Rob,
I've got some figures for the dursley 6/1 and 12/2 (iron pistons):
"piston to bore clearance at botton of skirt =.006 - .008 initially,.010 max."
I'd imagine that your alloy pistons will need more clearance just don't ask me how much!
Hope this doesn't confuse the situation...
Brian
Edit: come to think of it, are the alloy pistons (deliberately) oval ? .The original iron ones are a plain cylinder.This is contraryto GF's opinion but is easily measured
.I'd do your own measuring rather than going by the stamping.
Has anyone measured an indian (iron) piston? I would imagine that they are cylindrical but you never know!
brian