This is not about welding a stock IC up, but welding aluminum in general.
I have found that its best to use a TIG welder for any aluminum welding.
Aluminum must be very clean,then clean again with alky, before welding.
When welding cast aluminum parts. You can have contaminated welds
even if you clean the aluminum perfect. Besides contamination from not
cleaning the aluminum.The next largest reason for contamination is that
when the aluminum cast part was poured .The pour was to hot at begining
of pour, or aluminum cooled off before pour was finished.
This is why sometimes you can start a weld and its contaminated,
Aluminum to hot during pour, weld will be contaminated
aluminum to cold during casting pour weld will be contaminated.
So don't pull your hair out if you can't make a clean perfect weld
on cast aluminum parts sometimes.
Randy