ASME B1.20.1 Pipe Threads Q&A
General Questions
Q: What does ASME B1.20.1 cover?
A: ASME B1.20.1-2013 covers dimensions and gaging of pipe threads in the following series: NPT, NPSC, NPTR, NPSM, and NPSL.
Q: What is the current version of ASME B1.20.1?
A: The current version is ASME B1.20.1-2013 (Revision of ASME B1.20.1-1983, Reaffirmed 2018).
Q: What does NPT stand for?
A: NPT stands for National (American) Standard Pipe Taper.
Q: What does NPSC stand for?
A: NPSC stands for National Pipe Straight Coupling.
Q: What does NPTR stand for?
A: NPTR stands for National Pipe Taper Railing fittings.
Q: What does NPSM stand for?
A: NPSM stands for National Pipe Straight Mechanical.
Q: What does NPSL stand for?
A: NPSL stands for National Pipe Straight Locknut.
Q: How do you designate pipe threads according to ASME B1.20.1?
A: Specify the nominal pipe size, number of threads per inch, and thread series symbol. Example: 1/8-27 NPT.
Q: How do you designate left-hand pipe threads?
A: Add “LH” to the designation. Example: 1/8-27 NPT-LH.
Q: What is the default hand for pipe threads if not specified?
A: Right-hand threads are the default when “LH” is not specified.
Thread Form Questions
Q: What is the thread angle for American National Standard pipe threads?
A: The angle between the sides of the thread is 60 degrees when measured in an axial plane.
Q: What is the formula for pipe thread height H?
A: H = 0.86602540P, where P is the pitch of the thread.
Q: What is the maximum height of truncated pipe threads?
A: The maximum height h = 0.800P.
Q: What is the minimum truncation for pipe thread crests and roots?
A: The minimum truncation is 0.033P for both crests and roots.
Q: Can pipe thread crests and roots be radiused?
A: Yes, a radius form is acceptable as long as the crests and roots lie within the limits shown in Table 1.
NPT Taper Pipe Thread Questions
Q: What is the taper of NPT threads?
A: The basic taper is 1 in 16 or 0.75 in./ft measured diametrically relative to the axis.
Q: Do NPT threads require sealant?
A: Yes, where pressure-tight, leak-free joints are required, NPT threads should be made up wrench-tight with a sealant.
Q: What is the hand-tight engagement length L1 for NPT threads?
A: L1 is the basic length of engagement between external and internal taper threads when screwed together hand-tight. Values are specified in Table 2, column 6.
Q: What is the effective thread length L2 for NPT external threads?
A: L2 = (0.80D + 6.8) × 1/n, where D is the outside diameter of pipe and n is threads per inch.
Q: What is the pitch diameter formula for NPT threads at the small end?
A: E0 = D – (0.05D + 1.1) × 1/n, where D is the outside diameter and n is threads per inch.
Q: What is the gaging tolerance for NPT external threads?
A: ±1 turn (or ±1 pitch) from the small end of the L1 ring gage.
Q: What is the gaging tolerance for NPT internal threads?
A: ±1 turn (or ±1 pitch) from the basic step of the L1 plug gage.
Q: How do you gage NPT external threads?
A: Use an L1 ring gage or external L1-indicating gage, screwed hand-tight, and note the standoff position.
Q: How do you gage NPT internal threads?
A: Use an L1 plug gage or internal L1-indicating gage, screwed hand-tight, and note the standoff position relative to the basic step.
Q: What should you do about nicked pipe threads during gaging?
A: The product mated with a gage may be “rapped” and hand-tightened while engaged to reform nicks to their proper position.
Q: How do chamfers affect NPT thread gaging?
A: The gaging reference point depends on chamfer diameter. For external threads with chamfer smaller than minor diameter, reference is the last scratch of the chamfer cone.
Q: Do NPT threads include coating allowance?
A: No, ASME B1.20.1 specifications do not include allowance for coating or plating.
Q: How does coating affect pipe thread dimensions?
A: Coating buildup on thread flanks increases external pitch diameter and reduces internal pitch diameter by four times the coating thickness.
Q: How do you compensate for coating on NPT threads?
A: Make external threads smaller and internal threads larger before coating so that after coating, threads conform to the standard.
Q: What are the threads per inch options for NPT?
A: NPT threads are available in 27, 18, 14, 11.5, and 8 threads per inch, depending on pipe size.
NPSC Thread Questions
Q: What type of threads are NPSC?
