I have a 18hp B&S engine that is shearing flywheel keys. I replace the key and the engine may run fine for a while, but eventually it will shear the key again. I have replaced the key about four times now. Any ideas?

Make sure you are getting the flywheel torqued properly, if it is a little too loose you will continue to shear key's. Check and make sure whatever type of drive adapter you have on the pto side is also tight and not loose on the shaft.

I've been very careful about putting the flywheel on. About half of the time after I put everything back together, the engine will backfire through the carb. as soon as I try to start it and thats it. Then one time it ran for about 20 mins., backfired and sheared the key. Another time I ran it for a minute or so and I thought I would try mowing with it, began driving it to a place where I wanted to mow, and the flywheel key partially sheared only a few seconds after I began to move the tractor. Could a valve problem be causing this.

Well I guess anything is possible, but I have never seen a valve problem cause a flywheel key to shear.

It's most likely backfiring when the key shears and not before. Perhaps your flywheel keys are too soft. And the flywheel has got to be really good and tight.

If you post your engine model number I can tell you what the torque should be for your flywheel nut, as they range anywhere from 35 - 150 ft lbs.

I put a flywheel key in it that I use on push mowers... they looked the same. The engine model number is 31H777 020E1. Thanks for the help.

Try the flywheel key for that engine.
Maybe the ones your using are slightly smaller.

Briggs & Stratton Part Number for keyway is 222698S. I'm sure the keyway from the small push mower engine will not hold this heavier flywheel.

Do as 30yeartec said and torque flywheel to engine specs with Torque Wrench. Hand tight with a rachet won't get it tight enough.

I second tommy3s reply - not properly tightening the nut will result in the flywheel moving, which in turn messes up the engine timing and causes backfire before the whole thing goes kaput.
In my experience, the only way to get the flywheel nut tight enough is with an impact wrench.
Truth be told, I have never checked the torque when replacing a flywheel - it's something like 160 ft/lbs for a B&S twin - and is near impossible to obtain with a non-powered tool - hard to hold the shaft in place in order to apply the kind of pressure needed to torque it.
Hence, the impact wrench.

The proper torque for your engine's flywheel is 100 ft. lbs.

All Briggs engines use the same flywheel key to my knowledge, getting the flywheel tight is the key (pardon the pun) to getting it to quit shearing the key's.

I used an impact wrench each time.

Dang - there goes my big easy solution! :-)
Make sure there is no oil, grease, WD40 or anything slippery on the flywheel or the shaft - unlike almost everything else about the engine, that union needs to have a much friction going on as possible.
Other than that, set wrench for max torque and crank down on it!

Uh oh.... My flywheel was such a pain to get off, I cleaned up the shaft taper and tapered hole with a Dremel tool / rotary wire brush, and then greased it. Guess I'd better use degreaser and get Gretta the Hun on the end of the wrench when reassembling.