# Standard Hard Drive Mounting Dimensions

Some notes on mounting patterns and screw holes for standard 2.5" and 3.5" hard drives (and hard drive sized SSDs).

I had a nice little small-form-factor server at work that was at risk of going into the trash, because at some point the sheet-metal bottom plate of the thing had been lost.  (Probably someone just saw a random bit of sheet metal and either repurposed it for something else, or just tossed it out, thinking it was junk.)  To salvage it, I decided to fire up my laser cutter and make a replacement bottom plate -- easy enough!  But first, I had to figure out what the standard hole patterns were for the different types of hard drive that you could mount inside the case, screwed to that bottom plate.

It turns out that all the specs for hard drives are online, and they aren't too terrible to read.

## 3.5" Hard Drives

Traditional 3.5" hard drives (which are actually 4" wide; the 3.5" is presumably the diameter of the platters) are defined by the SFF-8300 spec, available directly from the SNIA. (Refreshingly, and unlike many other standardization bodies, they don't even try to charge you money for it, by which I mean they don't force you to download it from a sketchy website in China.)

=> https://members.snia.org/document/dl/25861 SNIA SFF-8300 (PDF)

The 3.5" spec is very clearly an inch-based specification, and many of the values are really ugly decimals when converted to millimeters.  The inch values, although they might at first seem to be equally-ugly decimals, tend to be traditional fractional-inch sizes.

And as befits an inch-based spec, the tapped holes used to mount 3.5" drives are ASME standard 6-32 UNC-2B.  Although they look very much like M3.5 screw holes and are very close in nominal diameter, they aren't the same.  (An M3.5 screw has a pitch of 0.024", while a 6-32 has a pitch of 0.031"; you'll know you have the wrong one if it starts to thread in but gradually stops after a turn or so.)  Don't force one into the other or you'll strip the threads from the drive -- they are almost always made from a softer metal than the average machine screw.

## 2.5" Hard Drives

The newer 2.5" spec is SFF-8200:

=> https://members.snia.org/document/dl/25850 SNIA SFF-8200 (PDF)

Unlike the 3.5" spec, 2.5" drives use metric M3 screws, despite many of the dimensions seemingly being drawn up originally in inches (although the values aren't quite as nice and round as the 3.5" spec's are).  The only real concern here is using a screw that's too long, which will bottom out in the blind hole and not hold the drive securely.

Thankfully there's not an ASME standard screw that's quite as close in size to M3 as the M3.5 is to the #6.  If you're using an ASME screw by mistake instead of metric, you'll know: the #4 screw is about 2.8mm and generally won't engage the threads of the M3 hole, while the #6 screw is almost 0.5mm too big and won't fit at all.  So that's kinda nice.