3/8 clearance hole size
A 3/8 bolt needs a 0.406″ (10.3 mm) clearance hole for a standard normal fit — the through-hole the bolt passes freely into, not the tapped hole it threads on the far side.
Nominal 0.375″ · through-hole for the bolt shank · ASME B18.2.8 fine series / medium series / coarse series
3/8 clearance hole — all fits
| Fit | Ø inch | Ø mm |
|---|---|---|
| Close fine series | 0.391″ | 9.93 mm |
| Normal medium series | 0.406″ | 10.3 mm |
| Loose coarse series | 0.422″ | 10.7 mm |
Hole diameters per ASME B18.2.8 (fine series / medium series / coarse series). Value is the recommended drill; the standard specifies a small tolerance band above it.
Clearance hole vs. tap drill — don't mix them up. The clearance hole is the plain through-hole on the part the bolt passes through; the tap drill is the smaller hole you thread on the part the bolt screws into. Drilling a tap-drill-sized hole where you need clearance will bind the bolt.
How to pick close, normal or loose
All three holes let a 3/8 bolt pass; the difference is how much room is left around the shank. Normal fit (0.406″) is the default for almost all general assembly — enough slack to line parts up without hand-fitting. Choose close fit (0.391″) when location matters and you want the bolt to hold parts in register; choose loose fit (0.422″) when you need to absorb hole-to-hole misalignment across a bolt pattern or thermal movement.
Why the hole is bigger than the bolt
A bolt clamps by tension, not by its shank touching the hole wall, so the through-hole is always drilled oversize. The extra room accommodates thread run-out, paint, plating and the real-world scatter of drilled-hole position. Making the hole a tight sliding fit on the bolt is a common mistake — it fights assembly and does nothing for joint strength, which comes from preload. If you need a bolt to precisely locate parts, use a reamed hole with a shoulder bolt or dowel, not a tight clearance hole.
Threads that use this hole
Clearance depends only on the nominal 3/8 diameter, not the thread pitch, so every 3/8 thread shares it: 3/8-16, 3/8-24. For the tapped side and tightening spec, see those pages.
Tapping the other part instead? 3/8-16 tap drill size · 3/8-16 torque spec
References
- ASME B18.2.8 — Clearance holes for bolts, screws and studs (close / normal / loose fit).
- Machinery’s Handbook — clearance holes and fastener assembly practice.
Last reviewed: 2026-07 · Calculation method & assumptions