I have a Diana Model 34 that is need of a new breech seal, I have heard to sand some material off the O.D or take some off the back of the seal, which way is best and how far should the new seal protrude from the the breech face ?
I forgotI guess it's a take off if too tall or shim up if not .
Here's a blog I guess you already found it by now
https://www.pyramydair.com/blog/2008/04/shimming-a-diana-breech-seal/
I guess the oring is a 8x2.5mm s70? If you can find 8.5x 3 may save shimming .
Also you may try flyingdragon and ask Mike if he has the orings and shims on the ready to sell and ship . Maybe sold in a 5 pack ??