On Sunday, April 5, 2015 at 5:47:16 AM UTC-7, Ignoramus10114 wrote:
I have an aluminum heatsink that I need to tap , I wanted to know if I
should use 6-32 or some less coarse thread like 8-32 would be better?

Either would work, assuming the aluminum is thicker than 1/16 inch..
you never want less than two threads.
Be sure to check/flatten the machined surface (I sometimes rub it down
with a whetstone) if there's any deformation, or the flat-to-flat
heat transfer interface will be compromised.