Hi all

Just rebuilt a 12hp Biggs and stratton model 252707 0026-01 7905** it was all going well had spark spun it over fired and then sheared the flywheel key. No worries got two news ones tried again, cleaned the surfaces better nipped it up tighter and sheared another. The last one didn't shear and flogged out the keyway of the flywheel and crank. It ran for 30 seconds or so.
[Linked Image]
The photo is the result.

What are the main causes?
Have I put it together wrong?
Did I miss something?

Any thoughts on how to fix it?

P.s the engine owes me little, I have a spare flywheel to suit.

Mick