The standard dimensions of common threads are classified into grades above 10, such as 3.6, 4.6, 4.8, 5.6, 6.8, 8.8, 9.8, 10.9, 12.9, etc. Among them, bolts of grade 8.8 and above are made of low-carbon alloy steel or medium-carbon steel, which are generally called high-strength screws after heat treatment, and the rest are generally called ordinary screws.

French engineer Renault saw that the wire rope on the hot air balloon was of various specifications, so he thought of a way. He powers 10 five times to get a number 1.6, and then multiplies it to get five priority numbers: 1.0, 1.6, 2.5, 4.0 and 6.3.

Extended data:

Screw model driving code: head grooving, characteristic model.

Length code: screw length (mm)

A- 1: thread code: screw model.

Metric screws directly indicate the screw type with the screw outer diameter, such as M3, that is, the screw outer diameter is 3.00mm；; M4 means that the outer diameter of the screw is 4.00 mm. ..

Metric thread size x pitch:

Note: Metric screws are behind the screw model, and sometimes the pitch is indicated. Such as M3x0.5, M4x0.70, M5x0.8, M6x 1. However, due to standard specifications, it is usually not mentioned.

A-2: length code: screw length:

Metric screw, directly indicating the length of the screw, in mm. Mark the total length of the screw, and only calculate the length below the head, excluding the height of the head. Except flat head screws, the total length of screws includes the height of screw head.

A-3: driving code/head groove, characteristics.