The calculator
Runs in your browser. Inches or millimetres at the top. Type an AN3 to AN8 part number and the grip comes from the AN bolt data sheets; for any other bolt, enter the grip from its own data. The washer thickness is always yours.
How the rule works
AC 43.13-1B par. 7-37: the grip length of a fastener “is the thickness of the material the fastener is designed to hold when two or more parts are being assembled.” The FAA Airframe Handbook measures it on the bolt, “from the underhead bearing surface to the first thread.”
The AC allows a bolt of slightly greater grip, “provided washers are placed under the nut or bolthead”, and caps them: “The maximum combined height of washers that should be used is 1/8 inch.” That is enough to bridge up to the next standard grip size, and no more.
The calculator adds the parts, compares the bolt's grip, and works out the washers: the difference, and how many of your washers it takes. Past 1/8 inch combined, the answer is a shorter bolt.
Worked example
A 0.250 in fitting and a 0.125 in flange, a bolt with a 0.4375 in (7/16) grip and 0.063 in washers.
- Stack = 0.250 + 0.125 = 0.375 in.
- Grip − stack = 0.4375 − 0.375 = 0.0625 in.
- Washers: 0.0625 ÷ 0.063 → 1 washer, 0.063 in combined, inside 1/8 in.
One washer under the nut or head, then check at least one thread shows through the nut.
Limits — when NOT to use this
- Use the bolt's data. Grip lengths depend on the bolt standard and dash number, and the AC notes specifications have changed over the years. Measure the bolt before ordering when replacing original hardware.
- Blind bolts are sized with their own grip gauges, one per system, not interchangeable (FAA Airframe Handbook).
- Plate nuts. Where the proper grip is not available, the AC says to add shims under the plate, not washers.
- Approved data governs where the maintenance manual specifies washers, shims or a particular bolt.
Questions people actually ask
Can I stack three thin washers to make up grip?
Only while the combined height stays at or under 1/8 inch. Past that, AC 43.13-1B par. 7-37 points you to a bolt with a shorter grip.
How much thread should show through the nut?
At least one thread, for self-locking and plain nuts alike (AC 43.13-1B par. 7-37).
Sources
- FAA AC 43.13-1B CHG 1, Chapter 7, par. 7-37, Grip Length — grip as the thickness of material the fastener holds; washers under nut or head for slightly greater grip, 1/8 inch maximum combined; measure the bolt before ordering; shims under plate nuts; at least one thread protruding through the nut.
- FAA-H-8083-31, AMT Handbook — Airframe, Volume 1, Chapter 4 — grip length of a bolt as the distance from the underhead bearing surface to the first thread; blind bolt grip gauges.