For comparison: of factory specs and pipefather's thumb rule based on clearance per 1 inch (25.4mm) the list below shows the factory specs converted to a measurement per 25.4mm or 1 inch of bore as pipefather thumb rule uses.

Example Zenoah G26 34 mm bore
34/25.4* .0224mm = .03mm minimum clearance
34/25.4*.000882" = .0012" minimum clearance
34/25.4*.03mm = .04mm (pipefather thumb rule)
34/25.4*.04mm =.055mm (pipefather thumb rule)
34/25.4* .0448mm = .06mm manual maximum
34/25.4*.001763" = .00236" manual maximum
and the numbers to guide on whether a new piston is required
34/25.4*.112mm = .15mm worn out
34/25.4*.004408" = .0059" worn out

Could anyone with factory specifications for engines larger than the 26cc post some additional specs. I'm curious how the thumb rule compares. The worn out clearances are important as most conversions are from used engines, (often well used by the time they make it to my local second hand shops!)