Does anyone know what could be wrong my os 91fx turns over by hand without the prop just fine, when I put the prop on it is very stiff. I crashe the plane not to long ago but it was a belly hit not nose, but I wonder if the crank could be bent or what please if anyone knows pease help.

their is a washer between the engine casing and the prop hub . check and see is it's there or missing

I have to think it's a bent crank or a bearing problem.

Contact Flyboy Dave (one of the RCU Moderators). He posted a thread offering to straighten bent cranks.
http://www.rcuniverse.com/forum/fb.asp?m=6948801
He should be able to give you info to help determine if it is bent, or if it's a different problem. If it is bent, he can fix it for you.

I think the problem is the bearing but I don't now to get it out, to you tap it out our what

The bearings do not see anything with the prop on that they don't see with it off.

And if the shaft is bent, it will show it with or without the prop on.

If tightening the prop causes any thing at all, then the problem has to do with tightening the prop. And just about the only way that can cause problems is for the prop drive washer to rub against the front of the engine or the front bearing. There are a couple of ways for that to happen. In any case, it's very, very simple to watch while you're tightening the prop to see if the washer is coming into contact with either the case or the front bearing.

One way is when a spacer washer that some engines have between the drive washer and front of the crankshaft is left off. Another way is if the drive washer is held by a key, as is the case with the 91FX and the key is left out. Another way is if the crash has injured the drive washer and the damage allows it to seat farther aft on the crank.

Thanks for the help guys when some said the thrust washer I noticed that it indeed was messing and that was the problem, so I took my motor out of my plane and tore it all down for nougthing that sucks but motor turns over fine with the washer that I noticed that was on the top of my shelf.

Sometimes the fix is close enough to bite you but it won't tap you on the shoulder and whisper in your ear!