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I actually measure the clearance directly with a feeler guage. If you take the rings off you can slide the pistons into the cylinders and slip a feeler blade between the skirt and the cylinder wall. The min clearance is toward the bottom of the cylinder. You need the blade in at least .5 inches or so to get a good feel. So push the piston down from the top over the feeler blade and see how hard it is to move. When it can just slip from gravity and doesn't rattle around I call that blade size the clearance.
Minimum safe clearance is .0015 inches. Wear limit is .004 inches. I like to have about .002 to .0025 if I can get it. I had a mismatched set last time I rebuilt and by measuring all the pistons in all the cylinders I was able average out the clearances. In other words match the largest piston with the largest cylinder and on down the line.
With your sets being close I bet you can match them up enough to be fine.
-Andy
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72 Carrera RS replica, Spec 911 racer
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