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90mm 'M21' piston details...

I've had a few enquiries regarding the fitting of the forged 90mm piston to the M21...

I fit them to late 'unlinered' M21 barrels which simply have to be bored out to accept the piston...This removes the location spigot from the base of the barrel but isn't a problem as it locates well on the barrel studs. A chamfer has to be machined (or filed by hand)to facilitate the entry of the piston rings into the barrel during refitting. A solid copper 1mm thick gasket will also be needed..

No alterations to the standard M21 jetting are needed in my experience (I've run 2 of my own bikes in this format for a total of approx. 20 years...)

Top speed with this set up is marginally improved but most of the gain is at the lower end and through the midrange with improved cruising speed and particularly, hill climbing capability.....This is entirely predictable taking into account the breathing restrictions inherent in the engines design...

Although the piston weight is close to that of the std M21 piston and the engine can be run without altering the crankshaft balance I have found that rebalancing to 60% (std 58%) does make the engine 'sweeter' generally...This may well also be down to production tolerances in balancing the standard engine originally...

Any further work to valve sizes, porting, head shape, flywheel weight, cams, carburation etc. is optional beyond the basic modification and will further change the engines characteristics/performance depending on the spec...

It may be possible to remove the liner from a 'linered' barrel and to bore it out to take a bigger liner. I haven't yet investigated this option...Liner wall thickness should ideally be 1.5mm but I have run 1mm liners in OHV engines without any dramas...This should be born in mind when making any measurements...

BSA used either a high quality casting without a liner fitted or a poorer quality barrel casting with a liner...Therefore, although it may be possible to run a piston in these lower quality barrels without a liner fitted I would personally regard that as a high risk strategy...

I think the alteration is well worth the effort and wouldn't consider running a standard engine again myself...Regarding longevity I recently removed the piston from my own M20 which was one from the first batch I had made approx. 15-20 years ago and it has done a lot of work in that time...Ian

email (option): ian@wright52.plus.com

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