circular. Once you do the short side radius this is not so much the case.
First I port matched using my small flame cut bur (1) I found the ports to be very close at the edge of the
head being only slightly smaller than the gasket. Once this was done I used an intake valve from a mk2,
you could use anything the same diameter as the port which is circular and rigid. As a gauge and worked
my way down the port. I would put the guide in and lower it into the port until it stopped, being careful
to keep the stem parallel to the port direction. Then I would mark the spots where it touched the wall
with a sharpie. An example can be seen in figure 3 showing 4 high spots. I’d remove the guide and give
that area a quick light shave with the small flame cut bur. This was repeated until I reached a depth of 1-
1/4” and the ports starts to make the turn at the short side radius. From here the port is no longer as
circular. I found the largest area which needed adjustment here was 3/8”-1/2” into the intake port
where there was a large flat intrusion., beyond this the port was quite circular.
Figure 3 Port Guide
Short Side Radius
For the next step see Neil’s guide to DIY head mods.
My tips are after the initial cut with the burn, widen it making long sweeping motions with the same bur
until to you are close to the depth of the cut. Then switch to the spherical stone and work your way up
the port towards the manifold with these same sweeping motions. The spherical stone allows you to
really follow the port around and eliminates a flat spot at the end of the radius which you would get
2 Triumph Spitfire Head Modifications by Alexander Christie
Cookie Head Modification 2011
using a flat stone. The larger diameter sphere also matches the port shape quite well and makes it
tougher to cut a divot. Figure 4 shows my final short side radius.
3 Triumph Spitfire Head Modifications by Alexander Christie
Cookie Head Modification 2011
Valve Guide Area
I used the spherical stone again here and just kept
gently working it around the area until it became
relatively smooth. I think it is important not to remove
more material than is necessary here which is why you
can see the untouched area behind my valve guides.
To me it was more important to create a nice arcing
path without large intrusions which followed the
original port path.
I also paid special attention to the sides of the port
here. I notice the ports cross section was more oblong
here with the roof having a tighter radius than the Figure 4 My final Short Side Radius
short side radius (Speaking to the cross section now). I
assume this is because more of the flow will follow the
shorter path along the short side radius. With this in
mind I tried to follow this and only just clean the area
up.
Figure 5 Valve Guide Detail
4 Triumph Spitfire Head Modifications by Alexander Christie
