You have it backwards. The backfiring is not shearing the key. The engine is backfiring because the key is sheared.
The key is sheared because the proper technique of inserting the key into the keyway has not been used and insufficient torque on the flywheel retention nut. Using other than a genuine Briggs key can cause it too.
To install the flywheel properly:
The mating tapers of the crankshaft and the flywheel must be clean and free of any oil, grease, or rust/dirt.
Set the flywheel onto the crankshaft with the keyways aligned.
Carefully insert the new key into the keyway and work the key down into place, tapping gently. Do not flare or mushroom the key.
Install the flywheel retention nut and torque it to 100 ft lb.

I have installed the key exactly as you have described except for torqueing the retention nut to 100 ft lbs.
I do not have a torque bar, but I do get it pretty tight with what I have. I will find a torque bar and torque it to the ft lbs as you have described. I got the keys (3 sheared) from a Briggs dealer, so they should have been OK.
Thanks for your response to my question. Is there something else it could be?

The only other thing that will cause the key to shear would be if the tractor has a gravity fed carburetor (tank higher than carb) and the float needle valve was leaking fuel into the cylinder, causing hydro-lock. If an engine is hydro-locked, the piston is stalled by the fuel in the cylinder. Since the starter turns the flywheel, and the flywheel turns the crankshaft, hydro-lock can overcome the grip of the tapered surfaces and result in shearing the key. But.........if you were getting liquid fuel into the cylinder, I'd think you would have mentioned it as your primary problem when you posted.
So, I'm sticking to the scenario where you just do not have enough torque on the nut to prevent the key from shearing when you crank the engine.
If you do not own a torque wrench now, you should get one.
You do not need an expensive professional version. You can get a beam type for around $30 or less.
For the cost of the 3 keys already sheared, plus the aggravation.........I bet you have more than $30 of your time already invested here :^)

I have a 1985 B&S 11hp. vertical shaft engine on my old garden tractor. It has developed an oil leak which seems to be coming from the area where the cylinder meets the block.

It's not coming from the head gasket but further back. It runs down the face of the block and pools on the engine base. It's been leaking for quite a while but seems to have gotten worse.

I just pulled the head, cleaned things up and installed a new head gasket. I ran the engine for about 10 minutes and after it sat awhile, the oil appeared once again.

Anyone have a similar experience? Could it be a cracked cylinder or block?

Upper seal? Model numbers and a pic could be good.

Post the model and type numbers you will find on the engine sheet metal shroud.
You might find a defective gasket under the crankcase breather cover plate, or that the rubber breather tube that connects to the cover has split or broken.
I really doubt a cracked block unless there has been some sort of past catastrophic engine failure you haven't mentioned.

"Install the flywheel retention nut and torque it to 100 ft lb."

Well this depends on WHICH 16 hp engine it is, it could be 150 ft. lbs., need to post the engine model number.

Walt Conner

This post was edited by walt2002 on Wed, Apr 10, 13 at 8:04

anyone have knowledge of Briggs Propane engines.(4248 Generator) basicly a lawn mower engine with an oil filter). I suspect mine may have a bad solinoid,Im getting spark, It tries to fire, a little when I spray starter fluid into the air intake at the air filter. But it usually took 5 sec for the engine to start when it was working so I didnt really expect it to fire properly.It has enough compression to blow my thumb off the sparkplug hole. any other test I might try.

loteksw - prolly best to start a separate thread and also include the full briggs model # and type # orff the ID decal or what's stamped on the cowling sheet metal so that the experts here know what the engine detail is.

See bottom of first page of this forum for details on how to start a new thread.