its a SAE 609A taper, pretty standard for a gen engine less than 20hp..might want to check ebay for a used tapered generator crank, might be easier than cutting a taper on yours..I did a refit on an old 8kw Generac generator, the brushed gen end went bad so I wanted to replace it with a new generator end, only to find that Generac back in those days had their OWN taper standard on the crankshaft! So I replaced the crank with a used one off eBay..$50..easy