The application of coating pistons is used to reduce wear and scuffing problems. The thickness of the coating is usually 0.0010".
There are two types of coatings molybdenum (moly) and moly graphite based that are used for coating the pistons.
Proper piston skirt clearances is a must especially with tapered skirts. Measure the piston to wall clearance 90 degrees to the wrist pin (known as the major axis) just below the pin centerline and then add the skirt clearance to this to get the proper bore size. Minimum clearances are based on non coating measurements but do not measure the skirt and compensate for the increase in diameter due to the skirt coating as this could result in excessive clearance and possible piston failure.
Coating the piston crown will enhaunce the flame propagation, reduce the carbon build up on the piston crown and increase the compression ratio (depending on thickness of coating).