Below is a basic use of a GO / NOGO gauge for straight threads, such as Unified National (UN), Metric threads, Acme / Stub Acme and other straight thread connections:
Using thread plug and ring gauges is a simple effective and economical way to pass or fail a threaded connection. This type of inspection is also called limit gauging as we don’t determine dimensions of the part thread but can confidently say it is within the upper and lower limit of the specification.
Essentially the GO gauge is designed to enter the part and the NOGO gauge is not. When the go plug gauge enters the female connection or box connection it is proven to be NOT too small. When the NOGO plug gauge fails to enter the part this ensure that female connection is NOT too large. Therefore, it is safe to determine that the female connection is in tolerance and between the known sizing of the GO plug member and the NOGO plug member. The inverse is true for ring gauges. When GO ring enter the pin or male threaded connection that ensures that the sizing of the pin is not too Big. When the NOGO ring doesn’t enter the pin that means that the pin is NOT too small.