Modifying Norton Pistons

When working on my ES2 we modified a 16H piston to increase the compression ratio whilst searching for more performance. Here is what we did and how we did it.

Shortening the skirt

The first stage is to shorten the skirt by 5/16" to ensure it clears the flywheels at BDC. The simplest way to do this is to turn the skirt down in a lathe as shown in the image.

Valve pockets need to be measured and cut individually as they will vary from one engine to the next depending on cams being used, valve timing and valve sizes.

This picture clearly shows a larger pocket on the left (inlet).

Adding the valve pockets

The next job is to machine the valve pockets. The best way to do this is to mount the piston on an angle plate and use a fly cutter on a milling machine. This image shows the piston being clamped down to the angle plate using a length of 7/8 bar through the gudgeon pin holes and a fixed pillar at each end. This allows the piston to be removed, tested and re-fitted in exactly the same position.

Pockets should be measured and cut as follows:

Do one valve at a time, do the inlet first. Remove the exhaust pushrod so the valve stays shut.
Assemble the engine and turn over slowly. If the piston and valve hit, machine a pocket approximately 0.050’’ deep.
Re-assemble and check again. If they still hit remove another 0.050’’. Repeat this process until they clear each other throughout the engine's rotation.
Now assemble with some Blu-Tack or similar on the piston crown and turn the engine over slowly.
Remove the cylinder head being careful not to disturb the Blu-Tack.
Using a vernier, measure the thickness of the Blu-Tack at its thinnest point, you may have to measure several spots to work out where the thinnest is. This bit can be fiddly, be patient and if you have to repeat the process a couple of times to be sure of results, do it. A little more work now could save a lot of problems later!
Put the piston back on the milling machine and machine the pocket out so that there is 0.040’’ minimum clearance between the valve and piston at all times. It’s a good idea to aim nearer 0.050’’ but remember, the deeper the pocket the thinner the piston crown is getting. The deepest part of the pocket should be out over the skirt and rings so a blow hole is unlikely – but don’t make the crown any thinner than necessary.
Once you are happy with the inlet valve pocket, repeat the process with the exhaust valve. Make sure you mark the piston so you know which way round to put it in. If using non-standard valves, cams or timing the pockets are likely to be different sizes so it’s important the piston always goes in the same way round!

A couple of things to remember when trying different pistons:

When selecting a piston to use it's worth considering the availability of the one you want, as well as a few over-sizes. You may need a rebore in a few years time so it would be useful to be able to get hold of an over-size piston.
It’s worth checking the engine's compression ratio before and after carrying out any piston modifications. More information on how to do this can be found here.
When changing or modifying pistons remember that altering the weight of a piston will affect the balance of your engine and could cause excessive vibration. Always check your balance factor before carrying out any work so that, if required, you can get back to your original figure.