Rebuilt motor won't turn

Are you running a really fat cam which might require valve guide machining and spring seat cutting. You could be in bind with the valve spring, interference between valve keepers and guide or just good old fashioned piston to valve contact. What pistons, flat tops, domes, valve reliefs cut in or not? Pull yer valve covers to check valve springs , if they're closed up , that's a problem . Not a 100% on wether you can see your retainers vs guide but something to look into. If the head side of things is ok. Then its time to clay your pistons to measure for clearance. Good luck.