A12: 73mm piston
A12A: 75mm piston
A14/A15: 76 mm piston

It depends on the exact piston size -- via direct measurement -- as to what the machinist should bore the cylinder to:
* 73mm piston has a tolerance of -0.033 mm to +0.017 mm
* cylinder should be finished to a clearance of 0.023 to 0.043 mm.


For 1.0 mm oversize pistons, "0.040 inch over" is not accurate enough (.040" is already a 0.016 mm difference from 1.0 mm). If the machinist does precise conversions that's OK ... but if he talks in inches I would be suspicious. Maybe he'll do it right, maybe not. Does he even know what the Datsun A12 or A14 piston clearances are? Or is he going to use generic clearances?