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Cotter Pin for AircraftInstalling Cotter Pin

Aircraft Nuts & Washers

Washers are used as a bearing surface between the bolt head and the part you install it in. Without the washer the head can crush the material damaging and possibly creating a structural failure after some time in service. This can cause accidents.

Depending on the application, engine compartment or airframe, you will need some form of locking that can resist heat and vibration without failing prematurely. Mechanical engineers have developed special nuts for that purpose.

So we continue with cotter pins and nuts. Castellated-, plain-, check- and self locking fibre nuts are discussed here. With some info about washers. Not your ordinary laundry washer but the washer type that goes with the nuts... Capice? Anyway, enjoy reading this 😉.

Cotter pins, Nuts & washers

The cotter pin used to lock this nut is a low carbon steel cadmium plated AN380 or the corrosion resistant steel AN381. Diameter is in 1/32 inch steps, for example: AN380-4 is 4/32" inch diameter or 1/8" inch. The length is noted in the second dash number (in 1/4 steps): AN381-2-4 is a corrosion resistant steel cotter pin of 1/16" inch diameter and 1" inch long. There is a special technique to install a cotter pin.

AN310 Castellated nut

The AN310 (AN320 is the small version) castle nut is designed to be used in conjunction with a cotter pin for safetying purposes. The nut may be used in tension and on assemblies that turn on the bolt or vice versa (think of a CM-5 bearing connecting to a flap or aileron). This nut is used primarily with the AN3 to AN20 series drilled shank (no A designation) bolt.

The nut is made of cadmium plated steel (no letter designation), aluminum alloy 2024-T4 (D designation) or corrosion resistant steel (C designation). These nuts all have the National Fine Thread (Class 3NF) and the dash number indicates the diameter of the bolt they fit on, in 1/16ths of an inch.

This is how a AN310 Castellated nut looks like (basically), the image on the right shows various ways of locking nuts (click for a larger version):

AN310 Castellated nutAN310 Castellated nut

More variants of the castellated nutsCastle Nut and more nutsOther Aircraft Nuts.

AN315 Plain nut

AN315 Check nut

The AN315 nut may be safetied through the use of a shake proof or lock washer - may be used in conjunction with the AN3 to AN16 series bolt. The nut may be had in right (R) or left (L) hand thread.

AN315 AN316 Check nut

AN316 Thin Check nut

This nut is used as a check nut to eliminate travel of other nuts or parts on the threads of a bolt or rod. This nut must not be used in tension or shear. The AN316 nut may be had in either right (R) or left (L) hand thread.

AN365 Self-locking fibre nut

AN365 / MS20365 Self-locking fibre nut

The fibre nut is used extensively throughout an aircraft. Because of the locking ability of the fibre in the nut cotter pins or lock washers are not needed. Do not use the same fibre nut over and over as the nylon wears with each tightening and does not give the same locking strength. If you come to a step that calls for repeated nut use on the same bolt use a plain nut until the final install (a couple of extra washers and the lock nut would do the same).

There is also a small version of this nut available, AN364 / MS20364.

Nut & Cotter pin sizes

Below you can see the sizes and numbers for some of the nuts and cotter pins.

AN NumberDiameterPlain Nut AN numberCastle Nut AN NumberCotter Pin MS Number

Washers, AN960 & AN970

Aircraft Grade Washer

The AN960 plain washer is furnished in the following materials: Steel (Cadmium plated), Aluminum alloy (D), Brass (B), and corrosion resistant steel (C). The code letter (none for steel) follows the basic number.

The inside diameter is indicated by the last dash number which is expressed in screw sizes as #6, #8 and #10, or; AN960-6, AN960-8, AN960-10. Washers requiring an I.D. or 3/8, 7/16 or 1/2 inch are called out as AN960-616, AN960-716, and AN960-816. The washer is made in a regular and light (L) series in all metals indicated except brass.

If the washer is of the light (L) series, the "L" will follow the dash number. Add the letter "P" before the dash number to obtain aluminum washers that have been anodized.
The AN-970 large area washer provides a greater bearing area than the plain type, and is used to prevent local crushing of the surface. These are available in -3 through -8, for bolt sizes no 10 (-3) to 1/2" (-8).

If you need a high strength steel washer then use a MS20002 washer. Available in -4 (4/16 or 1/4 inch) through -8 (8/16 or 1/2 inch) sizes for bolts 1/4 inch to 1/2 inch diameters.

Written by EAI.

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