M8 tap drill size
The tap drill for M8×1.25 is 6.8 mm. That’s the standard chart size and gives about 74% of full thread.
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Tap drill by thread percentage
75% is the usual workshop figure. Less thread is easier to tap; more adds little strength.
| Thread % | Ideal drill | Nearest metric drill | Nearest imperial drill |
|---|---|---|---|
| 60% | 7.026 mm 0.2766" | 7 mm (62%) | letter J 0.2770" (59%) |
| 65% | 6.945 mm 0.2734" | 6.9 mm (68%) | letter I 0.2720" (67%) |
| 70% | 6.863 mm 0.2702" | 6.9 mm (68%) | letter I 0.2720" (67%) |
| 75% | 6.782 mm 0.2670" | 6.8 mm (74%) | letter H 0.2660" (77%) |
| 80% | 6.701 mm 0.2638" | 6.7 mm (80%) | 17/64" 0.2656" (77%) |
About this size
ISO metric coarse is the default thread for general engineering in the UK and Europe. If a drawing just says “M10”, it means the coarse pitch.
- For stainless steel, duplex and other tough materials, go to about 65% thread: a 6.9 mm drill gives 68%. For mild steel, brass and cast iron the 6.8 mm is fine.
- For a blind hole with 1.5 × D thread engagement (12 mm), drill about 17 mm deep. That leaves room for the 3–4 tapered lead threads on a second or plug tap. In steel, 1 × D engagement is usually enough; in aluminium or cast iron allow 1.5–2 × D.
Clearance holes for M8
| Fit | Drill |
|---|---|
| Close | 8.4 mm (0.331") |
| Medium (normal) | 9 mm (0.354") |
| Free (loose) | 10 mm (0.394") |
Fine, medium and coarse series from ISO 273. Medium suits most jobs; use fine where parts must locate closely and coarse for quick assembly or coated bolts.
M8 thread data
| Designation | M8×1.25 |
|---|---|
| Standard | ISO 68-1 basic profile, ISO 261/262 series |
| Pitch | 1.25 mm pitch (20.3 TPI equivalent) |
| Thread angle | 60° |
| Major diameter | 8 mm (0.315") |
| Pitch (effective) diameter | 7.188 mm (0.283") |
| Minor diameter, external | 6.466 mm (0.255") |
| Minor diameter, internal | 6.647 mm (0.262") |
| Thread depth (internal) | 0.677 mm |
Basic sizes. Real screws are slightly under and real nuts slightly over these values because of tolerance classes.
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Nearest alternatives
If you don’t have the right drill, these give:
- 6.7 mm drill → about 80% thread
- 6.8 mm drill → about 74% thread
- 6.9 mm drill → about 68% thread
Other pitches of M8: M8 × 1.
Threads close in size (not interchangeable – the pitch and/or angle differ):
- M8 × 1 – 8 mm major, 1 mm pitch (25.4 TPI equivalent)
- 5/16" UNC – 7.94 mm major, 18 TPI (1.411 mm pitch)
- 5/16" UNF – 7.94 mm major, 24 TPI (1.058 mm pitch)
- 5/16" BSW – 7.94 mm major, 18 TPI (1.411 mm pitch)
- 5/16" BSF – 7.94 mm major, 22 TPI (1.155 mm pitch)
Questions
What size drill for a M8 tap?
The tap drill for M8×1.25 is 6.8 mm. That’s the standard chart size and gives about 74% of full thread.
What size clearance hole for M8?
For a M8 screw or bolt, drill 8.4 mm for a close fit, 9 mm for a medium (normal) fit or 10 mm for a free fit. These are the ISO 273 fine, medium and coarse sizes.
What is the pitch of M8?
M8×1.25 has a 1.25 mm pitch (20.3 TPI equivalent), a 60° thread angle and a 8 mm major diameter.
Sources: Chart drill: the common metric coarse tapping chart. Percent values worked out from the ISO 68-1 basic profile. Clearance holes: ISO 273 (checked 25/10/2026). Values are for reference only – check your tap maker’s chart. See the disclaimer.
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