Thanks for all the quick responses, I really appreciate it.

I did take the flywheel off, pushed the crankshaft as far out as possible, but it’s still rubbing very hard.

I’m thinking if it were just the crankshaft that needed to go forward a little bit, with nothing on the crankshaft it should spin relatively freely when I put the back plate back on because nothing is pushing it back, instead it pushes so hard on the plate that it seizes the engine right up.

Also, I checked the bearings and they seem to be ok.

The only part that baffles me is it ran fine for a few tanks, ever since then it has been locked up against the back plate.