日本語

Printed from Kezuriba (kezuriba.net/en/threads/m8/)

M8×1.25

ISO metric coarse thread

Nominal diameter 8 mm, pitch 1.25 mm. Tap makers recommend slightly different drill sizes, so they are listed side by side.

Tap drill, rule of thumb (d − P)
6.75 mmMaker recommendations below
Minor diameter D1 range (6H)
6.647 – 6.912 mmKeep the hole inside this range[4][5][3]
Pitch diameter d2 (basic)
7.188 mmd2 = d − 0.649519P
M8 basic profile and dimensions (schematic)
external thread (bolt)major dia. d = 8pitch dia. d2 = 7.188minor dia. D1 = 6.6476H range 6.647–6.912 (tap drill target)P = 1.25H160°

Thread angle 60°. H1 (basic thread engagement) = 0.541266P = 0.677. 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
OSG6.75min. 6.65, 6H max. 6.91
previous edition recommended 6.8 (changed in the current edition)		[3]
Nachi (Nachi-Fujikoshi)6.8high-performance drill 6.9[5]
Yamawa6.85[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 d8
Pitch P1.25
Pitch diameter d2, D27.188
Basic minor diameter D1 (nut), d1 (bolt)6.647
Fundamental triangle height H = 0.866025P1.083
Basic engagement H1 = 0.541266P0.677

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)13Agrees in 2+ sources[10][11][12]
Hex bolt width across flats s (old JIS, annex)131 source (reference)[13]
Hex nut width across flats s (ISO-based JIS, main body)13Agrees in 2+ sources[10][14][15]
Hex nut width across flats s (old JIS, annex)131 source (reference)[16]
Hex nut height m (style 1, max.)6.8Agrees in 2+ sources[14][15]
Socket head cap screw head diameter dk (max.)13Agrees in 2+ sources[17][18][19][20][21]
Socket head cap screw head height k (max.)8Agrees in 2+ sources[17][19][20][21]
Hex socket size s (hex key size)6Agrees in 2+ sources[17][19][20][21][10]
Hex socket depth t (min.)4Agrees in 2+ sources[17][19][20][21]

⊙Clearance holes and counterbores

ItemmmConfidence
Clearance hole d′ for socket head cap screws9Agrees in 2+ sources[17][23]
Counterbore diameter D′ for socket head cap screws14Agrees in 2+ sources[17][23]
Counterbore depth H″ (head sits below the surface)8.6Agrees in 2+ sources[17][23]
Counterbore depth H′ (head slightly proud)7.4Agrees in 2+ sources[17][23]
Clearance hole, class 1 (JIS B 1001)8.41 source (reference)[22]
Clearance hole, class 2 (JIS B 1001)91 source (reference)[22]
Clearance hole, class 3 (JIS B 1001)101 source (reference)[22]
Spotface diameter for hex bolts and nuts (JIS B 1001)201 source (reference)[22]

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.

≋M8 fine threads

SizeD1 range (6H)Recommended drill (cutting tap)
M8×16.917 – 7.153OSG 7 / Nachi (Nachi-Fujikoshi) 7 / Yamawa 7.09
M8×0.757.188 – 7.378OSG 7.25 / Nachi (Nachi-Fujikoshi) 7.3 / Yamawa 7.33

📚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. osg.co.jp https://www.osg.co.jp/media_dl/technical/file/p_8.pdf
  7. yamawa.com https://www.yamawa.com/Portals/0/images/download/holesize/jp/technical-contents-16.pdf
  8. baer.tools https://baer.tools/en/blog/core-hole-dimensions-for-screw-taps-everything-you-need-to-know
  9. orist.jp https://www.orist.jp/technicalsheet/98015.PDF
  10. eg.aktio.co.jp https://www.eg.aktio.co.jp/wp-content/uploads/R3T_1-5_TONE_bolt_nimen.pdf
  11. jp.misumi-ec.com https://jp.misumi-ec.com/pdf/fa/2015/p1_2367.pdf
  12. mikipulley.co.jp https://www.mikipulley.co.jp/en/resources/standards-hex-bolt
  13. tsurugacorp.co.jp https://www.tsurugacorp.co.jp/img/usr/pdf/B00000.pdf
  14. jp.misumi-ec.com https://jp.misumi-ec.com/pdf/fa/2015/p1_2369.pdf
  15. fabory.com https://www.fabory.com/en-GB/p/01103080001
  16. static.nbk1560.com https://static.nbk1560.com/images/ja-JP/product/technical_data/rokkaku_nut_NBK/rokkaku_nut…
  17. jp.misumi-ec.com https://jp.misumi-ec.com/pdf/fa/2015/p1_2363.pdf
  18. tsurugacorp.co.jp https://www.tsurugacorp.co.jp/img/usr/pdf/A00000.pdf
  19. static.nbk1560.com https://static.nbk1560.com/images/ja-JP/product/technical_data/rokkaku_bolt_NBK/rokkaku_bo…
  20. mikipulley.co.jp https://www.mikipulley.co.jp/EN/Services/Tech_data/tech10.html
  21. farnell.com https://www.farnell.com/datasheets/11699.pdf
  22. static.nbk1560.com https://static.nbk1560.com/images/ja-JP/product/technical_data/boltana_zagurikei_NBK/bolta…
  23. nbk1560.com https://www.nbk1560.com/resources/other/article/technical-22-dimensions-of-holes-for-hexag…