I remember that post. They are pretty small in terms of volume. I'm not an engineer or an expert in fluid dynamics, so I have no clue whats necessary, but in my redneck mind, more is betterer.
so i have a tap removal tool coming (don't listen to the internet kids, a 5mm hole is really too tight for an m6 tap, 5.5mm is much better), but even if I can't get that out, there are 2 other mounting holes nearby and a hold down plate will serve my purposes fine if it comes to that.
A word about drilling. The aluminum is soft and drills easily. The holes are roughly 15mm deep and the bolts extend about 12.8mm into the head, so drill accordingly. A piece of tape on the drill is sufficient to mark depth.
The drill block works fine but makes it hard to find your center punch divot, so there were 1 or 2 misstarts. Call it weight savings
To prevent further issue, I have done the following:
line up aluminum plenum with the ports
Use a 6mm drill bit with a sharp point as a punch. This fits very tightly in the mounting bosses of the plenum so you can accurately center. It works way better than trying to use a marker or paint.
Then I enlarge hole with center punch. Check and recheck alignment here.
Then I free drill a divot in that spot. Check alignment again
Then comes full drilling.
Drilling must be fairly accurate, but at the end of the day, the plenum aluminum is soft, so you can hog out the hole a bit to perfect your alignment, so if your hole is a little off center, no big deal.
hope this helps.