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Model 50 Rebore - where/which piston ?


Bike is 1959 Model 50. I'm looking to have it rebored and to fit a new +0.040" piston this winter. I live in the Bournemouth area. My Dominator rebore was undertaken by D.R. Massam Motorcycles and he did an excellent job, but it is a way to go !  Can anyone recommend a more local establishment that does a good job ?

What piston is recommended ? My Dommi pistons came from Andover Norton and these seemed to be good quality and are performing well. However I didn't research the options before buying these and I would like to do this before buying one for the M50. Any recommendations ? Different weights ? Compression ratio considerations ?

Thanks in advance,

Mike Haworth




Nobody has thrown a hat into the ring from a position of ignorance!  I suspect Andover Norton is as well placed as anyone (and better placed than most) to decide what's best to stock.  Unless you can find new old spare - there are still piston specialists around.


Hi Mike,

   Sorry, but I am not in the Bournemouth area. I guess the Old Bike Mart or the glossies would be a starting point to find the nearest place. I think getting a piston will be the biggest problem as I don't think any new ones are being made. Because of the engine design, for production reasons, these pistons have a very high gudgeon pin to crown height measurement, so there aren't any easy alternatives.


Dave Massam has partially retired, but is on here from time to time so if you get stuck he may or may not! be able to help. Another option is to courier the motor to Mike Pemberton, it will add £100 or so to the bill but you know you’ll get a good job. If you are feeling that you need a few more horse power you could get it increased to 520 with one of mikes aluminium barrel - easily reversed if you ever want to!  This will also get over the piston availabity issue! Mike might be able to advise on the availability of pistons but the main problem with the modern pistons (if you can find one) is that they are heavier than the original ones and may need a bit of machining to lighten them or the crank balanced to keep vibs down, depends how you ride it I guess! 

Sorry I don’t know any engineers in your neck of the woods.




I needed a piston for my M50 about 2 years ago, and spoke to Mr. Pemberton (via internet), and was told to use a Gandini piston. He told me not to touch the JP pistons. I was able to find one after much searching, and here is where I found one:

F.W. Thornton - email: - Tel:+44 (0) 1952 252892



I do believe MODAC Cycles in Australia still hold a stock of NOS GPM italian pistons.


Drop Dave a message via his web site



Hi Mike, it's a pity you have to go to +40, I have a new +30 for a M50, a club member let me have it a while ago........just after Mike P had got me a Gandini ! (+30)

Don't forget to weigh your old piston so you can compair with your new one and adjust if required, easier than re-balancing!

Regards John O


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