These covers are used on S&S engines. WW#64-551 covers are machined for ¼”-20 socket head screws. Screw holes in these covers are counterbored to allow the socket heads of the screws to bottom on a flat surface. WW#64-549 covers are machined for countersunk screws and are shipped with 1/4”-20 countersunk flathead screws. All gear covers for S&S crankcases are machined to accept the late style 9/16” diameter pinion shaft bushing end.