Pipe Thread Gages Questions & Answers

National Pipe gage questions and answers for NPT, NPTF NPSM and other thread questions received.

Q. I need to check 1-11 1/2 NPT external threads and looking on your site do not see any gages to check the taper of thread crests why not. all I see listed is a L1 ring gage.

A. Not an unusual question. First of all, most ( we'll always qualify this statement that there are always exceptions) NPT call outs require only a functional check that is accomplished with the L1 gage.  Because NPTs require some sort of medium to seal the threads it is usually acceptable to verify your cutting tooling is correct to comply with checking the thread crest.

We have had some customers who have been required to also do the crest check using a 6 step or plain tapered gage. This gage, in your case is a ring that touches the thread crests and verifies that the functional crest height and taper conforms. If you need this, since it is not called out by ASME, you can use the ANPT crest check gages. The dimensions on the ANPT 6 step and L2 gages are the same for the calculated dimensions for NPT 6 step so are interchangeable.

Q. I have to check a 2-1/2-8 NPT and our customer is calling our a L1 and L2 gage but I can not find this on the site.

A. As with the smooth 6 step gages, ASME does not specify the L2 gage for NPT. A similar logic is invoked. The NPT depends on sealing pipe dope or plumbers putty or Teflon tape to create the seal. The L2 is normally only used for Dry Seal thread forms like the NPTF where there is a metal to metal seal created by the male and female threads. The L2, when "hand tightened", checks that the projected wrench tight location is within specification as it relates to the position of the L1 gage previously established. Basically in simple terms turn in the L1 gage if it is perfect and exactly an the nominal position the L2 should also be at or near it's nominal radial position. We will not tell you not to check with L2 but you would have to use the ANPT L2.  By the way L2 or L3 depending on whether it is a Plug or ring gage.

 Remember when in doubt always refer to the actual ASME specification at www.ASME.org answers here are only suggestions and not to be taken as the absolute word. There are enough exceptions within the ASME specifications to write a couple of books about so if this helps good if it does not apply then refer to the specifications for definitive answers.

If you have questions feel free to address them to Technical@thegagestore.com

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