Okay, this is a new one on me.

I just assembled a CoMo 40 (SuperTigre) engine after it had been in storage. The bearings felt good enough to me so I re-used them.

When I attached a prop and tightened the prop nut, the engine got very tight to turn. Too tight to attempt running.

So blaming the bearings, I ordered a new set from RC-Bearings. They arrived today, and yes they felt much smoother. I figured I had solved the problem.

I re-assembled the engine, and have the same problem. If I tighten a prop nut so that the prop is secure, the engine is too tight to run.

I had thought it might be either the tapered cone jamming the front bearing inner race or the prop driver rubbing the outer race or the engine case itself so I made a spacer to go between the cone and the inner race. No joy.

I then removed the prop driver, and installed only the tapered cone, but backwards. Then I tightened the prop nut as much as I could, until the crank started turning because the taper cone was not locking onto the crankshaft without the prop driver. I was thinking this might eliminate the prop driver as the culprit. Even at this 'lighter' load the engine had become tight and hard to turn.

If I remove the taper cone, the crankshaft turns easily and has ~ .010" freeplay.

So I have run out of possible explanations.