How to Modify a Norton 16H Piston to Fit an OHV Engine
Whilst searching for a little more performance from my 1951 Norton ES2, I modified a Norton 16H piston to increase the compression ratio. This involved shortening the piston skirt and machining valve pockets to provide the necessary clearance in the OHV engine. This guide explains what I did and how the same modification can be applied to Norton ES2 and Model 18 engines.
Why I Use a Norton 16H Piston in My ES2
Whereas the Norton ES2 pistons have a flat crown with large pockets to clear the valves, the 16H uses a domed piston with no valve pockets. This means fitting a 16H piston in an OHV Norton such as the ES2 or Model 18 can increase the compression ratio from around 6-7:1 up to approximately 9:1.
Being another Norton, the 16H uses a 79 mm piston as standard, and can be bought in the same oversizes as the ES2 pistons. It also uses the same gudgeon pin, so is a relatively easy swap. It does, however, require some modifications.
Shortening the Piston Skirt to Clear OHV Flywheels
The first stage is to shorten the skirt by 5/16" to ensure it clears the flywheels when it is at the bottom of its stroke. The simplest way to do this is to turn the skirt down in a lathe as shown in the image.
Checking the Deck Height
The exact crown shape of the 16H seems to have varied slightly over the years, so it is important to check that the outside edge of the dome isn't going to come up and hit the cylinder head. The one pictured here protruded above the cylinder slightly so I put it in the lathe and continued the dome shape all the way to the outer edge.
Given the depth and shape of the dome in the Norton cylinder head, you do not need to worry about 'squish' clearance. However, if you are using the principles outlined here on other cylinder heads, please read up on squish bands and how to ensure you have the required clearances.
Machining Valve Pockets to Clear the Valves
The biggest job when fitting a 16H piston 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 fixing pillar at each end. This allows the piston to be removed, tested and re-fitted in exactly the same position.
The angle plate is set so that the bottom of the valve pocket is at the same angle as the valve head. The pockets need to be measured and cut individually because they will vary from one engine to the next depending on the cams, valve timing and valve sizes being used.
How to Measure and Cut the New Valve Pockets
The aim is to remove no more material than necessary, so I machine the pockets progressively and repeatedly check the clearance on the assembled engine. I aim for 0.040"-0.050" of clearance at all times on both the inlet and exhaust valves.
Before starting this process you need the bottom end of the engine assembled with the valve timing set. Remove the exhaust pushrod so you are working with the inlet valve only and assemble the barrel, head and rockerbox ensuring that your tappet clearance is set as it will be for running (pushrod free to rotate with no up and down movement).
Turn the engine over very slowly. If the piston and valve hit, machine a pocket approximately 0.050" deep.
Reassemble 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 caliper, measure the thickness of the Blu-Tack at its thinnest point.
You may have to measure several spots to work out where the thinnest part is.
I use a scalpel to slice the Blu-Tack in half as shown in the picture, then I can see where I need to measure.
This job can be fiddly, be patient and, if you have to repeat the process a couple of times to be sure of the 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 between 0.040" and 0.050" clearance between the valve and piston at all times.
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.
It is generally accepted that a minimum thickness of 0.180" should be maintained on the crown. Take some time to measure this if you are concerned.
When 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!
Things to Consider When Fitting a Different 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. On the long-stroke Nortons I have built, I have had good results with balance factors in the 65-70% range.
What Would You Like to Do Next?
Read about My Norton ES2 Engine Development
Compare OHV Norton Piston Options
View our Norton Technical Guides
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