A: NPSC are straight (parallel) internal threads with the same thread form as NPT.
Q: Where are NPSC threads used?
A: NPSC threads are used in pipe couplings to form pressure-tight joints when assembled with NPT external taper threads.
Q: Do NPSC threads require sealant?
A: Yes, NPSC joints should be made up wrench-tight with sealant for pressure-tight applications.
Q: What is the gaging tolerance for NPSC internal threads?
A: ±1½ turns (or ±1½ pitch) from the basic step of the L1 plug gage.
Q: What type of gage is used for NPSC threads?
A: Internal taper thread gages (L1 plug gage or indicating gage) are used to gage NPSC straight internal threads.
Q: How is the NPSC pitch diameter related to NPT?
A: The NPSC pitch diameter corresponds to ±1.5 turns from the NPT E1 dimension.
NPTR Thread Questions
Q: What are NPTR threads used for?
A: NPTR threads are used for railing joints that require a rigid mechanical taper thread joint.
Q: How do NPTR external threads differ from NPT?
A: NPTR external threads are the same as NPT but shortened to permit use of the larger end of the pipe thread.
Q: What is the gaging tolerance for NPTR external threads?
A: No turns large and one turn small from the modified basic position.
Q: Do NPTR internal threads have a recess?
A: Yes, a recess in the fitting provides covering for the last scratch or incomplete threads on the pipe.
Q: What gages are used for NPTR threads?
A: Standard NPT L1 gages are used for gaging NPTR threads.
Q: What pipe sizes are available in NPTR?
A: NPTR threads are available in sizes from 1/2 to 4 inches.
NPSM and NPSL Thread Questions
Q: What are NPSM threads used for?
A: NPSM threads are free-fitting mechanical joints for fixtures with no internal pressure or sealing requirements.
Q: What are NPSL threads used for?
A: NPSL threads are loose-fitting mechanical joints with locknuts, designed to produce the largest diameter thread possible on standard pipe.
Q: Are NPSM and NPSL threads tapered or straight?
A: Both NPSM and NPSL are straight (parallel) threads for both external and internal threads.
Q: What type of gages are used for NPSM and NPSL threads?
A: GO and NOT GO gages or indicating type gages are used.
Q: What is acceptable for GO gaging of NPSM/NPSL?
A: The GO gage must freely engage the full threaded length of the product.
Q: How many turns can the NOT GO gage enter for NPSM/NPSL?
A: The NOT GO gage should not enter more than three complete turns.
Q: What is the pitch diameter for NPSM and NPSL threads?
A: The basic pitch diameter equals E1 from Table 2, the same as the large end of internal NPT threads.
Thread Tolerance Questions
Q: What is the tolerance on taper for NPT threads?
A: +1/8 to -1/16 inch per foot on the pitch line.
Q: What is the lead tolerance for NPT threads in effective length?
A: ±0.003 inch for sizes 1/16 through 2, and ±0.003 inch per inch for larger sizes with effective length greater than 1 inch.
Q: What is the thread angle tolerance for NPT threads?
A: ±2½ degrees for 27 and 18 TPI; ±2 degrees for 14 TPI; ±1½ degrees for 11.5 and 8 TPI.
Q: What are the working gage pitch diameter tolerances?
A: Tolerances vary by size and threads per inch, ranging from ±0.0002 to ±0.0008 inch. See Table 9 for specific values.
Q: What is the maximum wear allowed on NPT working plug gages?
A: Maximum wear shall not exceed 1/4 turn small from basic when checked with a master gage.
Q: What is the maximum wear allowed on NPT working ring gages?
A: Maximum wear shall not exceed 1/4 turn large from basic when checked with a master gage.
Gaging Questions
Q: What is an L1 ring gage?
A: An L1 ring gage is used to inspect external taper pipe threads and has a length equal to dimension L1.
Q: What is an L1 plug gage?
A: An L1 plug gage is used to inspect internal taper pipe threads, has length equal to L2, with a gaging notch located at distance L1 from the small end.
Q: What are master gages used for?
A: Master gages are used to inspect working gages at manufacture and calibration, and to set indicating gages.
Q: What are working gages used for?
A: Working gages are used to inspect product threads.
Q: How accurately must master gages be made?
A: Master gages should be made to basic dimensions as accurately as possible, with cumulative variation not exceeding one-half of the total working gage tolerance.
Q: How should master plug and ring gages mate?
A: Master ring and plug should seat flush at the plug notch within ±0.002 inch (sizes 1/16-2), ±0.003 inch (sizes 2½-12), or ±0.005 inch (sizes 14+).
