Theoretically there are two or three types of screws used in PCs: 6-32s, M3s and standoffs. I need to secure the internal DVD drive in a HP Compaq 8100 Elite SFF PC. It was given to a family member by a relative who lives far across the country. Everything is perfect working order except the DVD drive is not secured. If you're familiar with HP DVD drives, they have a black screw installed on either side (looks like maybe a Torx head) where the head is only there to guide the drive through an L-shaped path during installation/removal. About 3/4 of an inch from each of these is another threaded hole in the DVD case. Anything inserted there will keep the drive locked in place during normal use.
But this unit came without the locking screws. Online research (including the HP site) told me two things; the missing screws are likely M3s and HP provides spare fasteners for customers by inserting them in extra holes on the back of the case. This PC has no extra holes on the case, so I went to the hardware store and bought 6-32 and M3 screws. Both were too large to fit the hole for the locking screw. I got curious and removed the black screw being used as a guide. The threaded portion was visibly smaller than the M3 I had purchased. I tried putting the black "guide" screw in the locking screw hole and it fit just fine.
The obvious problem is that I need two more, maybe even some spares, but I don't know what type of screws these are. If I want to try M2.5 or M2 I'll have to mail order, and one will be a waste of money. Tips?