With engine apart, soak parts in acetone after scraping th tops of the pistons off with a knife or anything with a good edge. After soaking use scotch brite and clean the remaining stains off piton sides and top/dome. Go lite on the scotch bright and the machining marks will still show up on piston sides...if not worn much. If there is any hard carbon in piston ring groves, use a broken piston ring that has the cross section squared off on a grinder ...then touch up square end of ring with a file. Carb cleaner works also in place of acetone. Best Regards capt,n