Really? How about an M8-1.25 or maybe M8-1.0, take a bolt with you to the store to make sure you get the right one. Then read the package to get the correct drill bit size.

 

have you tried looking into that stuff called
alumaloy "I think thats how its spelt"
all you need is a torch to melt it onto your part and let it cool then you can grind it down to the way you want it and then tap it.
I hade a co-worker use that on his radiator for his bike and he said that its holding up pretty good

 

Mixed results. I've used some of the stuff before on some plate aluminum to try it out and it worked great. on the other hand I tried to use it to fix my oil tank before I had it tigged and it did not work AT ALL. No idea why, but it would stick fine to the bung and not at all to the sheet. Tried more heat, different techniques, etc. Nothing worked.

You might be lucky or you might just make a mess with it.

 

When I crashed my 88 cbr600f I cracked the stator cover and the clutch case I tig welded both with great success. But, at the time I made a living with welding. Cast aluminum does required special skill but, is in no way impossible