Clearance holes, reamers & key sizes
The four tables that stop a drawing going back for revision: bolt clearance holes, reaming stock, parallel keyways and the edge conventions everyone assumes but nobody writes down.
Metric clearance holes (ISO 273)
All dimensions in millimetres.
| Bolt | Close | Medium | Free |
|---|---|---|---|
| M1.6 | 1.7 | 1.8 | 2.0 |
| M2 | 2.2 | 2.4 | 2.6 |
| M2.5 | 2.7 | 2.9 | 3.1 |
| M3 | 3.2 | 3.4 | 3.6 |
| M4 | 4.3 | 4.5 | 4.8 |
| M5 | 5.3 | 5.5 | 5.8 |
| M6 | 6.4 | 6.6 | 7.0 |
| M8 | 8.4 | 9.0 | 10.0 |
| M10 | 10.5 | 11.0 | 12.0 |
| M12 | 13.0 | 13.5 | 14.5 |
| M14 | 15.0 | 15.5 | 16.5 |
| M16 | 17.0 | 17.5 | 18.5 |
| M20 | 21.0 | 22.0 | 24.0 |
| M24 | 25.0 | 26.0 | 28.0 |
| M30 | 31.0 | 33.0 | 35.0 |
| M36 | 37.0 | 39.0 | 42.0 |
Reaming stock allowance
Material left on diameter for the reamer to remove. Drill undersize by this amount.
| Finished hole Ø | Allowance on diameter |
|---|---|
| up to 5 mm | 0.1 – 0.2 mm |
| 5 – 20 mm | 0.2 – 0.3 mm |
| 20 – 50 mm | 0.3 – 0.5 mm |
| over 50 mm | 0.5 – 0.8 mm |
Parallel keys (DIN 6885-1 / ISO 773)
Key cross-section and keyway depth by shaft diameter. Keyway depths are measured from the shaft or bore surface.
| Shaft Ø (mm) | Key b × h | Shaft depth t1 | Hub depth t2 |
|---|---|---|---|
| 6 – 8 | 2 × 2 | 1.2 | 1.0 |
| 8 – 10 | 3 × 3 | 1.8 | 1.4 |
| 10 – 12 | 4 × 4 | 2.5 | 1.8 |
| 12 – 17 | 5 × 5 | 3.0 | 2.3 |
| 17 – 22 | 6 × 6 | 3.5 | 2.8 |
| 22 – 30 | 8 × 7 | 4.0 | 3.3 |
| 30 – 38 | 10 × 8 | 5.0 | 3.3 |
| 38 – 44 | 12 × 8 | 5.0 | 3.3 |
| 44 – 50 | 14 × 9 | 5.5 | 3.8 |
| 50 – 58 | 16 × 10 | 6.0 | 4.3 |
| 58 – 65 | 18 × 11 | 7.0 | 4.4 |
| 65 – 75 | 20 × 12 | 7.5 | 4.9 |
| 75 – 85 | 22 × 14 | 9.0 | 5.4 |
| 85 – 95 | 25 × 14 | 9.0 | 5.4 |
| 95 – 110 | 28 × 16 | 10.0 | 6.4 |
Edge and chamfer conventions
| Feature | Standard value | Note |
|---|---|---|
| Countersink for M-screws | 90° included angle | ISO 10642 socket countersunk heads; 82° is the UNC/imperial convention. |
| General edge break | 0.3 × 45° to 0.5 × 45° | Use when the drawing says 'break sharp edges'. |
| Thread lead-in chamfer | 1 × pitch at 45° | Prevents cross-threading and burr pick-up. |
| Press-fit lead-in | 1.5 mm × 15° | Shallow angle protects the interference surface during insertion. |
Frequently asked
What size clearance hole for an M8 bolt?
ISO 273 gives 8.4 mm for a close fit, 9.0 mm for a medium (normal) fit and 10.0 mm for a free fit. Medium is the default on general engineering drawings; use close only when the bolt also locates the part.
How much material should be left for reaming?
About 0.1–0.2 mm on diameter under 5 mm, 0.2–0.3 mm from 5 to 20 mm, 0.3–0.5 mm from 20 to 50 mm, and 0.5–0.8 mm above 50 mm. Too little and the reamer rubs instead of cutting; too much and it chatters and drifts.
What key size fits a 30 mm shaft?
A 30 mm shaft sits in the DIN 6885 22–30 mm band, taking an 8 × 7 mm parallel key, with 4.0 mm keyway depth in the shaft and 3.3 mm in the hub.
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