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When you buy a case for assembling a new PC it comes with several screws and other small components. Even seasoned technicians have trouble in telling what is the exact use of each screw that comes with the PC case. In this tutorial we will present a detailed explanation about all screws used on a PC and when to use each one of them, so by reading this tutorial you will enhance your PC assembling skills.
The screws vary in three aspects: their thread, the shape of their head and their length. The length and especially the shape of their head can vary a lot, but there are only three types of thread used on the PC. So we think the best way to classify the screws found on a PC is through their thread. In Figure 2, you can see these three thread types.
The first thread type, shown on the left hand side from Figure 2, is a self-tapping screw used to fasten fans to the case (and also ventilation-related plastic parts like ducts, if your case has them). That is the only use of this kind of screw in the whole PC, so it is pretty easy to identify this kind of screw. This screw is usually 10-mm long and usually has a flat-head, like shown in Figure 3, and its head is cut to accept a #2 Phillips screwdriver.
The second thread type found on screws used on the PC uses UNC (United National Coarse) 6-32 standard, which means a thread using a #6 wire (0.1380”-diameter wire) with 32 threads per inch (0.0313” pitch). They are cut to accept a #2 Phillips screwdriver. To simplify, let’s call this thread as “thick thread” from now on, as the third thread type is thinner than this one.
The third thread type found on screws used on the PC uses M3x0.5 standard, which is a standard set by ISO using the metric system. It means the use of a 3-mm wire with a 0.5-mm thread pitch. They are cut to accept a #1 Phillips screwdriver. To simplify, let’s call this thread as “thin thread” from now on, as it is thinner than the other types of thread used by PC screws.
Screws using both thin- and thick-thread can be found using several different types of heads and several different lengths, and here is where most people get confused. So let’s talk deeply about all screws found on a PC.
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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.
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 at Figure 5 to see what we are talking about.
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.
[nextpage title=”Thin-Thread Screws”]
Traditionally there were three thin-thread screws on the PC: one with 7/32” hex head and 5 mm in length – used to fasten optical drives and I/O brackets to the case –, another with a round head and the same length, 5 mm – used to fasten the motherboard to the case –, and a third one also with a round head, but shorter, with 4 mm in length – used to fasten the floppy disk drive to the case. These three traditional thin-thread screws are pictured in Figure 7.
Since they all use the same thread, you can fit a 5 mm-long screw where a 4 mm-long should be used, so play close attention. As mentioned, 4 mm-long screws must be used to fasten floppy disk drives to the case.
Using 5 mm-long round head screws where 5 mm-long hex head ones should be used doesn’t make any difference. The real problem is the length, not the shape of the head.
We also said that I/O brackets should be fastened using thin thread hex head screws. Well, this isn’t always true; this will depend on the case. There are cases that require thick-thread hex head screws to be used, others require thin-thread hex head screws, and there are cases that don’t require any screw at all to fasten the I/O brackets.
The same goes for the screws used to fasten the motherboard to the case. Some cases will require thin-thread screws, some will require thick-thread screws and some will not require any screw at all. We will talk more about that in the next page.
You can find more thin-thread screws than the ones we shown in Figure 7 (the first one on the left in Figure 8 and on the right in Figure 9 is the traditional hex head thin-thread screw, for comparison). Usually screws like these come with the optical drive.
All thin-thread screws should be screwed using a #1 Phillips screwdriver.
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Together with the screws come several devices for installing the motherboard to the case, especially some stands that need to be installed between the metallic chassis from the case and the motherboard. These stands can be either plastic (Figure 10) or metallic (Figure 11). It is important to note that some cases have already the equivalent of these devices already mounted on the metallic chassis.
If you pay close attention to Figure 11 you will see that the metallic stands can use thick- or thin-thread. If in your case they use thick thread, you must use thick-thread screws on them (use the short ones, the same used by hard disk drives). If your case uses thin-thread ones, you will need to use thin-thread screws (we prefer using the round head one, but you can use any thin-thread screw here).
In Figure 11 we also show a screw that does not belong to the case. The last one on the right is used by expansion boards such as video cards to hold their connectors to their I/O shields. We put it here just for you to see the difference between it and the motherboard stands.
Together with the screws the case also comes with several red card box washers that should be used to isolate non-metalized holes. This subject goes beyond the scope of our present tutorial, so we recommend you to read our How To Install a Motherboard tutorial for a complete explanation about them.
Fore a more in-depth explanation about the devices shown on this page, please read read our How To Install a Motherboard tutorial.