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Printed from Kezuriba (kezuriba.net/en/threads/pipe/)

Pipe thread tap drills: G, R, Rc, Rp and NPT

Threads for piping and fittings. ISO pipe threads (BSP) have a 55° thread angle, NPT (the US taper pipe thread) 60°; both taper threads taper 1:16 on diameter. On older Japanese drawings, PF means parallel (G) and PT means taper (R/Rc).

∥Parallel pipe thread G / BSPP (ISO 228-1, JIS B 0202)

SizeTPIMajor dia. dCut tap drill range (OSG) = internal minor dia. D1 rangeYamawa drillOSG form tap
G1/8289.7288.566–8.8488.789.24–9.35
G1/41913.15711.445–11.8911.7812.41–12.62
G3/81916.66214.95–15.39515.2815.92–16.12
G1/21420.95518.631–19.1721919.93–20.15
G3/41426.44124.117–24.65824.525.41–25.64
G11133.24930.291–30.93130.831.919–32.205
G1-1/41141.9138.952–39.59239.4—
G1-1/21147.80344.845–45.48545.3—
G21159.61456.656–57.29657.1—

In mm. OSG gives only a range (100–80% thread engagement), not a single recommended size. Yamawa’s recommendation sits near the top of the OSG range.

Pipe threads (55°) vs NPT (60°), taper vs parallel
Pipe threads G, R, Rc, Rp (BSP): 55°55°PhRounded crests and roots (r = 0.137P)h = 0.640P P = 25.4 ÷ TPINPT (US taper pipe thread): 60°60°PhFlat crests and roots (h = 0.8P)P = 25.4 ÷ TPIParallel vs taper (external thread, side view)1°47′ per side(1:16 on diameter)G, Rp: parallelSame diameter all alongG does not seal on the thread (use a gasket)R: taper externalMates with Rc (taper internal) or Rp;seals on the threadTaper angle is true to scale; fewer threads are drawn.

The two profiles are drawn to the ratios in the standards (BSP-type pipe threads are 55° Whitworth form, NPT is 60°). Thread angle and depth both differ, so BSP and NPT threads do not mate. On a taper thread the diameter changes by 1 for every 16 of length.

◢Taper pipe threads R, Rc, Rp / BSPT (ISO 7-1, JIS B 0203)

External / internalTPIMajor dia. at gauge planeGauge length aRc drill (OSG)Rc max. straight hole (Yamawa)Rp drill (OSG / Yamawa)
R1/8Rc1/8, Rp1/8289.7283.978.2calculated 8.235with incomplete threads 8.1 / without 8.28.5 / 8.5
R1/4Rc1/4, Rp1/41913.1576.0110.9calculated 10.941with incomplete threads 10.7 / without 10.911.4 / 11.4
R3/8Rc3/8, Rp3/81916.6626.3514.4calculated 14.428with incomplete threads 14.2 / without 14.414.9 / 14.9
R1/2Rc1/2, Rp1/21420.9558.1618calculated 17.95with incomplete threads 17.6 / without 17.918.5 / 18.5
R3/4Rc3/4, Rp3/41426.4419.5323calculated 23.349with incomplete threads 23 / without 23.324 / 24
R1Rc1, Rp11133.24910.3929calculated 29.423with incomplete threads 29 / without 29.330 / 30.2
R1-1/4Rc1-1/4, Rp1-1/41141.9112.738calculated 37.94with incomplete threads 37.6 / without 37.939 / 38.8
R1-1/2Rc1-1/2, Rp1-1/21147.80312.744calculated 43.833with incomplete threads 43.5 / without 43.845 / 44.7
R2Rc2, Rp21159.61415.8855calculated 55.412with incomplete threads 55 / without 55.456 / 56.5

In mm. R = taper external, Rc = taper internal, Rp = parallel internal (mates with R). Yamawa recommends pre-reaming a taper hole with a 1:16 pipe reamer to reduce the load on the tap. OSG’s Rc drill sizes are rounded from calculated values; for 1/2, 1-1/4 and 1-1/2 the rounding goes up, slightly exceeding Yamawa’s maximum straight hole. Yamawa’s Rp 1-1/4 to 2 values come from the old JIS PS thread table.

◣NPT (ASME B1.20.1)

SizeTPIPipe OD mmHand-tight engagement L1 mmOSG: with reamerOSG: without reamerYamawa: max. straight hole
1/16 NPT277.9384.0645.94 mm6.15 mm6.05
1/8 NPT2710.2874.102121/64 (8.33 mm)8.43 mm8.39
1/4 NPT1813.7165.786127/64 (10.72 mm)7/16 (11.13 mm)10.85
3/8 NPT1817.1456.0969/16 (14.27 mm)9/16 (14.27 mm)14.27
1/2 NPT1421.3368.12811/16 (17.48 mm)45/64 (17.86 mm)17.6
3/4 NPT1426.678.610657/64 (22.63 mm)29/32 (23.01 mm)22.91
1 NPT11.533.40110.161-1/8 (28.58 mm)1-9/64 (28.98 mm)28.78
1-1/4 NPT11.542.16410.6681-15/32 (37.31 mm)1-31/64 (37.69 mm)37.5
1-1/2 NPT11.548.2610.6681-45/64 (43.26 mm)1-23/32 (43.66 mm)43.57
2 NPT11.560.32511.0742-11/64 (55.17 mm)2-3/16 (55.58 mm)55.58

OSG’s values are the recommended drill sizes from the ANSI/ASME B1.20.1 appendix (as listed in OSG’s technical data). Without a reamer, OSG’s size can be larger than Yamawa’s maximum straight hole (e.g. 1/2: 17.86 vs 17.60). The two reflect different approaches, so both are shown.

📚Sources

  1. nbk1560.com https://www.nbk1560.com/images/ja-JP/product/technical_data/heikounezi_NBK/heikounezi_NBK_…
  2. osg.co.jp https://www.osg.co.jp/en/media_dl/technical/file/t_3.pdf
  3. swagelok.com.cn https://www.swagelok.com.cn/downloads/webcatalogs/en/MS-13-77.pdf
  4. yamawa.com https://www.yamawa.com/Portals/0/images/download/holesize/jp/technical-contents-15.pdf
  5. products.naganokeiki.co.jp https://products.naganokeiki.co.jp/userfiles/files/technology_document/Technical080402.pdf
  6. osg.co.jp https://www.osg.co.jp/media_dl/technical/file/drill_hole_size.pdf
  7. tw.misumi-ec.com https://tw.misumi-ec.com/pdf/fa/p2843.pdf
  8. nbk1560.com https://www.nbk1560.com/images/ja-JP/product/technical_data/tapernezi_NBK/tapernezi_NBK_1.…
  9. ns2.alliedtitanium.com https://ns2.alliedtitanium.com/technical/threads/npt/NPT_Threads.pdf
  10. blog.misumiusa.com https://blog.misumiusa.com/npt-thread-fitting-chart/
  11. willrich.com https://willrich.com/wp-content/uploads/2014/10/NPTF-Thread-gage-dimensions.pdf
  12. api.dixonvalve.com https://api.dixonvalve.com/en/media/1493771