Example 1: GB/T197-2003 specifies the basic deviations for internal and external threads to determine the position of the tolerance zone relative to the basic tooth profile.
Common basic deviation designations are:
Internal threads: G, H (where H represents a basic deviation of zero and is the most common);
External threads: e, f, g, h (where h represents a basic deviation of zero and is the most common).
For example, M10-6H is a metric internal thread, where M10 is the major diameter and the pitch is a standard 1.5mm. 6H is the thread tolerance zone designation, where 6 is the tolerance grade and H is the basic deviation designation. In tolerance zone designations, internal threads are usually represented by the uppercase letter H, and external threads by a lowercase letter.
M is the thread designation. Generally, threads are classified into two types according to pitch: coarse threads and fine threads.
M10: Coarse thread with a nominal diameter of 10.
Coarse threads do not specify the pitch; the pitch of M10 is 1.5mm.
Fine-pitch threads must be marked, such as: M10×1: nominal diameter 10, pitch 1 (fine-pitch thread).
M10×1-LH indicates: nominal diameter 10, pitch 1, left-hand thread (left-hand threads are always represented by LH).
There is no difference in the nominal diameter marking for internal and external threads; the distinction only applies when specifying thread tolerances. Uppercase letters indicate internal threads, lowercase letters indicate external threads. H indicates internal thread, h indicates external thread.
Example 2: Thread markings like M10-6H, where the letter H is uppercase, indicate an internal thread; similarly, M20-6g, where the letter g is lowercase, indicates an external thread.
