Fastener Specifications

Tooth shape number is called the diameter, commonly used in the metric system are M2, M3, M4, M5, M6, M8, M10, M12, etc.

Commonly used in the American system are 4#-40, 6#-32, 8#-32, 10#-24, 1/4-20, 5/16-18, 3/8-16, 1/2-13, etc.

The length of the screw is the appropriate size to be embedded in the object being loaded. Therefore, for example, the loading length of the countersunk screw is the total length, the semi-countersunk screw plus half the head length, and the length of the cylindrical head screw does not include the head size, for example.

It is best to add the pitch to the full name of the specification, for example, M4-0.7×8, 4 means the outer diameter of the tooth is 4mm, 0.7 means the distance between the two tooth tops is 0.7mm, and 8 means the effective length embedded in the object being loaded is 8mm.

For simplicity, the pitch is not written; we assume that coarse teeth are standard teeth because they are the most common; so there is no need to mark them. This is only available in metric systems, and American products still need to retain the pitch.

Here we focus on the specifications of American screws, such as 6#-32*3/8, 6# is the outer diameter of the thread, which is close to 3.5mm; 32 is the length of the thread per inch in 32 threads (equivalent to the pitch of metric screws); 3/8 is the length of the screw (exactly the same as metric screws).

There are 2 formulas to remember here: Tooth outer diameter A#=(Ax0.013+0.06)x 25.4 (mm), 1 inch=25.4mm.

Among them, 2#=2.2mm, 4#=2.9mm, 6#=3.5mm, 8#4.2mm, and 10#=4.8mm are the data that need to be memorized. In addition, the number of threads corresponding to each specification of screws must also be familiar: 2#-56, 4#-40, 6#-32, 8#-32, 10#-24, 1/4-20, 5/16-18, 3/8-16, 1/2-13 (American standard number of threads).

Note: American UNC teeth are standard teeth, UNF are fine teeth, and we default coarse teeth to regular teeth.