OHV Norton Piston Options

It’s not uncommon when trying to improve the performance of an engine to want to change the compression ratio. There are a number of options when working with an OHV Norton and this page looks at a few of these.

Standard ES2 Piston

As you'd probably expect, standard ES2 pistons offer the simplest option during an engine overhaul as they fit straight into the engine. They are available new in a couple of different crown heights and can be used for compression ratios of between 6 and 7:1.

Modified 16H Piston

Slightly harder to find and more involved to fit than standard ES2 pistons, the side valve 16H piston can be modified to fit the OHV engine and offer a compression ratio of up to 9:1. However, they normally end up at nearer 8.5:1 as valve pockets are usually required.

I have fitted on of these to my own ES2 and there is another page in the Workshop section of this website showing how I did it.

Modifying Norton Pistons

BSA B40 Scrambles Piston

A BSA B40 scrambles piston has a high dome and large valve pockets so can be fitted to an OHV Norton to take the compression ratio all the way up to 10:1. There are however a couple of things that need changing in order to make it fit.

A B40 piston uses a 3/4" gudgeon pin whereas an OHV Norton uses a 7/8" one. As such a new small end bush will need to be fitted in order to fit the piston to the Norton conrod.
A B40 piston also has a lower compression height than the Nortons. As such up to 1/8" needs to be machined off the base of the barrel to lower the cylinder head. The exact amount removed can be altered in order to achieve the required compression ratio. Pushrods and tubes will also need to be reduced by the same amount.

Due to the considerable and irreversible work that needs to be done to the engine in order to fit the B40 piston it is an uncommon modification. It is a lot of work for a minor gain over the 16H piston. Certainly in the case of a ‘hot road engine’ there is unlikely to be a noticeable gain by going down this route. If building an out and out racer other options, as shown below, are preferable.

Custom Made Pistons

There are a number of companies out there that can manufacture a one off piston to your exact specifications. Just be prepared to give a LOT of information to the manufacturer, there are an incredible number of variables when making a piston. For this reason it is often better to look through a catalogue and pick one that is close to what you want.

A one-off will never be cheap but if you want it, and are willing to pay for it, there will be someone out there capable of making it for you. It’s all been done before so it can all be done again.

Big Bore Options

OHV Norton barrels can be bored out to around 83mm before beginning to risk the integrity of the cylinder wall. However there are options available if you want to go even bigger. Large bore barrels are available from engine tuners such as Mike Pemberton or can be machined from the billet. The options here are endless so we won’t go into it now.

The picture here is of an 88mm Yamaha piston using a modern design of oil control ring and with a low friction coating on the skirt. The offset valve pocket will be widened to bring it into the centre and make it suit a larger inlet valve. Paired with a specially made 82mm crank this setup will give a 500cc short stroke pushrod Norton!

Things to Consider

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, if required, you can get back to your original figure.