If the flywheel is loose on the taper (or parallel if it doesn't have a taper), the key will be chewed and broken quite quickly - that is a chainsaw, not a whipper snipper, it is a fairly high performance engine. Is it a taper or a parallel shaft, and how tightly does it fit/how tightly are you doing up the nut/what is the condition of the bearing area like? Is the key too tall for the space between the bottom of the shaft and flywheel keyways? In my experience the most common cause of sheared keys on shafts which have had amateur repairs, is that too tall a key has been fitted. Of course that prevents the bore fitting properly on the shaft: it is distorted in the region around the key, reducing the bearing area and introducing the potential for slippage.
I do not think a small amount of crankshaft end-float is likely to be the problem here.
Last edited by grumpy; 11/12/14 07:25 PM. Reason: Add detail
