The diff is on hold until I can get hold of a pinion depth shim that’s on backorder so I decided to do some work on the cylinder head. Having read a fair bit about porting and polishing I thought I’d give it a go. There’s loads of videos on the subject on YouTube. Some better that others, quite a few of them involve methods that seem to be a bit yee-haw. I’ve taken what I can from it and had a go myself.
Here’s what I was starting with. It wasn’t quite this bad while it was on the car as it’s been off the car and stood for some time.
I removed the valve guides first. The valve guides can be tapped out but I didn’t have anything particularly suitable so I made up a puller. The valve guides are 18.5mm high from the surface of the head on the rocker side. An M8 bolt (or threaded bar in my case) fits through the valve guides on the Spitfire just right. Under the valve on cylinder side I used a rivet nut with the lip ground off so it is just under the diameter of a guide. On the rocker side of the head I used an old gudgeon pin over the guide with a few washers and a nut, slowly but surely wind each guide out.
Next was to give the whole thing a good clean. I used some sodium hydroxide based over cleaner for this which is fine for use on cast iron heads (alloy heads won’t last long with sodium hydroxide). I left it for 1 or 2 hours. This loosened the dirt enough to brush off without too much effort.
After giving it a good rinse and dry I started with an air powered die grinder and various carbide burrs and stones. I’d borrowed some bits from a fella at work as well as a flexible extension without which the task would have been ten times harder. With the die grinder in one hand controlling the speed and the other doing the grinding you can get pretty fine control. Air is definitely the best way to go if you can. The die grinder uses a lot of air though so anything short of about 8CFM won’t keep up. Having a regulator is pretty essential too, it will want setting to 50-60psi.
Below is a photo taken before any cleaning. You can see how coked up the exhaust port is. The inlet port above it has a fairly visible casting mark down the side and at the bottom of the port (left in the photo) it has a pretty abrupt edge.
- Keep the ports roughly the same size. The larger they are, the further up the rev range the power is made. I’m aiming for low end torque on this car. The 1500 engine in the Spitfire is actually a stroked 1300 and it doesn’t like being revved on a crank with only 3 bearings!
- Remove potential obstructions that would reduce flow. There’s a few areas that I noticed. The casting marks down the sides of all the ports, around the valve guide and near the gasket side as mentioned above.
- Keep the finish on the inlet ports a bit rough and the exhaust ports smooth. The idea is that the rough surface on the inlet ports helps air flow. Similar to this idea. The exhaust port is kept smooth to try and help reduce carbon build up (which was clearly an issue on my car!) and to help prevent heat loss. Hotter gas, higher velocity.
Starting with a flame shaped carbide burr I removed most of the casting marks and carbon build up. I found that keeping it moving all the time was best to avoid deep areas forming. You’ll also need a lot of light, I had a 500w flood lamp sat on the head most of the time shining through the ports which worked really well. I finished the exhaust ports up with a white stone to give it a smoother finish. The slightly dimpled effect on the inlet side is just the result of the carbide burr which you can see close up below.
There is a possibility you can go through the side of a port and open it up too far which means you’ve gone through to the water jacket. I don’t think I got anywhere near this point here but it helps to remove the two core plugs from the end of the cylinder head as it will give you an idea where the outside of the ports are.
I gave the whole lot a clean up with the wire wheel attachment after I’d finished grinding. I’m pretty pleased with the result and I hope it flows better than it did before. Next steps with this are new valve guides, new valves and core plugs. Then it can be painted and ready to fit.
Before and after…