Rack screws are used to attach equipment to the vertical uprights of a 19” rack. There is no universal standard thread type for server racks, however, there are three thread types that are most commonly used: 12-24, 10-32, and M6.
The term 10-32 originates from Unified and American Screw Threads for Bolts, Nuts, and Machine Screws standards published in 1974 by ANSI B1.1. The number “10” is used as a size designator and has no numerical meaning. “32” refers to the 32-thread pitch in threads per inch. You can identify a 10-32 screw by measuring the diameter with a ruler at exactly 3/16 inches ().
The 12-24 thread type is more commonly seen in pre-threaded racks. The fasteners are slightly larger than those seen on 10-32 and the threads are more course. The term 12-24 also come from the Unified and American Screw Threads for Bolts, Nuts, and Machine Screws standards. The number “12” is used as a size designator with no numerical meaning. “24” references the 24 threads per inch. You can identify a 12-24 screw by measuring the diameter with a ruler at slightly less than 7/32 inches (5.6mm).
M6 threads are 6mm metric screws. The standard metric rack screw is called M6 x 1. The ‘M’ means it’s metric. The ‘6’ is the outside diameter measured in millimeters, and the ‘1’ is the distance between adjacent threads, also in millimeters. Metric threads are used outside of the USA, and commonly in products by HP and other global entities. You can identify an M6 screw by measuring the diameter with a ruler. It’ll be 6 mm, or slightly more than 7/32 inches (0.228″).
The main difference between 10-32 and 12-24 threads is their dimensions. 10-32 screws have a diameter of 3/16″ (4.8mm) where as 12-24 screws have a diameter of 7/32″ (5.6mm). M6 threads are 6mm in diameter, only .4 mm larger than 12-24 screws.
When it comes to racking equipment, 12-24 threads are used with most racks, 10-32 threads are used with Dell equipment, audio rack and M6 metric threads are used with HP equipment. You will want to purchase the corresponding cage nuts to go along with these screws.