Gages and Gaging for Unified Screw Threads Per ANSI/ASME B1.2-1983 (R2017)

Not Go ( Lo ) Thread Ring Gage

A Not Go (Lo) thread ring gage inspects the Not Go (Lo) functional diameter limit of product external thread. The Not Go (Lo) thread ring gage, when properly set to its respective calibrated thread setting-plug, represents the Not Go (Lo) functional diameter limit of the product external thread. Not Go (Lo) thread ring gages must be set to the applicable W tolerance setting plugs.

Not Go (Lo) thread ring gages when applied to the product external thread may engage only the end threads (which may not be representative of the complete product thread).Starting threads on Not Go (Lo) thread ring gages are subject to greater wear than the remaining threads. Such wear in combination with the incomplete threads at the end of the product thread may permit further entry in the gage.

As stated in ANSI/ASME B1.2-1983, the "Not Go (Lo) functional diameter is acceptable when the Not Go (Lo) thread ring gage applied to the product external thread does not pass over the thread more than three complete turns. The gage should not be forced. Special consideration such as exceptionally thin or ductile material, small number of threads, etc., may require modification of this practice.” 

Not Go ( HI ) Thread Plug Gage

The Not Go (HI) thread plug gage inspects the Not Go (HI) functional diameter limit of product internal threads. The Not Go (HI) thread plug gage represents the Not Go (HI) functional diameter limit of the product internal thread.

Thread plug gages when applied to the product internal thread may engage only the end threads (which may not be representative of the complete thread). Entering threads on product are incomplete and permit gages to start. Starting threads on Not Go (HI) plugs are subject to greater wear than the remaining threads. Such wear in combination with the incomplete product threads permits further entry of the gage. As Stated in ANSI/ASME B1.2-1983, the “Not Go (Hi) functional diameter is acceptable when the Not Go (Hi) thread plug gage applied to the product internal thread does not enter more than three complete turns. The gage should not be forced. Special requirements such as exceptionally thin or ductile material, small number of threads, etc., may require modification of this practice.