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Gapping Piston Rings

David Wallace
New Member Customer

I am doing a top end job on my '72 R75/5.  After decoking, pistons and cylinders look good and measure approx 3.227 (pistons) and 3.230 (cylinder), so I've ordered a standard size set of replacement rings.  Will the gaps on these new rings need to be checked and possibly filed, or are the new rings manufactured such that the gap is correct (.012 - .020) for a nominal cylinder diameter?

Topic starter Posted : 02/19/2021 19:00
Scot Marburger
Member Moderator

ALWAYS check end gap when replacing rings!

Having said that, though, I can't recall the last time I had to open one up. Usually they're at or above the tolerance range, and that doesn't seem to matter much.

Posted : 02/20/2021 05:33
Richard Whatley
Member Moderator

Sorry to be so long on the reply, but I've knocked off the system for months.

I got to go with Scot on this one. There may be 200 piston ring makers in the world and BMW is probably buying from 3 of them. And the other 197 are packaging their rings to look exactly like BMW product. The bottom line is that you simply don't know what the gap will be like, because they all make their rings a little bit different. Therefore, it's better to be safe than sorry. 

And look at it from the "risk standpoint". It adds maybe 10 minutes to a 2 hour job. If you don't do it and are forced to do it over, then you've made it a 4 hour job. Which is worse: 2hr and 10 min, or 4 hr and 10 min ??

So spending the extra time to check IS the "shortcut".

[color=blue]Don't hide 'em, Ride 'em !!

Posted : 03/28/2021 17:44


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