M42 tap drill size
The tap drill for M42×4.5 is 37.5 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% | 38.493 mm 1.5155" | 38.5 mm (60%) | 1-33/64" 1.5156" (60%) |
| 65% | 38.2 mm 1.5039" | 38.25 mm (64%) | 1-1/2" 1.5000" (67%) |
| 70% | 37.908 mm 1.4924" | 38 mm (68%) | 1-1/2" 1.5000" (67%) |
| 75% | 37.616 mm 1.4809" | 37.5 mm (77%) | 1-31/64" 1.4844" (74%) |
| 80% | 37.323 mm 1.4694" | 37.25 mm (81%) | 1-15/32" 1.4688" (80%) |
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 38.25 mm drill gives 64%. For mild steel, brass and cast iron the 37.5 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 (63 mm), drill about 81 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 M42
| Fit | Drill |
|---|---|
| Close | 43 mm (1.693") |
| Medium (normal) | 45 mm (1.772") |
| Free (loose) | 48 mm (1.890") |
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.
M42 thread data
| Designation | M42×4.5 |
|---|---|
| Standard | ISO 68-1 basic profile, ISO 261/262 series |
| Pitch | 4.5 mm pitch (5.6 TPI equivalent) |
| Thread angle | 60° |
| Major diameter | 42 mm (1.654") |
| Pitch (effective) diameter | 39.077 mm (1.538") |
| Minor diameter, external | 36.479 mm (1.436") |
| Minor diameter, internal | 37.129 mm (1.462") |
| Thread depth (internal) | 2.436 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:
- 37.25 mm drill → about 81% thread
- 37.5 mm drill → about 77% thread
- 37.75 mm drill → about 73% thread
Questions
What size drill for a M42 tap?
The tap drill for M42×4.5 is 37.5 mm. That’s the standard chart size and gives about 77% of full thread.
What size clearance hole for M42?
For a M42 screw or bolt, drill 43 mm for a close fit, 45 mm for a medium (normal) fit or 48 mm for a free fit. These are the ISO 273 fine, medium and coarse sizes.
What is the pitch of M42?
M42×4.5 has a 4.5 mm pitch (5.6 TPI equivalent), a 60° thread angle and a 42 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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