Q: What is the crest truncation for NPT gages?
A: Gage crests are truncated by 0.140P for 27 TPI, 0.109P for 18 TPI, and 0.100P for 14, 11.5, and 8 TPI.
Q: What is the root clearance for NPT gages?
A: Gage roots shall clear 0.0381P width, with sharp V or undercut clearance acceptable.
Q: Should partial end threads be removed from gages?
A: Yes, partial end threads shall be removed on both ends of ring and plug gages by chamfering or convolution.
Q: How should gages be marked?
A: Each gage shall be marked with nominal pipe size, threads per inch, and proper thread series designation.
Q: Can indicating gages be used for NPT threads?
A: Yes, indicating gages set with master gages can be used to measure diametral variation from basic.
Dimension Questions
Q: What is the outside diameter of 1/8 NPT pipe?
A: 0.405 inches.
Q: What is the outside diameter of 1/4 NPT pipe?
A: 0.540 inches.
Q: What is the outside diameter of 1/2 NPT pipe?
A: 0.840 inches.
Q: What is the outside diameter of 3/4 NPT pipe?
A: 1.050 inches.
Q: What is the outside diameter of 1 inch NPT pipe?
A: 1.315 inches.
Q: What is the outside diameter of 2 inch NPT pipe?
A: 2.375 inches.
Q: How many threads per inch for 1/8 NPT?
A: 27 threads per inch.
Q: How many threads per inch for 1/4 NPT?
A: 18 threads per inch.
Q: How many threads per inch for 1/2 NPT?
A: 14 threads per inch.
Q: How many threads per inch for 1 inch NPT?
A: 11.5 threads per inch.
Q: How many threads per inch for 2 inch NPT?
A: 11.5 threads per inch.
Q: How many threads per inch for 3 inch NPT?
A: 8 threads per inch.
Q: What is the pitch diameter E1 for 1/2 NPT?
A: 0.77843 inches.
Q: What is the pitch diameter E1 for 1 inch NPT?
A: 1.23863 inches.
Q: What is the hand-tight engagement length L1 for 1/2 NPT?
A: 0.3200 inch or 4.48 threads.
Q: What is the hand-tight engagement length L1 for 1 inch NPT?
A: 0.4000 inch or 4.60 threads.
Application Questions
Q: Do NPT mating threads always contact at crests and roots?
A: No, mating crests and roots may clear, contact, or interfere. Mating threads should always contact on thread flanks.
Q: Should NPT joints be tightened beyond hand-tight?
A: Yes, joints should be tightened beyond hand-tight to create interference between flanks and produce a seal with sealant.
Q: Can overtightening damage NPT joints?
A: Yes, overtightening may be detrimental to the sealing function of the joint.
Q: Does out-of-roundness affect NPT sealing?
A: Yes, out-of-roundness of mating parts can negatively affect their ability to seal when made up wrench-tight.
Q: Are there dryseal pipe threads that don’t require sealant?
A: Yes, dryseal threads are covered in ASME B1.20.3 and may be used without sealing compounds.
Q: What is the related standard for hose coupling threads?
A: ASME B1.20.7 covers hose coupling screw threads (NPSH series).
Q: Can NPT threads be used for special high-pressure applications?
A: Yes, for applications like high-pressure flanges, longer thread engagement is needed by maintaining E1 and making E0 smaller.
Tap Drill Questions
Q: What diameter should the tap drill be for NPT threads?
A: The hole size should approximate diameter K0 (Basic Minor Diameter at Small End) from Table 2, column 24.
Q: Can tap drill holes be tapered or straight?
A: Both tapered and straight holes are acceptable as long as the product thread dimensions are met after tapping.
Q: Why might tap drill size vary from K0?
A: Hole sizes may vary from K0 so the tap can cut an acceptable pipe thread with the required thread height.
Special Gaging Methods
Q: What is the turns of engagement method?
A: A gaging method that determines adequate thread availability at hand engagement by counting turns to remove the L1 gage. Tolerance is basic turns ±1 turn.
Q: Is the turns of engagement method mandatory?
A: No, it’s described in Nonmandatory Appendix A and may be used when parties agree, but it’s not a required method.
Q: Why was turns of engagement moved to an appendix?
A: Due to inherent gage correlation issues between different gages.
Q: Can optical projection be used to check pipe threads?
A: Yes, optical projection is commonly used to measure truncation equivalent width of flat to ensure limits are not exceeded.
