日本語

Printed from Kezuriba (kezuriba.net/en/threads/m20/)

M20×2.5

ISO metric coarse thread

Nominal diameter 20 mm, pitch 2.5 mm. Tap makers recommend slightly different drill sizes, so they are listed side by side.

Tap drill, rule of thumb (d − P)
17.50 mmMaker recommendations below
Minor diameter D1 range (6H)
17.294 – 17.744 mmKeep the hole inside this range[4][5][3]
Pitch diameter d2 (basic)
18.376 mmd2 = d − 0.649519P
M20 basic profile and dimensions (schematic)
external thread (bolt)major dia. d = 20pitch dia. d2 = 18.376minor dia. D1 = 17.2946H range 17.294–17.744 (tap drill target)P = 2.5H160°

Thread angle 60°. H1 (basic thread engagement) = 0.541266P = 1.353. The pitch diameter is where the thread ridge and groove are equally wide. For cutting taps, pick a drill that lands inside the 6H tolerance of the minor diameter D1. Enlarged to show the shape; proportions are not to scale.

⌀Tap drill size for cutting taps (maker recommendations)

For internal thread class 6H. Values as given in each maker’s technical data (mm).

MakerRecommended drillNotesSource
OSG17.5min. 17.3, 6H max. 17.74
[3]
Nachi (Nachi-Fujikoshi)17.5high-performance drill 17.6[5]
Yamawa17.6[4]
Calculate tapping RPM for this size →

Forming (roll) tap drill sizes differ by maker and product line; see the Japanese page for the table.

⌇Basic dimensions

Nominal (major) diameter d20
Pitch P2.5
Pitch diameter d2, D218.376
Basic minor diameter D1 (nut), d1 (bolt)17.294
Fundamental triangle height H = 0.866025P2.165
Basic engagement H1 = 0.541266P1.353

Basic dimensions are theoretical values without tolerance. Try other pitches with the thread dimension calculator.

⬡Bolt and nut sizes

ItemmmConfidence
Hex bolt width across flats s (ISO-based JIS, main body)30Agrees in 2+ sources[9][10]
Hex bolt width across flats s (old JIS, annex)301 source (reference)[11]
Hex nut width across flats s (ISO-based JIS, main body)30Agrees in 2+ sources[9][12]
Hex nut width across flats s (old JIS, annex)301 source (reference)[13]
Hex nut height m (style 1, max.)181 source (reference)[12]
Socket head cap screw head diameter dk (max.)30Agrees in 2+ sources[14][15][16][17][18]
Socket head cap screw head height k (max.)20Agrees in 2+ sources[14][16][17][18]
Hex socket size s (hex key size)17Agrees in 2+ sources[14][15][16][17][18][9]
Hex socket depth t (min.)10Agrees in 2+ sources[14][15][16][17][18]

⊙Clearance holes and counterbores

ItemmmConfidence
Clearance hole d′ for socket head cap screws22Agrees in 2+ sources[14][20]
Counterbore diameter D′ for socket head cap screws32Agrees in 2+ sources[14][20]
Counterbore depth H″ (head sits below the surface)21.5Agrees in 2+ sources[14][20]
Counterbore depth H′ (head slightly proud)18.5Agrees in 2+ sources[14][20]
Clearance hole, class 1 (JIS B 1001)211 source (reference)[19]
Clearance hole, class 2 (JIS B 1001)221 source (reference)[19]
Clearance hole, class 3 (JIS B 1001)241 source (reference)[19]
Spotface diameter for hex bolts and nuts (JIS B 1001)431 source (reference)[19]

Socket-head values are from the reference table of JIS B 1176; clearance-hole classes are from JIS B 1001. The two standards give different clearance holes for some sizes.

≋M20 fine threads

SizeD1 range (6H)Recommended drill (cutting tap)
M20×217.835 – 18.21OSG 18 / Nachi (Nachi-Fujikoshi) 18 / Yamawa 18.1
M20×1.518.376 – 18.676OSG 18.5 / Nachi (Nachi-Fujikoshi) 18.5 / Yamawa 18.6

📚Sources

Most sources are Japanese technical documents from the tap and fastener makers.

  1. iwata-fa.jp https://www.iwata-fa.jp/contents/assets/pdf/technicaldata/2022_tec_other_13.pdf
  2. nbk1560.com https://www.nbk1560.com/images/ja-JP/product/technical_data/meternezi_NBK/meternezi_NBK_1.…
  3. osg.co.jp https://www.osg.co.jp/media_dl/technical/file/drill_hole_size.pdf
  4. yamawa.com https://www.yamawa.com/Portals/0/images/download/holesize/jp/technical-contents-15.pdf
  5. nachi-fujikoshi.co.jp https://www.nachi-fujikoshi.co.jp/dcms_media/other/2501-12.pdf
  6. yamawa.com https://www.yamawa.com/Portals/0/images/download/holesize/jp/technical-contents-16.pdf
  7. baer.tools https://baer.tools/en/blog/core-hole-dimensions-for-screw-taps-everything-you-need-to-know
  8. orist.jp https://www.orist.jp/technicalsheet/98015.PDF
  9. eg.aktio.co.jp https://www.eg.aktio.co.jp/wp-content/uploads/R3T_1-5_TONE_bolt_nimen.pdf
  10. jp.misumi-ec.com https://jp.misumi-ec.com/pdf/fa/2015/p1_2367.pdf
  11. tsurugacorp.co.jp https://www.tsurugacorp.co.jp/img/usr/pdf/B00000.pdf
  12. fabory.com https://www.fabory.com/en-GB/p/01103200001
  13. static.nbk1560.com https://static.nbk1560.com/images/ja-JP/product/technical_data/rokkaku_nut_NBK/rokkaku_nut…
  14. jp.misumi-ec.com https://jp.misumi-ec.com/pdf/fa/2015/p1_2363.pdf
  15. tsurugacorp.co.jp https://www.tsurugacorp.co.jp/img/usr/pdf/A00000.pdf
  16. static.nbk1560.com https://static.nbk1560.com/images/ja-JP/product/technical_data/rokkaku_bolt_NBK/rokkaku_bo…
  17. mikipulley.co.jp https://www.mikipulley.co.jp/EN/Services/Tech_data/tech10.html
  18. farnell.com https://www.farnell.com/datasheets/11699.pdf
  19. static.nbk1560.com https://static.nbk1560.com/images/ja-JP/product/technical_data/boltana_zagurikei_NBK/bolta…
  20. nbk1560.com https://www.nbk1560.com/resources/other/article/technical-22-dimensions-of-holes-for-hexag…