Small Engines (Lawn Mowers, etc.): B&S motor on Craftsman Lawn Tractor

Question
I Have a sears rider mower with the Briggs 15.5 hp OHV IC platinum engine, model 28N707 type 0173-01. I had replaced the flywheel gear ring, as the original plastic one had some teeth sheared off.  After reassembly the engine started up ok, then would not restart.  I found that it had sheared off the flywheel key. The engine is backfiring as well.  I adjusted the mag air gap as well as the lifter gaps.  It starts up and quickly shears off another key.

Answer
I'd start by lapping the flywheel taper to the crankshaft taper.  This only takes a minute or two and you only need a little lapping compound.

Second, were the any washers under the flywheel nut/bolt?  Most of the time there is a washer and if you forget to install it the key will shear almost every time.

Let me know if lapping the parts helps.

Eric