I am rebuilding a little 2HP, Dorin compressor. I’ve replaced the bearings, rods and pistons, valve plate and had the motor rewound. It is all back together again….However, with the head on and the front still off, I can only hand turn the crank until one of the pistons comes up to hit the valve then it stops. it will not hand turn on through and continue back down. My question is….Is it supposed to be able to turn freely all the way through? I know it will with the head off, but with the head on, should it? This is a Dorin compressor on a blast freezer. The pistons have a tit on top which fits into a hole on the valve plate. It looks like it is designed to push the suction valves open. I’ve never seen this before. Every piston I’ve ever witnessed was flat on top. 30 years experience.