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Pipe Thread Gages Questions & Answers

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

Q. We need a NPT L3 gage. I can't find this on your web site can you help?

A. We still seem to keep getting different variations of this question about NPT gages so once again here are the facts as outlined in ASME. NPT ASME B.20.1 which calls out the requirement for NPT plugs and rings has NO CALL OUT FOR L2 or L3 or PLAIN TAPER gages.

If you or your customer requires these checks your only option is to use the ANPT version of the plug or ring which has the same diameters as the NPT Plugs and Rings. This is because the NPT gage does not have a wrench tight requirement. These NPT products use a variety of sealing media to seal the threads, unlike the NPTF which uses the actual threads themselves to create the sealing. Thus the name Dry Fit for NPTF.


Q. We have a NPT thread ring and looked at your website which said that the face of the ring that was marked was the large end and we are failing our products. Now we find out or ring is not marked on the large end it is marked on the small end. What are the facts here.

A. Ouch is all I can say to start with. As we state in all of our How To Do pages when in doubt go to the appropriate specification for the absolute facts. This is a question that is hard to answer without a smart quip about the old joke of "Green side up" but to be serious, on some smaller NPT rings it is not always easy to visually confirm the larger end. As to the facts, I have to clarify that the ASME B 1.20.1 does not, in fact, state the face that these thread rings must be marked on. It has never been modified to make that statement however this is a requirement in ASME B 1.20.5 for NPTF rings and the general acceptable marking convention of our OEMs is to carry this unstated marking convention over from the NPTF to the NPT in lieu of a firm requirement. I can assume the thread ring you used is not from our OEMs though exceptions can be found based on a customer requirement, not enough surface so marking is carried over to both faces, mistakes at the OEM or that some OEMs may not follow this unstated marking rule.


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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NOTICE: All information provided on this reference sheet is intended to be for general guidelines and may not represent the full specifications or instructions as outlined within ASME requirements or other technical specifications which govern the manufacturing and use of thread measuring gages. thegagestore.com/LEECO INC, does not represent this information as anything except a common sense general set of guidelines. You must always follow the technical specifications relative to the gage, thread design or other related gage attributes and usage that apply. When in doubt always acquire the current specification from http://www.asme.org/ codes & standards.


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