Author Topic: M21 rebore  (Read 337 times)

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M21 rebore
« on: 04 May, 2024, 16:59:24 »
My M21 has oversize piston rings of 0.080" of an inch. This I think is the limit. Does anyone know what I should do after this? Is it still possible to rebore and what size piston and rings should I fit afterwards?


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Re: M21 rebore
« Reply #1 on: 05 May, 2024, 07:17:02 »
Hi , just have a look to see how thick it is in certain areas , what you can do is match up another  type of piston it's not as easy as in the eightys we used to have piston and bearing books so you could check the pin height to the top of the piston and the diameter and pin diameter  , there is a custom piston option more expensive though , or a sleeve . My b33 has a ( hate to mention it on this forum,  I'll be ostracized)🤣🤣  Harley shovel head piston and I think it's a 6mm over bore,  b33 88mm std,  now 94mm !  And it has a M20 crank fitted , I think for your side valve ,i would google search for a piston in millimeters and if you can't find a flat top ,search for a turbo piston for lower comp , then try to match up pin size and pin height .you might get lucky . Cheers


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Re: M21 rebore
« Reply #2 on: 05 May, 2024, 12:34:12 »
If it were mine, I would have it resleeved back to standard.