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
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.
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.
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
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.
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
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