Traditionally there were only two thick-thread screws on the PC: one with ¼” hex head and ¼” in length (6.35 mm), used to screw the case cover and all other metallic parts to the case, and another using pan head and 3/16” in length (4.7625 mm), used to screw the hard disk drives to the case.
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Figure 4: Traditional thick-thread screws.
Since they both use the same thread, a case screw fits a hard disk drive and vice-versa. The problem here is the length. If you try to use a screw longer than 3/16” (4.7625 mm) to fasten your hard disk drive to your case, you can chip or even break the hard drive’s printed circuit board. We’ve seen this problem happening more than once. So you must always use the 3/16”-long pan head screws to fasten hard disk drives to the case.
This would be easy to keep in mind if case manufacturers didn’t ship several other kinds of screw using thick-thread nowadays. Take a look on Figure 5 to see what we are talking about.
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Figure 5: Several case manufacturers use different thick-thread screws.
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Figure 6: Same screws, view of their heads.
As you can see, the problem is not the different head shapes available mainly the different available lengths. Besides ¼”- and 3/16”-long screws, a third kind, 5 mm-long, can be found (it has a length between the two standard lengths). Screws with ¼” in length should be used only to fasten the case cover and other metallic parts to the case, while 3/16”-long screws should be used to fasten the hard disk drives to the case. 5-mm screws should be also used to fasten hard disk drives to the case, but only when no 3/16”-long screws are available.
All thick-thread screws should be screwed using a #2 Phillips screwdriver.
