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PISTON

Hi everyone.

Looking at BSA pistons & came across this one:-

"PETER HEPWORTH COMPONENTS"
"B.S.A Ref. 12659"
"082=S.T.D +0,75mm"

Piston is stamped: "82 7S"

My question is this:- Is this a piston for an M20 or an M21??

Cheerz
Ian

email (option): fozzie001@hotmail.com

Re: PISTON

This doesn’t answer your question, but assuming you are looking at an actual piston and not reading a number on a page, both WM20 and 21 will have the same 82mm bore plus whatever overbore the piston in question might be.

The difference between the two will be in gudgeon pin location and overall piston height....

Because of the WM21 longer stroke, the piston is “shorter” with the pin location moved towards the crown of the piston. The 70mm length WM21 piston is much more “squat” in appearance in relation to the 85mm length of the WM20 piston. Hope this is helpful.


08-D45422-3-F4-B-4258-82-E6-9-AB6-EA392-EBC

email (option): teladelujo@msn.com

Re: PISTON

John D Edwards
This doesn’t answer your question, but assuming you are looking at an actual piston and not reading a number on a page, both WM20 and 21 will have the same 82mm bore plus whatever overbore the piston in question might be.

The difference between the two will be in gudgeon pin location and overall piston height....

Because of the WM21 longer stroke, the piston is “shorter” with the pin location moved towards the crown of the piston. The 70mm length WM21 piston is much more “squat” in appearance in relation to the 85mm length of the WM20 piston. Hope this is helpful.


Hi John,
Thanks for that.
It is difficult to tell at a glance as I can only see the one piston (the one in question) and so, I think I'm gonna give it a miss, as I think it may well be for the M21 & therefore not much cop for my M20.
Thanks for the helpful picture
Ian

email (option): fozzie001@hotmail.com

Re: PISTON

My 52 M21 has a dished crown to reduce compression due to longer stroke, where as the M20 had a flat top piston. Don't know about the earlier M21's they may well be different.

email (option): tknalder@iinet.net.au

Re: PISTON

Hi Ian all the M21 pistons I have seen dont have the top grove as per M20 (they have four groves), M21 pistons have only three groves for the compression and oil rings, I know some pattern pistons have a extra oil ring fitted to piston skirt.
Dave

Re: PISTON

The M20 top groove is not a 'ring' groove! Is it a heat sink or something? Also is the 15mm difference in height split evenly between the bottom and top from the gudgeon pin? Ron

email (option): ronpier@talk21.com

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