M27 tap drill size
The tap drill for M27×3 is 24 mm. That’s the standard chart size and gives about 77% 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% | 24.662 mm 0.9709" | 24.75 mm (58%) | 31/32" 0.9688" (61%) |
| 65% | 24.467 mm 0.9633" | 24.5 mm (64%) | 31/32" 0.9688" (61%) |
| 70% | 24.272 mm 0.9556" | 24.25 mm (71%) | 61/64" 0.9531" (72%) |
| 75% | 24.077 mm 0.9479" | 24 mm (77%) | 61/64" 0.9531" (72%) |
| 80% | 23.882 mm 0.9402" | 24 mm (77%) | 15/16" 0.9375" (82%) |
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 24.5 mm drill gives 64%. For mild steel, brass and cast iron the 24 mm is fine.
- At this size many people bore the hole and screwcut the thread on the lathe instead of using a hand tap. See the thread calculator for depths and the infeed.
- For a blind hole with 1.5 × D thread engagement (40.5 mm), drill about 52.5 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 M27
| Fit | Drill |
|---|---|
| Close | 28 mm (1.102") |
| Medium (normal) | 30 mm (1.181") |
| Free (loose) | 32 mm (1.260") |
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.
M27 thread data
| Designation | M27×3 |
|---|---|
| Standard | ISO 68-1 basic profile, ISO 261/262 series |
| Pitch | 3 mm pitch (8.5 TPI equivalent) |
| Thread angle | 60° |
| Major diameter | 27 mm (1.063") |
| Pitch (effective) diameter | 25.051 mm (0.986") |
| Minor diameter, external | 23.319 mm (0.918") |
| Minor diameter, internal | 23.752 mm (0.935") |
| Thread depth (internal) | 1.624 mm |
Basic sizes. Real screws are slightly under and real nuts slightly over these values because of tolerance classes.
Quick calculator
Nearest alternatives
If you don’t have the right drill, these give:
- 23.75 mm drill → about 83% thread
- 24 mm drill → about 77% thread
- 24.25 mm drill → about 71% thread
Other pitches of M27: M27 × 2.
Threads close in size (not interchangeable – the pitch and/or angle differ):
- M27 × 2 – 27 mm major, 2 mm pitch (12.7 TPI equivalent)
- G3/4 (3/4" BSP) – 26.44 mm major, 14 TPI (1.814 mm pitch)
- 1" UNC – 25.4 mm major, 8 TPI (3.175 mm pitch)
- 1" UNF – 25.4 mm major, 12 TPI (2.117 mm pitch)
- 1" BSW – 25.4 mm major, 8 TPI (3.175 mm pitch)
Questions
What size drill for a M27 tap?
The tap drill for M27×3 is 24 mm. That’s the standard chart size and gives about 77% of full thread.
What size clearance hole for M27?
For a M27 screw or bolt, drill 28 mm for a close fit, 30 mm for a medium (normal) fit or 32 mm for a free fit. These are the ISO 273 fine, medium and coarse sizes.
What is the pitch of M27?
M27×3 has a 3 mm pitch (8.5 TPI equivalent), a 60° thread angle and a 27 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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