Porting and port-matching a head means a more efficient engine — and it can be done at home. Here’s how to port cylinder heads.
For an engine to work as efficiently as possible, the airflow through the cylinder head is crucial. The most critical area lies around the valve seat. This is where the flow is most disrupted, on both the intake and exhaust ports. A step or ridge on the inlet port or bowl can cause an ‘eddy’, leading to atomised fuel dropping out of the air during induction. Small burrs of metal heating up and igniting the mixture at the wrong time are a common cause of pinking and running on.
Malcolm Darlow and Mike Bedford of Bell Classics showed us the cleaning-up process on a typical overhead-valve cylinder head. Shown here on the inlet and exhaust ports, it can be repeated to match the carburettors to the manifold.
1. This head has returned from a machine shop after having unleaded valve seats inserted. Malcolm inspects it for wear and cracks. With an unknown head, have it skimmed, then crack- and leak-tested before any machining takes place.
2. Use a block of wood to prevent damage to the opposite face of the head and stop it sliding around on the bench while you’re working on it.
3. Malcolm runs his fingers around the inside of the valve head, feeling for any ridges on the inner walls that will disrupt airflow. He’s also checking for any lip between the head and the valve seat
4. Starting with a coarse-grade cutting stone and a low speed setting, Malcolm begins smoothing out the edges of the inlet port and the inner rim of the seat. The aim is to make the two inner surfaces level, with no step between them.
5. It’s important to go slowly and do a little at a time, checking your progress with a bright torch. It‘s far easier to remove material than to put it back. Removing too much or breaking into a waterway would almost certainly turn the head into scrap.
6. Here the exhaust port (farthest left) and inlet port of number-one cylinder have been matched to their respective valve seats. Continue until the ports for the other cylinders are done. Malcolm now turns the head to work on the port faces.
7. On this head, the factory have machined smooth only the first half-inch of the inlet port — no doubt because of time and cost restraints. We’ll want to remove the rougher textures and ridges visible further in to aid air flowing in from the carburettor.
8. Having cleaned and checked the cylinder head studs, carefully put four or five of them into the head using two nuts locked together and a quality spanner — anything else could damage the thread. Wind each stud in gently.
9. Put the manifold gasket in place to use as a guide — note the perforations to stop the gasket from blowing out. Malcolm prefers to use the aluminium side facing the exhaust manifold since it’s the exhaust that will move about, not the head.
10. The ports are rarely the same size as those in the gasket; we’ll use the gasket as a guide as to how much to remove. You could mark the area for removal using a sharp scribe but Malcolm prefers…
11. …to use bright-coloured spray paint. Just dust it on — you want it to dry quickly and you don’t want any paint runs.
12. The area to be removed is now clearly marked, but we also need to ensure the ports in the inlet manifold will match those in the cylinder head.
13. Place some masking tape on the head sticky side up and put the gasket over it. Place the inlet manifold on top, using the studs for precise alignment.
14. Carefully tape the gasket to the inlet manifold. Lift the manifold off — it’ll bring the gasket with it, enabling you to use it as a guide. Repeat the trick with the spray paint.
15. The amount of material to be removed from the inlet manifold is now shown. The key to success is that the manifold ports should precisely match those of the head – gasket fit is less important provided the gasket doesn’t make a step between them.
16. Malcolm repeats the porting procedure, carefully removing the material using coarse then finer grades of sanding stones…
17. …and does the same on the inlet manifold. Here, only the port on the left has been resized. After the process is complete, remove any remaining paint overspray with thinners.
18. It’s vital to remove all the swarf and debris. Malcolm recommends blowing everything through with an airline followed by a thorough wash with paraffin or brake cleaner. Then it’s ready to fit together.
This guide to how to port cylinder heads first appeared in the October 2018 issue
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