If the outer diameter of the thread is 6.25mm, it is a non-standard thread. The marking method is the same as the standard thread, just mark M6.25 on the dimension line.
The dimensioning method of common thread: the common thread code is M, which is divided into coarse thread and fine thread, and the pitch of coarse thread can be omitted; When the codes of tolerance zones of middle diameter and top diameter are the same, they shall be marked only once; Right-handed thread may not be marked with rotation code, and left-handed thread may be marked with rotation code LH; When the tightening length is medium (n), don't pay attention. The long type is represented by L and the short type by S. ..
In design, the first series of threads is usually preferred, followed by the second series. The third series of threads and non-standard threads are not used as much as possible.