Stock lifter bore = 19mm Harley roller diameter = 18.6mm The cam followers used in Sportster engines, K models, big twin side valve models, and the side-valve W model series were a slightly shorter version of the followers used in the larger motors, but with the same 0.731-inch (18.6 mm) diameter body and 0.855-inch (21.7 mm) diameter roller follower since 1929. The company used similar cam followers for decades with minor changes, from the 1929 to the Eighties. Chev roller lifter diameter = 21mm