A lot of assembly problems start with a simple mistake: drilling the bolt hole too tight (or too loose).
This post answers: How big should a bolt clearance hole be? and explains when to use close / normal / large (loose) clearance.
Quick summary:
- Don’t drill “exactly M” for clearance holes (e.g. M10 → 10 mm)—clearance holes are usually larger.
- Normal clearance is the practical default for most shop assemblies.
- Use large clearance when alignment is difficult (field installation, multi-layer stacks, thick paint/coatings).
1) What is a bolt clearance hole?
A clearance hole is a hole that allows the bolt shank and threads to pass through so the joint can be tightened with a nut (or threads in another part).
Don’t confuse it with:
- A tap drill hole (for cutting internal threads)
- A reamed/fitted hole used for precise locating with dowel pins or fitted bolts
2) Close vs normal vs large clearance (plain meaning)
A practical interpretation:
- Close clearance: minimal gap → better alignment, harder assembly.
- Normal clearance: general-purpose balance.
- Large clearance: easiest assembly, useful for field installation and larger position variation (often needs washers that adequately cover the hole).
Many tables reference ISO 273 for clearance holes, but your final choice should reflect real conditions: hole position tolerance, coatings/paint, and installation constraints.
3) Quick reference table (mm)
A practical “shop table” for common metric bolt sizes:
| Bolt size | Close | Normal | Large |
|---|---|---|---|
| M4 | 4.3 | 4.5 | 5.0 |
| M5 | 5.3 | 5.5 | 6.0 |
| 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 | 14.0 |
| M16 | 17.0 | 17.5 | 18.0 |
| M20 | 21.0 | 22.0 | 24.0 |
Rule of thumb:
- Normal clearance is typically M + (0.5 to 2 mm) depending on size.
- Larger bolts usually need more clearance to compensate for position tolerances and ease of assembly.
4) How to choose in practice
Quick examples (most common sizes)
- M8: normal clearance is often around 9.0 mm; large clearance 10.0 mm.
- M10: normal clearance is often around 11.0 mm; drilling 10.0 mm commonly causes tight/no-fit issues.
- M12: normal clearance is often around 13.5 mm; large clearance 14.0 mm.
Choose normal clearance when:
- Typical shop assembly
- Reasonable hole-position control
- You want fewer “won’t fit” surprises
Choose close clearance when:
- You need better alignment and can control hole position well
- Small plates/brackets where placement matters
Note: If shear transfer and precise location are critical, the solution may involve design changes (fitted bolts, dowels, reaming), not just “tighter clearance”.
Choose large clearance when:
- Field installation (site work)
- Multi-layer stacks and difficult alignment
- Painted/thicker coated surfaces that reduce effective clearance
With large holes, use appropriate washers so you don’t concentrate load at the edge or risk the washer slipping into the hole.
5) Common mistakes (and fixes)
- Drilling exactly M (M10 → 10 mm)
- Often leads to tight/no-fit conditions.
- Ignoring hole position tolerance
- Two “correct” close holes can still fail assembly if centers are slightly off.
- Burrs and sharp edges
- Burrs steal clearance and cause interference.
- Paint/coatings
- Powder coat or thick coatings can noticeably reduce clearance.
- Mixing up clearance holes vs tap drill holes
- If you need internal threads, use a tap drill chart based on pitch—not this table.
6) A quick checklist before you commit to a hole size
- Is the bolt passing through + nut, or threading into the part?
- Shop assembly or field installation?
- Do you need alignment/location, or just clamping?
- Any thick paint or coating?
- Washer OD: does it properly cover the hole?
If you’re also confirming metric callouts like M6/M8/M10 and pitch P: How to Read Metric Bolt Sizes (M6/M8/M10) and Thread Pitch (P).
If you’re selecting bolt length and engagement for reliable clamping: Common Mistakes Choosing Bolt Length: Thread Engagement Formula.
Product category: Bolts.
