Or maybe these are #2 56 pitch screws?
I've got a new thread pitch gauge coming... but in parallel I did order a pack of #2 56 pitch (imperial) screws. They aren't metric M3s (they are smaller) and bigger than M2s. Might be M2.5 but I've found most of Apple's stuff is in the imperial / US standard system as opposed to metric. Anyhow, will figure it out.
I was asking because a few screws are missing, and a couple have some rust on them. The internal screws (some of which match the pitch of the chassis screws I am asking about -- and some which are different) are all good. It's the external facing ones that have some rust or are missing.
I had this surface corrosion on my 540c, too..in the process of refinishing the machine I also lightly sanded the internal frame, it is made of a very light alloy.
I am not familiar with this corrosion on magnesium. Looked at a few discussions online on treatment and I don't have the interest (or means) to take approaches like sand blasting it or utilizing strong acids to clean it. What's not clear to me is whether like rust, it will continue to spread even when dry or will stop in its current state. There's some folks who have used WD-40 to help clean it ... not sure how effective that is. I guess I will try and see. With rust I know some folks have painted over it with non-water based paint which seals it and stops it. Regardless if any future damage from this corrosion is slow, then maybe I just won't worry about it as other things will die first before the chassis corrodes away.
Maybe I will try this WD-40 thing and see if it does anything. And then after that, degrease it and just put a coat of paint over it to seal it?