These 3-1/2" +.020" bore pistons fit 1979-'84 80" HD^® big twin engines, producing 8:1 compression with stock unmodified cylinder heads.. The modern alloys used in these pistons, coupled with the advanced skirt profiles, allow them to be fit tighter resulting in low engine noise and long engine life. In the past, this could only be achieved with weaker cast pistons. S&S^® will fit these pistons to your existing cylinders (when purchasing new cylinders, pistons can be fit to them as an option)