AN Bolt Torque Values Chart: How to Read AC 43.13-1B's Torque Tables for Self-Locking Nuts

AC 43.13-1B has two torque tables, and they answer different questions. Table 7-1 gives the recommended installation torque in inch-pounds when the manufacturer supplies no specific value (for a 3/4-16 bolt with an MS20365/AN310 tension nut: 2300-2500 inch-pounds). Table 7-2 gives the minimum prevailing torque a reused self-locking nut must still develop before you can put it back on (for 3/4-16: 27 inch-pounds). If the nut can't reach that figure, it goes in the scrap bin.

This guide walks through both tables as they appear in AC 43.13-1B, Chapter 7 (Section 3, Bolts, and Section 4, Nuts), explains the torque procedure in paragraph 7-40, and lists the hardware the AC says you must never reuse. All values below are copied from the AC; the Kopik base on AC 43.13 and 14 CFR Part 43 lets you query the full tables and the surrounding paragraphs.

First: where AC 43.13-1B sits in the hierarchy

Under 14 CFR 43.13(a), anyone performing maintenance must use the methods, techniques, and practices in the current manufacturer's maintenance manual or Instructions for Continued Airworthiness, "or other methods, techniques, and practices acceptable to the Administrator." The FAA AMT General Handbook (FAA-H-8083-30B, Chapter 2) explains that when neither manufacturer document gives the guidance needed, AC 43.13-1 or AC 43.13-2 contain examples of those acceptable methods, and that these ACs apply to non-pressurized areas of civil aircraft weighing 12,500 lb gross weight or less.

Two consequences for torque work. First, if the maintenance manual specifies a torque, that value wins: Table 7-1 itself says it is "the recommended torque to be used when specific torque is not supplied by the manufacturer." Second, AC 43.13-1B is acceptable data, not approved data, and it does not approve a major repair on its own. It is also no substitute for the manufacturer's manual.

How Table 7-1 (Recommended Torque Values) is organized

Table 7-1 is titled "Recommended Torque Values (Inch-Pounds)" and carries a caution: the values are derived from oil-free cadmium-plated threads. It is split into a fine thread series (8-36 up to 1-1/4-12) and a coarse thread series (8-32 up to 7/8-9), with four value columns:

• Torque limits recommended for installation (bolts loaded primarily in shear): one column for tension-type nuts MS20365 and AN310 (40,000 psi in bolts) and one for shear-type nuts MS20364 and AN320 (24,000 psi in bolts). These are ranges.
• Maximum allowable tightening torque limits: one column for nuts MS20365 and AN310 (90,000 psi in bolts) and one for nuts MS20364 and AN320 (54,000 psi in bolts). These are single ceiling values.
• A footnote states the values may be used for all cadmium-plated steel nuts of the fine or coarse thread series that have approximately equal number of threads and equal face bearing areas.
• Values marked with an asterisk (for 1-14, 1-1/8-12 and 1-1/4-12 shear nuts) are "estimated corresponding values."

Read the row by the thread size of the bolt shank, then pick the column for the nut type actually installed. A 1/4-28 bolt with an AN310 castle nut sits in the 50-70 in-lb range; the same bolt with an AN320 shear castle nut is 30-40 in-lb. Mixing up the nut column is one of the easiest ways to overtorque a shear nut.

Applying torque the AC 43.13-1B way (paragraph 7-40)

Paragraph 7-40 opens bluntly: "The importance of correct torque application cannot be overemphasized." Undertorque causes unnecessary wear of nuts, bolts and the parts they secure; overtorque can fail a bolt or nut by overstressing the threads. The procedure it lists:

1. Use the torque for the size of the bolt shank, not the wrench size.
2. Calibrate the torque wrench at least once a year, or immediately after it has been abused or dropped.
3. Make sure bolt and nut threads are clean and dry, unless the manufacturer specifies otherwise.
4. Run the nut down to near contact with the washer or bearing surface and check the friction drag torque needed to turn it. Whenever possible, torque the nut rather than the bolt.
5. Add the friction drag torque to the desired torque: the AC calls the sum the "final torque," which is what should register on the indicator (or the setting of a snap-over wrench).
6. Apply a smooth, even pull. If chattering or jerking occurs during final torque, back off the nut and retorque.
7. For a castle nut, start aligning with the cotter pin hole at the minimum recommended torque plus friction drag, and do not exceed the maximum plus friction drag. If the hole and castellation don't line up, change the washer or nut and try again.
8. If an adapter changes the effective length of the wrench, adjust the setting accordingly (Figure 7-2).

