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Pipe Plug Gages

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There are three (3) styles of pipe screw thread plug gauges:

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

The NPS (American National Standard Straight Pipe) screw thread pipe plug gages consist of two (2) members, a GO and a NOGO member. They follow the same gauging principles as the working screw thread plug gages. The part being checked is generally acceptable when the GO screw thread plug gage member enters completely and the NOGO screw thread plug gauge member does not enter more than 1½ to 3 turns. In some situations, the customer may elect more of less turns of entry for the NOGO screw thread plug gage member.

The NPT (American National Standard Taper Pipe) gauge consists of one L1 Tapered screw thread gage 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 tapered screw thread plug gauge checks the first four (4) screw threads.

NPT (American National Standard Taper Pipe) gage NPT L1 Plug Gage

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

L3 gage NPTF L3 Plug Gage

The NPTF and ANPT screw thread also requires the 6-Step Crest-Check plain taper pipe plug gage which consists of one member which checks the minor diameter of an internally threaded NPTF and ANPT screw thread hole. Plain taper 6-Step Crest-Check pipe plug gauges are used in tapered threaded holes. In order to use the plain plug gage, you first must determine with the threaded L1 plug gauge the actual size of the hole in relationship to basic, minimum and maximum screw thread tolerance.

6-Step plain taper pipe plug gage NPTF 6 Step Plug Gage

EXAMPLE: Insert your NPTF thread plug gage into the hole. If the part comes flush with the notch, you must use the '"Basic" notches marked "B" and "BT" on your 6-step plain plug gauge. 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 "Minimum" notches marked "MN" and "MNT" on the 6-step plain plug gauge. 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 the "Maximum" notches marked "MX" and "MXT" on your 6-step Crest-Check plain plug gauge.

Remember, first determine tapered screw thread depth relative to basic and minimum or maximum screw thread with the L1 threaded plug gage before using the plain plug gauge which would be checking the "crest" of the internal threads.

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This page is a modified page extracted from Dynamics of Gaging published by Regal-Beloit Corporation.The data is provided for general information only. The intention is to provide accurate information; regardless; errors may exist in the supplied information. If accuracy is critical, base your final decisions on the data provided in the root documents: ANSI/ASME B1.20.1; ANSI B1.20.5;and ANSI SAE AS71051 which are copyrighted documents. To purchase copies visit an Authorized Reseller.

Original Posting: 5/4/2005
Last Revision: 5/5/2016
Error corrections in, or comments about, the above data can be sent to: office@gagecrib.com

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