1. The character code of the thread uses the letter "m": the bolt specification M14 indicates the metric bolt with the thread diameter of 14mm. M stands for "metric, metric" thread. The complete thread mark consists of: thread feature code + dimension code + tolerance zone code + and other information + Standard Code (for example: add national standard code to domestic screws)

2. Size code: the size code of single thread is "nominal diameter × screw moment". For coarse thread, the screw moment can be omitted. The dimension code of multi thread thread is "nominal diameter × pH lead P bolt moment".

3. The code of tolerance zone is indicated by numbers plus letters (upper case for internal thread and lower case for external thread), such as 7h, 6G, etc. it should be specially pointed out that 7h, 6G, etc. represent thread tolerance, while H7, G6 represent cylinder tolerance.

4. The screw in length is specified as short (s), medium (n) and long (L). Generally, the screw in length is not marked, and the thread tolerance zone is determined by the medium screw in length (n). If necessary, the screw in length code s or l can be added, such as "m20-5g6g-l". In case of special needs, the spin length value can be indicated, such as "m20-5g6g-30".

5. The left-hand thread shall be marked with "LH" after the screwing length. The screwing length code and the screwing direction code are separated with "-" and the right-hand thread is omitted.