Any engine designed by a sane engineer should have tolerances that can handle a 20 micrometer difference. I have a digital caliper, and the tolerances I've seen are always in the 100-200 micrometer range, because otherwise heat expansion alone would make the engine parts bind-up.

If the conrod only *rubs* on the crankcase, as opposed to clanging against the side and getting jammed, then you should be able to file-down the area of the crankcase where more clearance is needed. This is a hobby after all, some end-user customization is always required.