Ah thanks.
M3 is 3mm and M4 is 4mm. They just use the Mx rating for screws. Maybe ill try with both. Its best if the screw attaching the heatsink is as snug as possible...
Nope, stay with M3 screws. For two reasons:
- drilling tolerances
- thermal expansion of the PCB.
Yes, I believe I started out with 3mm and made them bigger to accommodate standard ATX screws with some clearance.