I have always used the traditional figure of bore size in inches times three.
ie 3″ bore for Silk Pistons x 3 = 0.009″ (Nine thou)
So I work on the safe side and make it 10 thou.
When I am building engines, I put each ring on the piston, fit it in the bore, then twist the piston LH & RH.
You can feel the piston move and the ring stay still till the ring end hits the stop. You should be able to feel and see the free movment.
We usually put a clock on to check this in situ figure.
Usually, folks put the ring in the bore, then push it down a bit with the skirt of the piston to be sure the ring is square, then use feelers to check the gap. This is a tried and proved method, but I cannot resist belt and braces!
I will never win a prize for Brevity!
Piston ring gap is ten “thou” – although in reality, a little extra will not affect the engine diddly-skwatt, being a two-stroke. Thats one gap of ten thou, not two of five thou each if you break a ring fitting them!!!!
I didn’t think you knew the difference between a piston ring gap and Watford Gap.