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PIPE PLUG GAGES

There are three (3) basic styles of pipe plug gages:

Straight Threaded Pipe Plug Gages
Threaded Taper Pipe Plug Gages
Plain Taper Pipe Plug Gages

The NPS (American National Standard Straight Pipe) threaded pipe plug gages consist of two (2) members, a GO and a NOGO member. They follow the same gaging principles as the working thread plug gages. The part being checked is generally acceptable when the GO member enters completely and the NOGO member does not enter more than 1½ to 3 turns dependent upon customer specifications.

The NPT (American National Standard Taper Pipe) gage consists of one L1 member and is used to check an internal threaded hole. Parts being checked are generally acceptable when they come flush with the notch to plus (+) or minus (-) one turn. The L1 plug gage checks the first four (4) threads.

NPTF (Dryseal) and ANPT (Aeronautical) thread tolerances are more closely controlled requiring an L3 gage as well as an L1. The L3 checks the fifth (5), sixth (6) and seventh (7) threads.

The NPTF and ANPT thread also require the 6-Step plain taper pipe plug gage which consists of one member which checks the minor diameter of an internally threaded NPTF and ANPT hole. Taper 6-Step pipe plugs are used in tapered threaded holes. In order to use the plain plug, you first must determine with the threaded plug the actual size of the hole in relationship to basic, minimum and maximum thread tolerance.

EXAMPLE: Insert your NPTF thread plug gage into the hole. If the parts comes flush with the notch, you must use the notches marked "B" and "BT" on your 6-step plain plug gage. If you insert the NPTF threaded plug gage into the hole and the notch is one (1) turn small from being flush then use the notches marked "MN" and "MNT" on the 6-step plain plug gage. If after inserting the NPTF threaded plug gage into the hole the notch is one (1) turn large (that is, into the part deeper) from being flush, then use notches marked "MX" and "MXT" on your 6-step plain plug gage.

Remember, first determine thread depth relative to basic and minimum or maximum thread with the threaded plug gage before using the plain plug gage which would be checking the "crest" of the internal threads.

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