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Oversize piston update....

OK, I have sorted out some provisional details for the 90mm pistons...

They are forged pistons so of a superior spec. to any original type...The gudgeon pin diameter will be the same as a standard BSA pin. The ring set is a 'modern' one comprising of two narrow compression rings and a '3 piece' oil scraper...Pin clips are the normal 'plain' wire type using 1/16" diameter wire..

The top face of the piston is flat and has valve pockets machined into it to make it suitable for use in B33 motors...In that application the standard stroke (88mm) gives 559cc..At 94mm stroke (using modified M20 flywheels) the result is 598cc..

It will also fit the M21 and when used with the side valves 112mm stroke capacity comes out at 712cc...A solid copper 1mm thick head gasket will be needed for this application..

Pistons are complete with rings, clips and pin...

Price will be £172.50 per unit (inc.vat) plus shipping at cost...Availability will be approx. 3 weeks from date of order.

The viability of the batch at this price is entirely dependant on the minimum number of orders being placed so if you have an interest please get in touch off forum so that I can collate a list with the aim of achieving that...Ian:sunglasses:

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Re: Oversize piston update....

On an M21, with this piston, what is the compression ratio?


Re: Oversize piston update....

Hi Bruce,

Quote (Ian W)
"The 712cc engine I used in the bike I carried out compression ratio tests on, without any head alteration, resulted in a 6.6-1 compression ratio due to the increased bore size.. "


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