Somehow my engine (and airplane) hit the ground inverted wide open. After cleaning it out (removed everything except the crankshaft) and re-assembling it runs fine except it make a loud grinding metal to metal sound (like the crankshaft is rubbing against the rear plate). I've ran it about six tanks now and it is not getting any better. I've inspected the back plate and do not see any wear..
The engine seems to be running fine. It just sounds bad.
BTW: there is no slop or play in the crankshaft side to side or front to back.

Should I be concerned or just run the damn thing and not worry about it?

FWIW: it's an OS 61FX

Must have bad bearings? Does it make the funny noise when you hand flip it?

Not really (or it is not very noticeable). It is not as smooth as my other engines when hand spinning it back and forth - but it is definately not rough.

It has bad bearings then

If you've already run that many tanks through it with noisy bearings, you've likely damaged the crank already also... I'd just run it until it snaps. The money you might save by replacing the crank/bearings now would be likely be spent again soon.... you've had metal shavings cycling through the engine this whole time.

I suspect that the rear bearings have simply been pushed back or out of alignment. Bad bearings make a whining noise and will have radial (side to side) play. There should be some thrust (front to back) play. Often you cannot feel any roughness from a bad bearing from turning the prop over by hand. If you have no thrust play it may be contacting the back plate just slightly. I would take the engine apart heat the crank case and see if you can push the rear bearings back in.

The front bearing is the one that takes most of the shock load in a crash. The balls tend to get flat spots on them when hammered in a crash. Be sure to check both front and rear bearings.

Great suggestions.. I'll check out the bearings for flatspots and make sure they are seated properly.

New bearings are relatively inexpensive. If you've got a good amount of time on this engine, it'd be worth installing a new set, just to be safe.

RC-Bearings.com is the best place to get your new bearings at a reasonable price .plus your crankshaft should be fine unless it got bent in the crash which is another possibility where your noise it at.Easy to tell if you look at it when running and the end of the crank is not running true or if you have a spinner and it wobbles .

If the engine still has good compression and runs ok ,just makes noise stick some bearings in it and go fly.

I have heard "bad bearings' only a couple tmes. In both cases, they both had a wicked rattling noise.

well - I took out the old bearings (the oven trick worked great). The bearing under the piston was pretty smooth. The bearing next to the prop was rough - like it had flat spots. I ordered a new set.

Thanks for the advise everyone.

Good idea! Haven't had that happen, but been a long time since I had a crash that was straight in.

Same here but in my case they simply whined and you could not feel any roughness, but some radial play. But I suppose there are probably much more than several failure modes.