Engine locking up??

Whaaat?
- Didnt seem to be turning? Is the cam turning?
Yes and no. When everything is fresh, that oil-pump drive can be rotated out with a screwdriver off the cam and plucked out easy.Later as miles accumulate the shaft varnishes up and shaft removal becomes more difficult. Somewhere after 130,000 miles they become really sticky and are sometimes stubborn.
- Sounds to me like the shaft to bushing clearance may be on the tight side or the shaft is bent, or its varnished up.Or ?, IDK. I think it would be prudent to take the pump apart also, but Im not there yet.
-Also should mention that if you flipped the motor upside down, then whatever might have been sitting on top of piston 5/8 may have migrated to a different place with extra clearance and now is no longer interfering. Im kinda stuck on something being in there, or a valve may be hanging up in one of those holes.Depending on your parts combo, there might be between .040 and .180 inch in the quench area. Not much can fit in there and not do damage. I have seen machine screws get in there and be jammed into the piston top and do just what yours is doing. Im not saying yet that the head(s) needs to come off. But it may come to that soon.
-I would pop the covers and check the valve action, and also do a cylinder leakage test in at least those two holes.
-I hope Im wrong.