Aircraft Bolt Torque Specs: The Chart Every A&P Keeps at the Bench

Torque values for aircraft fasteners are not suggestions. Under-torqued joints lose clamp load under vibration and thermal cycling; over-torqued bolts yield the shank or strip the nut, destroying the clamping ability they're supposed to provide. The values here come directly from FAA AC 43.13-1B, Chapter 7, Table 7-1 — the authoritative reference for AN hardware in general aviation and experimental applications. Post this chart on the shop wall. Cross-reference it against your aircraft's maintenance manual when a specific torque is called out there, since the manual takes precedence for its own hardware.

Where These Numbers Come From

FAA AC 43.13-1B Table 7-1 provides torque ranges for AN standard hardware under clean, dry conditions. The values assume:

• Cadmium-plated alloy steel bolts (standard AN3–AN20 series)
• Plain steel or castle nuts (AN310, AN365, MS20365) in clean condition
• Torque applied to the nut, not the bolt head, unless the application specifically requires bolt-head torque
• No thread lubricant, sealer, or corrosion inhibitor — those require a lubricated-torque reduction

These are the dry torque values. If you're torquing into a fuel-wet environment or have applied any lubricant to the threads, see the lubricated adjustment section below before applying these numbers.

AN Bolt Torque Values — Dry, Cadmium-Plated Alloy Steel

Note that the AN7 minimum (450 in-lb) is higher than AN8 minimum (480 in-lb) — this is correct in AC 43.13-1B. The AN7 bolt has a different grip-to-thread geometry than AN8. Always verify against the current revision of the advisory circular for your specific application.

Lubricated Torque Adjustment

AC 43.13-1B specifies that the dry torque values must be reduced when any lubricant is present on the threads. The standard reduction is 20–30% from dry values. Use the lower end of the reduction (20%) for light lubricants like cadmium plating interaction; use the higher end (30%) for heavy grease, wet sealers, or anti-seize compounds.

• Fuel-resistant sealer on threads (PR-1422, Pro-Seal, etc.): reduce dry torque by 25–30%
• Corrosion inhibitor applied (MIL-PRF-16173 or equivalent): reduce by 20–25%
• Engine-temperature grease on exhaust hardware (Jet-Lube, nickel anti-seize): reduce by 25–30%
• Standard cadmium plate on standard AN nut: the dry values already account for cadmium-plate lubricity — no further reduction needed
• Never apply dry torque values to zinc-plated or chrome-plated bolts — the friction coefficient is different from cadmium, and the actual clamp load delivered will not match the design intent

Special Cases

Several hardware variants require adjusted torque handling beyond the standard table:

• CRES bolts (AN4C series, "C" suffix): Same torque range as alloy steel of the same nominal size. The CRES material has similar friction characteristics to cadmium-plated alloy steel at these torque levels.
• Aluminum bolts (AN4A, two raised dashes on head): Reduce torque by 25% from the alloy steel value. Aluminum bolts are used in non-structural or low-load applications; check the repair manual before using them structurally.
• NAS close-tolerance bolts with UNJF threads: The higher-precision thread form and higher tensile strength may be accompanied by a higher specified torque in the NAS drawing or the aircraft manufacturer's maintenance manual. Do not default to AC 43.13-1B values for NAS hardware without confirming no specific torque is called out.
• Self-locking nuts (AN365 / MS20365): The prevailing torque from the deformed fiber insert adds to the torque reading. Do not confuse break-loose torque (spin-off test) with installation torque. The installation torque values in Table 7-1 apply to self-locking nuts; the torque wrench reading includes prevailing torque resistance.

Torque Sequence and Technique

How you apply torque matters as much as the value. Thread galling, embedment, and elastic interaction between adjacent fasteners can all cause actual clamp load to differ significantly from what the torque wrench reads. Use a three-step approach:

1. Snug the bolt finger-tight. Verify the bolt is not cross-threaded, the washer is seated, and the mating surfaces are in full contact.
2. Apply torque in three equal increments (approximately 30%, 65%, and 100% of the final value). This allows the joint to seat and the bolt to develop consistent tension without shock loading.
3. Apply final torque. Do not exceed the maximum by backing off and re-applying — if you exceeded maximum, document it and evaluate whether to replace the fastener.

For castle nuts: torque to the low end of the specified range, then advance (never back off) to the nearest cotter pin hole that aligns with the bolt shank hole. If no hole aligns within the torque range, the grip length is wrong — change the bolt, not the torque. Backing off a castle nut to find a cotter pin slot removes the preload you need and creates a joint that will loosen in service.

Use a calibrated torque wrench for final torque on all structural fasteners. An open-end wrench or box wrench gives no indication of applied torque. Snap-action click wrenches are accurate in the middle of their range but lose accuracy at the extremes — for AN3 bolts (20–25 in-lb), use a beam-type wrench in the 0–100 in-lb range for best accuracy.

Torque Wrench Calibration

A torque wrench that hasn't been calibrated is a false confidence instrument. It feels like precision and may be nowhere near it. Calibration requirements:

• Torque wrenches must be calibrated at intervals per your shop's QA manual — typically annually or after any suspected overload event
• Store beam wrenches horizontally, click wrenches wound back to minimum setting — internal springs take a set if stored under tension
• Calibrate or replace after any wrench is dropped — the mechanism can be damaged with no visible indication
• For AN3 work (20–25 in-lb), a 0–100 in-lb range beam wrench is the appropriate tool. A 0–600 in-lb click wrench at 20 in-lb is operating at 3% of full scale — accuracy at that end of the range is poor regardless of calibration.
• Digital torque wrenches: more accurate at low values than click wrenches; require battery maintenance and annual calibration same as mechanical