The AN bolt numbering system encodes diameter directly: the AN number is the bolt diameter in 16ths of an inch. AN3 = 3/16", AN4 = 1/4", AN5 = 5/16", AN6 = 3/8", AN8 = 1/2". Once you internalize that rule, the sizing system becomes intuitive. This reference covers every AN bolt size from AN3 through AN10 with thread specifications, typical grip lengths, hardware pairings, and application context for aircraft construction and repair.
Master AN Bolt Sizing Table
All AN bolts use UNF (Unified National Fine) thread form. Fine threads are standard for aircraft hardware — they provide more thread engagement per inch and resist vibration loosening better than coarse threads.
| AN # | Diameter | Thread Spec | Thread Length | Common Grip Range | Typical Applications |
|---|---|---|---|---|---|
| AN3 | 3/16" (0.1875") | 10-32 UNF-3A | 0.406" | 1/16" – 1‑1/2" | Control cable brackets, light attach fittings, instrument panel brackets |
| AN4 | 1/4" (0.250") | 1/4-28 UNF-3A | 0.469" | 1/16" – 2‑1/2" | Rib attach, flight controls, firewall attach, most general structural |
| AN5 | 5/16" (0.3125") | 5/16-24 UNF-3A | 0.531" | 1/16" – 3" | Spar attachments, engine mount brackets, main bulkhead attach |
| AN6 | 3/8" (0.375") | 3/8-24 UNF-3A | 0.641" | 1/16" – 3‑1/2" | Wing attach fittings, spar carry-through, heavy structural joints |
| AN7 | 7/16" (0.4375") | 7/16-20 UNF-3A | 0.656" | 1/16" – 4" | Heavy structural, tailwheel attach, high-load fittings |
| AN8 | 1/2" (0.500") | 1/2-20 UNF-3A | 0.680" | 1/16" – 4" | Engine mount, main landing gear, primary structural truss joints |
| AN10 | 5/8" (0.625") | 5/8-18 UNF-3A | ~0.750" | 1/16" – 5" | Heavy structural, gear box attach, large-diameter pivot points |
AN9 (9/16") exists in the standard but is rarely specified in light aircraft construction. AN3 and AN4 account for the majority of bolts in a typical homebuilt airframe by count. AN5 and AN6 carry the highest structural loads in wing attach and spar carry-through. AN8 and larger appear in engine and landing gear applications.
Understanding the Tolerance Range
AN bolts are standard-tolerance, not close-tolerance like NAS hardware. Understanding the actual shank dimensions matters for hole sizing and clearance fit verification.
| AN # | Nominal Diameter | Shank Tolerance | Clearance Drill | Close-Fit Drill |
|---|---|---|---|---|
| AN3 | 0.1875" | 0.186" – 0.189" | #12 (0.189") | #11 (0.191") |
| AN4 | 0.250" | 0.246" – 0.249" | Letter F (0.257") | Letter E (0.250") |
| AN5 | 0.3125" | 0.309" – 0.312" | Letter P (0.323") | Letter O (0.316") |
| AN6 | 0.375" | 0.371" – 0.374" | 3/8" (0.375") | Letter W (0.386" close) |
| AN8 | 0.500" | 0.496" – 0.499" | 33/64" (0.516") | 1/2" reamer |
For standard (clearance) fit, the hole should be 1/64" to 1/32" larger than the bolt shank. For a structural joint relying on bearing load transfer, use the close-fit drill and verify fit. Never force a bolt through a tight hole — the threads will pick up galling and the shank geometry will be compromised.
The "A" Suffix Rule
AN bolt part numbers include a grip length code, and some include an "A" suffix. This suffix is commonly misunderstood.
AN4-7 vs AN4-7A: Both are AN4 (1/4") bolts with a 7/16" grip length. The difference is the shank below the threads:
- AN4-7 (no A suffix): The unthreaded shank has a hole drilled through for a cotter pin, used with castle nuts (AN310 or AN320). The cross-hole engages the castle nut slots.
- AN4-7A (A suffix): The shank is undrilled — smooth with no cross-hole. Used with self-locking nuts (AN365, MS21044) that do not require a cotter pin.
Always follow the plans. If the drawing calls AN4-7A, use AN4-7A with a self-locking nut. If it calls AN4-7, use AN4-7 with a castle nut and cotter pin. Substituting A for non-A (or vice versa) affects the locking mechanism. See our castle nut and cotter pin pairing guide for the full torque-and-back-off procedure.
What Hardware Goes With Each Size
Each AN bolt size has a corresponding family of washers, lock nuts, castle nuts, and shear nuts. Using mismatched hardware degrades the joint — an AN365-1032 nut on an AN4 bolt will strip immediately.
| AN # | Flat Washer | Lock Nut (AN365) | Castle Nut (AN310) | Shear Nut (AN320) |
|---|---|---|---|---|
| AN3 | AN960-10 / AN960-10L | AN365-1032A | AN310-3 | AN320-3 |
| AN4 | AN960-416 / AN960-416L | AN365-428A | AN310-4 | AN320-4 |
| AN5 | AN960-516 / AN960-516L | AN365-524A | AN310-5 | AN320-5 |
| AN6 | AN960-616 / AN960-616L | AN365-624A | AN310-6 | AN320-6 |
| AN7 | AN960-716 / AN960-716L | AN365-720A | AN310-7 | AN320-7 |
| AN8 | AN960-816 / AN960-816L | AN365-820A | AN310-8 | AN320-8 |
| AN10 | AN960-1016 / AN960-1016L | AN365-1018A | AN310-10 | AN320-10 |
The "L" suffix on washers (e.g., AN960-416L) denotes a thin washer — same OD and ID, reduced thickness. Thin washers are used when stack height is tight or when the grip length calculation requires fine adjustment. The grip length of the bolt must be chosen so that the bolt threads engage the nut but the unthreaded shank occupies the structural joint — not thread in bearing. See our AN bolt grip length chart for the calculation procedure.
How Many Do You Actually Need for a Full Build?
This is the question every first-time EAB builder has when staring at a hardware order form. The answer varies by aircraft design, but the RV-7 is the most-built homebuilt in the world and a reasonable benchmark.
A typical RV-7 build uses approximately 3,800 to 4,200 AN bolts across the airframe (not counting rivets, which are counted separately by the tens of thousands). The distribution by size:
- AN3 (~800 bolts): The smallest structural bolt. Control hardware, light attach points, instrument panel, avionics trays, fairings.
- AN4 (~1,200 bolts): The primary workhorse. Rib attach, flight control connections, firewall attach, most general structural work. More AN4s than any other size.
- AN5 (~400 bolts): Spar area, wing root ribs, main carry-through, engine mount structure.
- AN6 and larger (~150 bolts): Wing attach fittings, spar pins, main landing gear, engine mount primary joints.
Buying in bulk quantities reduces per-unit cost significantly on new surplus hardware. If your plans call for 1,200 AN4 bolts in six different grip lengths, ordering by the 100-pack per grip length is more efficient than ordering individually. Our catalog carries AN3 through AN8 in the most common grip lengths as in-stock items.