#6-40 UNF 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
0.1700" (#18)
Close fit: 0.1540" (#23) · Loose (free) fit: 0.1850" (#13)
All three fits
| Fit class | Hole | When to use it |
|---|---|---|
| Close | 0.1540" (#23) | You need the fastener to help locate the part. Costs you assembly time and punishes any hole-position error. |
| Normal / medium | 0.1700" (#18) | General assembly. Absorbs ordinary drilling and hole-pattern tolerance without fighting you. |
| Loose / free | 0.1850" (#13) | 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.