I hate to step on any toes 5twins, but your misquoting the specs. The .0039 is the max allowable clearance, the normal clearance is .0020 to .0022. in inches
In metric the max is .1 mm, the normal clearance is .050-.055 mm
arborman, as 5twins said those numbers were when they assembled the engine at the factory. Useing those numbers your clearance was 75.013 - 74.960 = .050 just at the minumum spec.
I have pistons from two engines, one set marked 963, the other 956. Both factory pistons.
At the factory pistons come out of the molds at many sizes. They don't all come out the same. They measure them and mark them with the digits after the decimal point. They then match the sizes and use them as pairs. When the pistons sets get to the assembly line they bore the cylinders to match the pistons, and mark the bore size on the lower cylinder sleeves.
If it was not the standard size then the marking on the piston would be two or maybe three digits, as in 25, 50, 75, 100. These represent the over sizes. You put in a decimal point, as in .25, .50, .75, 1.00. That gets added to the 75, as in 75.25, 75.50, 75.75, 76.00 So if you have a three dgit number it's a standard piston.
The rings as stock have an R or RN on them. I don't think they represent much more than that the lettered side goes up. The over size are marked with the letters and a number to match the over size, see above for the over size info.
Now back to your pistons and cylinders. Most machine shops that do cylinder boring will measure your pistons and cylinders for a small fee, maybe free if you have them do the boring later. Once you know just what you have then you can decide if you can just do rings or you need a rebore. If you need a rebore get the pistons first, then the machine shop can do as the factory did and bore to match the pistons for proper clearance.
I got a set bored for $70 both holes.
