M33 tap drill size
The tap drill for M33×3.5 is 29.5 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% | 30.272 mm 1.1918" | 30.25 mm (60%) | 1-3/16" 1.1875" (62%) |
| 65% | 30.045 mm 1.1829" | 30 mm (66%) | 1-3/16" 1.1875" (62%) |
| 70% | 29.817 mm 1.1739" | 29.75 mm (71%) | 1-11/64" 1.1719" (71%) |
| 75% | 29.59 mm 1.1650" | 29.5 mm (77%) | 1-11/64" 1.1719" (71%) |
| 80% | 29.363 mm 1.1560" | 29.25 mm (82%) | 1-5/32" 1.1563" (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 30 mm drill gives 66%. For mild steel, brass and cast iron the 29.5 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 (49.5 mm), drill about 63.5 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 M33
| Fit | Drill |
|---|---|
| Close | 34 mm (1.339") |
| Medium (normal) | 36 mm (1.417") |
| Free (loose) | 38 mm (1.496") |
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.
M33 thread data
| Designation | M33×3.5 |
|---|---|
| Standard | ISO 68-1 basic profile, ISO 261/262 series |
| Pitch | 3.5 mm pitch (7.3 TPI equivalent) |
| Thread angle | 60° |
| Major diameter | 33 mm (1.299") |
| Pitch (effective) diameter | 30.727 mm (1.210") |
| Minor diameter, external | 28.706 mm (1.130") |
| Minor diameter, internal | 29.211 mm (1.150") |
| Thread depth (internal) | 1.894 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:
- 29.25 mm drill → about 82% thread
- 29.5 mm drill → about 77% thread
- 29.75 mm drill → about 71% thread
Threads close in size (not interchangeable – the pitch and/or angle differ):
- G1 (1" BSP) – 33.25 mm major, 11 TPI (2.309 mm pitch)
- M30 × 2 – 30 mm major, 2 mm pitch (12.7 TPI equivalent)
- M36 × 3 – 36 mm major, 3 mm pitch (8.5 TPI equivalent)
Questions
What size drill for a M33 tap?
The tap drill for M33×3.5 is 29.5 mm. That’s the standard chart size and gives about 77% of full thread.
What size clearance hole for M33?
For a M33 screw or bolt, drill 34 mm for a close fit, 36 mm for a medium (normal) fit or 38 mm for a free fit. These are the ISO 273 fine, medium and coarse sizes.
What is the pitch of M33?
M33×3.5 has a 3.5 mm pitch (7.3 TPI equivalent), a 60° thread angle and a 33 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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