The calculator
Runs in your browser. Inches or millimetres at the top.
The rule it applies
AC 43.13-1B paragraph 7-39: “Generally, it is permissible to use the first-lettered drill size larger than the nominal bolt diameter, except when the AN hexagon bolts are used in light-drive fit (reamed) applications and where NAS close-tolerance bolts or AN clevis bolts are used. A light-drive fit can be defined as an interference of 0.0006 inch for a 5/8 inch bolt.”
Letter drills run from A (0.234 in) to Z (0.413 in), per the drill chart in the FAA Airframe Handbook. Above a 1/4 in bolt the first is F, 0.257 in; above 5/16 in, O, 0.316 in; above 3/8 in, W, 0.386 in. A 3/16 in bolt has no letter drill near it, and bolts over 0.413 in have none at all, so there the hole comes from the manufacturer's data.
For elongated holes the same paragraph allows reaming or drilling for the next larger bolt, keeping edge distance, clearance and structural integrity, and asks you to consult the SRM, the manufacturer or the FAA first in a critical structural member.
Worked example
An AN5 bolt (5/16 in) in a general, non-reamed application.
- Nominal 0.3125 in. First letter drill above it: O, 0.316 in.
- Clearance 0.0035 in.
Drill O.
Limits — when NOT to use this
- Reamed fits, NAS close-tolerance and AN clevis bolts are excluded from the rule by the AC itself.
- The drawing governs wherever it gives a hole size and tolerance.
- Critical structure: consult the SRM or the manufacturer before drilling or reaming.
Questions people actually ask
What about AN3 (#10) bolts?
The first letter drill above 0.190 in is A, 0.234 in: far too big. The AC's rule does not serve that size; use the hole size in the manufacturer's data.
Sources
- FAA AC 43.13-1B CHG 1, Chapter 7, par. 7-39 Bolt fit — the first-lettered drill rule, its exceptions, the light-drive fit example, and next-size bolts for elongated holes.
- FAA-H-8083-31, AMT Handbook — Airframe, Volume 1, Figure 4-47 — letter drill sizes and decimal equivalents.