I have a 4mm Crankworks crank under a Dasa power valve billet Cyl. No spacer Req, but if you go BIG piston's a spacer is needed from what Ive seen. Have mine pretty dialed in now. Better start hitting the Gym for some arm workouts. Its a BEAST! Kyle at Dasa was my go to guy on this build. Good peeps at Dasa.