There are a few types of case screws from my experience.
1) Normal case screws. These are threaded slightly bigger then the holes inside in front bay drives like a dvd drive or hdd. These will not fit in those holes but are used to hold parts of the case together, including back mounting of pci cards and back covers.
2) Drive screws. Slightly thinner threaded. used almost exclusively to mount drives on the front of the case.
3) Fan Screws. These thick thread screws are used to cut into the plastic to hold the fan in place. they are big and chubby and sort of hard to confuse with any other screw in a case.
4) Custom screws. alot of components, heatsinks, fans and radiators can come with different sized screws used to mount them to whatever they are intended to go on. these screws can be quite different in length and thread from other computer screws, but from my experience alot of them use the same thread as (2) drive screws, just longer.
This is just a rule of thumb, there may be other types I am not aware of in cases I have not worked with.
You have your coarse thread and fine threaded screws pretty much. Coarse for case, mobo, pci cards, hard drives. Fine for cd/dvd and floppy. Thats about it, other then that they pretty much just have different heads.
When I get home I will take my old guide from school and pdf it if I can find it.
For fan to rad work 6-32 works out well,there is a metric equivalent. Be real careful with the lenght of fan to rad screws, swiftech likes to use plastic push rivets on their stackable units,very safe if nothing else.
WC fittings may initally look complex but a little experience will clear up any confusion.
For Antec hdd cages and side cover retainers are not the same thread.
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