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3/8-16 UNC Clearance Hole Size
Per ASME B18.2.8. The hole in the part the bolt passes through — not the tapped hole.
Normal (medium) fit — use this by default
0.4060" (13/32)
Close fit: 0.3910" (25/64) · Loose (free) fit: 0.4220" (27/64)
All three fits
| Fit class | Hole | When to use it |
|---|---|---|
| Close | 0.3910" (25/64) | You need the fastener to help locate the part. Costs you assembly time and punishes any hole-position error. |
| Normal / medium | 0.4060" (13/32) | General assembly. Absorbs ordinary drilling and hole-pattern tolerance without fighting you. |
| Loose / free | 0.4220" (27/64) | Weldments, laser-cut or punched stacks, long bolt patterns, anything where positional error accumulates. |
A clearance hole is not a locating feature.
A bolt in a normal-fit hole can shift by the full clearance before the shank touches
anything. If the joint has to hold position under load, the friction from correct preload
does that job — or you add dowel pins. Relying on bolt shank contact means the joint
has already slipped.