Aircraft Cotter Pins and Clevis Pins: Selection and Installation

A cotter pin is a split pin with two prongs that passes through a drilled bolt, pin, or shaft and is bent to lock a nut or part in place. A clevis pin is a straight, headed pin with a drilled shank [3] that is held in place with a cotter pin [4]. The FAA's AC 43.13-1B notes that cotter pins are favored because they can be removed and installed quickly [2].

WHAT MONROE STOCKS

Cotter pins: MS24665 in cadmium-plated steel and stainless steel, plus legacy AN380 and AN381 pins. Assortments are sold as cotter pin kits.
Clevis pins: MS20392 and AN392 through AN406 series, along with clevis ends and clips such as MS21252 and MS21256. The clevis pins category also lists AN21 through AN29 clevis bolts.

COTTER PIN SPECS

AN380 - Cadmium-plated, low-carbon steel. Used for safetying bolts, screws, nuts, and other pins [4].
AN381 - Corrosion-resistant steel. Used where nonmagnetic material or corrosion resistance is needed [4].
MS24665 - The current number that replaces both AN380 and AN381 [5]. It's now maintained as NASM24665 [6].

Boeing Distribution publishes an MS24665 interchangeability chart that cross-references AN380 and AN381 dash numbers to MS24665 dash numbers by diameter and length [7]. Use a chart like this, or your parts catalog, rather than guessing at a conversion.

WHERE THEY'RE USED

The most common use is locking a castellated nut on a drilled bolt. The FAA handbook explains that castellated nuts are used with bolts drilled for cotter pins [4]. AC 43.13-1B says to lock or safety all bolts and nuts except self-locking nuts [1], and notes that self-locking castellated nuts used with cotter pins may be used in any system [1].

When aligning a castellated nut with the cotter pin hole, AC 43.13-1B says to start at the minimum torque plus friction drag and tighten to align, without exceeding maximum torque. If the hole won't line up within the torque range, change the washer or nut rather than over- or under-torquing [1]. For castle nut styles and torque, see how castellated nuts are torqued and safetied in our aircraft nuts guide. They also secure pins, screws, and shafts [2]. Shop castellated nuts.

SIZING, INSTALLING, AND REPLACING

Size: AC 43.13-1B says to use the largest pin that will fit the cotter pin hole and/or the nut slots [2]. The FAA handbook adds that the pin should fit neatly into the hole with very little sideplay [4].

Installation: These rules come from AC 43.13-1B paragraph 7-127 [2] and the FAA handbook [4]:

1. Insert the largest pin that fits, with little sideplay [2][4].
2. Bend the prong over the bolt end so it seats firmly against the bolt shank. It must not extend beyond the bolt diameter. Trim it if needed [2][4].
3. Bend the other prong down, flat against the nut. It must not touch the washer surface. Trim it if needed [2][4].
4. If you use the optional wraparound method, the prongs must not stick out from the sides of the nut [4].
5. Bend prongs over a reasonable radius. Sharp bends invite breakage. Tapping lightly with a mallet is the best way to bend them [4].
6. To prevent injury, the ends can be rolled and tucked [2].

Replacement: AC 43.13-1B states that cotter pins should not be reused on aircraft [2], and says not to reuse cotter pins or safety wire [1]. Always install a new one.

CLEVIS PIN SPECS

The FAA lists MS20392 as a straight, headed pin with a drilled shank [3]. MS20392 clevis pins supersede the AN392 through AN396 series and the AN400, AN402, AN404, and AN406 series [8]. Univair, for example, lists MS20392-1C19 as superseding AN392-19 [9].

USE AND INSTALLATION

The FAA handbook describes the flathead (clevis) pin, MS20392, as used with tie rod terminals and in secondary controls that aren't subject to continuous operation [4].

- Install the pin head up. The FAA explains that if the cotter pin fails or works out, a head-up pin will stay in place [4].
- Secure it with a new cotter pin through the drilled shank, following the rules above [2][4].

CLEVIS PINS VS. CLEVIS BOLTS

They look alike but aren't interchangeable in function. AC 43.13-1B says clevis bolts are used only for shear loads [1]. For head styles and uses, see our guide to clevis bolts and other special purpose bolts. Clevis bolts sit in the misc bolts category.

OTHER WAYS TO SAFETY HARDWARE

The FAA handbook lists safety wire, cotter pins, lockwashers, snap rings, and special nuts such as self-locking nuts, pal nuts, and jam nuts as the most widely used safetying methods [4]. Snap rings can be reused as long as they keep their shape and spring action [4]. Shop spring and retaining rings.

FAQ

Q: What replaced AN380 and AN381 cotter pins?
A: MS24665 replaced both. AN380 was cadmium-plated steel and AN381 was corrosion-resistant steel [5].

Q: Can I reuse a cotter pin?
A: No. AC 43.13-1B says cotter pins should not be reused on aircraft [2].

Q: What size cotter pin should I use?
A: The largest one that fits the hole and nut slots, with very little sideplay [2][4].

Q: Which way should a clevis pin face?
A: Head up, so the pin stays in place if the cotter pin fails [4].

Q: Does MS20392 replace AN392?
A: Yes. MS20392 supersedes the AN392 through AN396 series and AN400, AN402, AN404, and AN406 [8].

Shop: Cotter Pins | Cotter Pin Kits | Clevis Pins | Castellated Nuts | Spring & Retaining Rings

Related guides: Aircraft Nuts | Special Purpose Bolts | Aircraft Bolts

This guide is part of the Monroe Aerospace Guides library. Browse the full library, or contact us to cross-reference an AN pin number.