In California Bill Fisher's GMC Speed Manual there are references to "knurlized" pistons. The purpose is to make the piston fit tighter I suppose. He said that 1/2 the clearance could be removed. I remember my dad knurling the piston on my Cushman to make it fit. That was because we couldn't afford boring and a new piston. My question is does anyone use this any more? Do the newer piston metals change the expansion characteristics of pistons allowing a tighter fit to start with? If so can we use less clearance than shown in the old shop manuals?
