Piston clearance

What is the recommended formula for piston-to-bore clearance for an
alloy piston in an air-cooled diesel? X thou per inch diameter, sort of
thing.
For the Aussies, this is a Southern Cross ET-B, 2.5 hp @ 1500 rpm. There
is no figure given in the recently published Southern Cross book.
I am setting up such an engine as a 32 volt lighting plant for our Club.
It is part of a set of 3 engines donated to the club by a recently
deceased member's family, and I have undertaken to prepare it for
presentation at the upcoming Royal Show. The engine starts and runs but
grabs after a few minutes. A few minutes later, it becomes free again
and will restart. It was previously overhauled, but I suspect the piston
clearance is insufficient. I have not yet pulled the piston. It does
smoke quite a bit, too.
Jack in Oz