Table 7-2: minimum prevailing torque for reused self-locking nuts

Paragraph 7-64 covers self-locking nuts. They come as all-metal nuts (used primarily in high-temperature areas) or fiber/nylon insert nuts, which are not installed where temperatures exceed 250 ºF. The rule that matters at reassembly: "DO NOT reuse a fiber or nylon locknut if the nut cannot meet the minimum prevailing torque values (See Table 7-2)." Paragraph 7-122c repeats the idea for all self-locking nuts: repeated removal wears out the locking feature, and they should be replaced when no longer capable of maintaining the minimum prevailing torque.

Note that Table 7-2 starts at 7/16 inch. It gives no values for smaller sizes such as 1/4-28 or 3/8-24, so don't extrapolate from it: for those sizes, look to the nut or aircraft manufacturer's data.

Worked example: reusing a nylon locknut on a 3/4-16 bolt

1. Check that the location is suitable for a self-locking nut at all (see the next section).
2. Thread the used nut on and check its prevailing (locking) torque. Table 7-2 requires at least 27 inch-pounds for a 3/4-16 fine thread. Below that, discard the nut.
3. If it passes, run it down to near contact and note the friction drag torque (step 4 above).
4. Final torque = friction drag + the Table 7-1 value. With an MS20365/AN310 tension nut, the installation range is 2300-2500 in-lb, and the maximum allowable is 5000 in-lb. With an MS20364/AN320 shear nut, it is 1300-1500 in-lb (maximum 3000).
5. After tightening, make sure at least one thread of the bolt shows past the nut (paragraphs 7-37 and 7-64f).

The point of the example: 27 in-lb is a go/no-go test of the nut's locking feature, not a tightening value. Confusing the two columns of Chapter 7 is a classic error in student logbooks.

What you can't reuse, and where self-locking nuts don't belong

Chapter 7 is explicit on single-use hardware:

• Cotter pins: "Cotter pins should not be reused" (paragraph 7-103), and again in 7-127: "Cotter pins should not be reused on aircraft." Paragraph 7-38 adds: "Do not reuse cotter pins or safety wire."
• Safety wire: never reused, and it must not be overstressed: wire twisted too tightly will break under vibration (7-124e).
• Pal or speed nuts: "should never be reused and should be replaced with new ones when removed" (7-122d).
• Self-locking nuts: do not use them on parts subject to rotation; not with bolts on turbine airplanes where a loose part could be drawn into the engine intake; and not to attach access panels or doors or parts routinely disassembled before or after each flight (7-64).
• Lock washers (AN935/AN936): not on primary or secondary structures or accessories where failure might cause damage or danger, nor where frequent removal is required, in areas subject to corrosion, or in areas exposed to airflow (7-87).

Two more bolt rules from Section 3 often come up during inspections: steel bolts smaller than No. 10-32 and aluminum alloy bolts smaller than 1/4 inch diameter should not be used in primary structure (7-41), and the maximum combined height of washers used to compensate for grip length is 1/8 inch (7-37).

Quick checklist before you sign the logbook

• Torque value taken from the manufacturer's manual first; Table 7-1 only if none is given.
• Correct column for the nut type (tension AN310/MS20365 vs shear AN320/MS20364).
• Wrench calibrated within the last year; threads clean and dry unless otherwise specified.
• Friction drag measured and added to obtain final torque.
• Reused self-locking nut tested against Table 7-2 (7/16 inch and larger).
• At least one thread protruding; new cotter pin or new safety wire.

You can ask the base questions such as "What's the minimum prevailing torque for a reused self-locking nut on a 3/4-16 fine-thread bolt?" and get the answer with the table reference. The source document is the FAA's AC 43.13-1B.