Q: What is supplemental gaging?
A: Additional gaging beyond prescribed methods to evaluate conformance to individual thread elements like taper, truncation, lead, and flank angle.
Q: When should supplemental gaging be used?
A: When agreed upon by supplier and purchaser to ensure all basic design dimensions are met.
Thread Engagement Questions
Q: What is wrench make-up length L3?
A: The internal thread length beyond L1 allowing wrench-tight engagement, shown in Table 2.
Q: What is effective thread length L2?
A: The length of effective external thread, equal to L1 plus wrench make-up threads.
Q: How does gage tolerance affect thread engagement?
A: The ±1 turn tolerance can potentially increase engagement by up to 2 turns or reduce it by up to 2 turns.
Q: Does gage wear affect thread engagement?
A: Yes, working gage wear has the additional effect of reducing thread engagement by the amount gages wear past basic.
Q: What is the overall thread length L4?
A: The overall length of external thread, including vanish threads.
Q: What is the complete thread length L5?
A: The length where the cone of major diameter intersects the outside diameter of pipe, where thread form is incomplete at the crest.
Standards and References
Q: What standard covers gage blanks for pipe threads?
A: ASME B47.1 covers gage blanks.
Q: What standard covers screw thread nomenclature?
A: ASME B1.7 covers screw threads nomenclature, definitions, and letter symbols.
Q: What standard covers gaging for unified screw threads?
A: ASME B1.2 covers gages and gaging for unified screw threads.
Q: What standard covers rounding dimensions for threads?
A: ASME B1.30 covers standard practice for calculating and rounding dimensions.
Q: When was ASME B1.20.1-2013 issued?
A: November 15, 2013.
Q: When was ASME B1.20.1-2013 reaffirmed?
A: It was reaffirmed in 2018 (R2018).
Q: What standard did ASME B1.20.1-2013 revise?
A: It revised ASME B1.20.1-1983 (R2006).
Coating and Plating Questions
Q: Must coated NPT threads meet the standard after coating?
A: Yes, gaging requirements must be satisfied after coating.
Q: How can you indicate threads are sized for coating?
A: Add “AFTER COATING” to the thread designation for finished product specs.
Q: How do you specify pre-coating thread dimensions?
A: Specify gage limits before coating followed by “BEFORE COATING” or “PRE-PLATE.”
Q: How much does coating increase external thread pitch diameter?
A: Coating on flanks increases external pitch diameter by four times the coating thickness.
Q: How much does coating decrease internal thread pitch diameter?
A: Coating on flanks decreases internal pitch diameter by four times the coating thickness.
Q: Can special pre-coating gages be used?
A: Yes, modified ring and plug gages may be designed based on standard gages with compensated dimensions.
Q: Does barrel coating affect pipe threads?
A: Yes, barrel coating can damage thread form, especially the crests of external threads.
Technical Calculation Questions
Q: How do you calculate diameter equivalent of angle variation?
A: Diameter equivalent = 1.53811978P × tan(α), where α is variation in half angle in minutes. See Table 10.
Q: How do you calculate diameter equivalent of lead variation?
A: Diameter equivalent = 1.73205081 × ΔP, where ΔP is variation in lead. See Table 11.
Q: How do you convert gage variation to turns?
A: Longitudinal variation (diameter equivalent for lead + angle + pitch diameter × 16) divided by pitch equals decimal turns from basic.
Q: What is the taper constant for standoff calculations?
A: 16 (from the taper of 1 in 16).
Q: What is the 60-degree thread constant for coating compensation?
A: 4 (coating thickness × 4 affects pitch diameter on flanks).
Historical and Changes Questions
Q: What significant change was made regarding chamfered threads?
A: The gaging reference point was moved for external threads with chamfer diameters smaller than the minor diameter.
Q: What change was made to gage wear reference point?
A: Changed from size at manufacture to basic size as the reference for checking wear.
Q: Were any calculated dimensions revised in 2013?
A: Yes, calculated data were revised following rounding rules of B1.30M, though changes were generally beyond measurement margin of error.
Q: When was an acceptability section added?
A: In the 2013 revision, acknowledging acceptable variation in prescribed gages and gaging methods.
Q: What guidance was added about coatings?
A: Detailed explanations on effects of coatings on threads and guidance for precoating size compensation were added in 2013.
This comprehensive Q&A set covers the major aspects of ASME B1.20.1-2013 and should help improve your website’s SEO for pipe thread-related searches.
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