First off...we have the PS rear piston absolutely clean. Head gaskets are intact. Secondly...we have a frost plug in the water jacket behind the PS rear cylinder. Why would the top of the piston be clean...and will the frost plug sitting in the water jacket behind the cylinder affect anything? I doubt we can get it out. This is how the engine was handed to us...how it got there...is a mystery. The short block is mint. Pistons and bearings are mint...and the cylinders still have crosshatch marks on them. Crank end play is within tolerance.