I am trying to put a 3 valve head on a gpx but the cam gear and gear on the crank are different. the bigger gpx gear does not fit in the 3 valve head so i need to change the gear on the crank. how does it come off? does it need to be pressed off or will it un screw. when you put it back on how do you know it is going to line up with the gear on the head when cam timing is set.