172 piston..

ITS ON THE BOTTOM OF THE BORE NOT ON THE GASKET FACE

and not all 172's bores come with grade a pistons

mine was stamped grade b

 

The barrel grade marking is on the bottom gasket face (base of barrel) on the edge of the boost port opening at the very back of the barrel, you need to remove the base gasket and clean up the face to see it.

It will be marked with a letter "O" "A" "B" "C" or "D" and followed by four numbers eg A 4056.

The letter marking is the plating dimension and the following numbers are production stamps.

The dimensions (as I recall) go up in extremely small increments of 0.05mm or 2 thou.

"O" is the same as "A" the rest are going up in size by the above amount.

The corresponding piston letter stamp is in the centre of the top of the piston, you will need to clear any carbon off to see it and if the pistons been damaged in that area (detonation/hole etc) you wont have any chance of reading it.

Always replace the piston with the matching grade, it will run with the next grade up but piston skirt clearance will be tight and you risk seizure, the thickness of the nicasil plating is measured in microns and dos`nt wear as a cast barrel does, if it`s scored you dont re-bore...you replace or re-plate sending a new piston off to the company so they can match the plating to the piston.