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1972 Engine Seized Post Top End Rebuild

gatorforge

CB750 Enthusiast
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Fredericksburg, VA
So I'm currently working on a top end rebuild for a 1972 CB750. When I got the bike, the engine turned over easily, but as it hadn't been run in a while and I had no idea on the internal condition of the engine, I decided to do a top end rebuild just to be safe. I tore it down with minimal issues, replaced the piston rings, honed the cylinders, and got to work on the reassembly (after measuring all parts of course). I cleaned the pistons (they are in very good shape) and put the cylinders back on (kinda a nightmare). While I was working to get the cylinders back on, I noticed the motor was not moving completely freely, It only had a partial range of motion in one direction, but moved completely freely in the other direction. All turning of the crank was done via the bolt under the points cover. Once the cylinders were on, it only moved a little in either direction, and with the heads mounted, it will not turn either direction at all. There aren't any spark plugs installed so there shouldn't be any compression. Anyone who could provide insight into why the motor is no longer turning over would be extremely helpful. Especially if it does not require removing cylinders/cracking case.

Thanks,
Lance
 
I would guess you probably broke a oil ring while putting the cylinder on. Or you got a piece of trash inside that is binding up maybe in the timing or primary chain. The moment you started having issues you should have stop and taken it apart and looked for the problem. Also turning the motor over with the nut on the point side is a big no no. You can easily bend the point shaft and cause timing and point issues. I see no option other then to start tearing it back down to the point where the motor turns free again.
 
@dirtdigger, you were right about the oil ring, the lower section of the ring in the #1 cylinder had become slightly kinked. I'm going to double check in the bottom end and make sure theres no other issues. Thankfully I'd bought spare ring sets, so that was an easy replacement.
 
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