post back with full model # usually under tilting seat..

My clutch pully fell off,had to get a new bolt for it.Now how do I keep the pully from turning when I try to tighten up the new pully bolt?It's hard turning because the threads inside the shaft are slightly stripped.
Model#of the tractor is 917.254661

if it's one of the clutch idler pulleys, get a new bolt. If it's the engine driving pulley, may hafta remove engine shroud and use a large dia strap wrench to keep the engine crankshaft from rotating - be careful not to break cooling fins !!...
Also put some no-seize on the threads first and use an air wrench to tighten up.

"threads inside the shaft are slightly stripped."

If it was me I would tap the threads on the crank and get another NEW BOLT Or you may never get it off again or tighten fully (if you seize the bolt threads to the crankshaft you will have to cut it off flush, try to hit the middle of the bolt and drill it out and retap the threads (no easy task for someone that know how to do it in hard metal).

That Fine threaded bolt should free spin up till it bottoms out and the lock washer on the bolt (and you want to put locktite on the threads of the bolt) I would also verify that the lock key is there.

You did get the right thread pitch per inch right?

Need to add this? if you do deside to tap the threads "DO NOT FORCE THE TAP) If you break it off in the crankshaft it will be harder to get out than a broken bolt. So go slow back and forth, use plenty of oil (lubricant) to help remove any fine slivers of metal you maybe removing. Naturally make sure you tap is the same thread per inch as the threads in the crank and the bolt or you will recut new threads.