Aircraft Hardware – Threaded Fasteners Part Five (Nuts Part Two)

in Aircraft Materials Processes and Hardware

Sheet Spring Nuts

Sheet spring nuts, such as speed nuts, are used with standard and sheet metal self-tapping screws in nonstructural locations. They find various uses in supporting line clamps, conduit clamps, electrical equipment, access doors, and the like, and are available in several types. Speed nuts are made from spring steel and are arched prior to tightening. This arched spring lock prevents the screw from working loose. These nuts should be used only where originally used in the fabrication of the aircraft.

Internal and External Wrenching Nuts

Two commercial types of high strength internal or external wrenching nuts are available; they are the internal and external wrenching elastic stop nut and the Unbrako internal and external wrenching nut. Both are of the self-locking type, are heat treated, and are capable of carrying high strength bolt tension loads.

Identification and Coding

Part numbers designate the type of nut. The common types and their respective part numbers are: Plain, AN315 and AN335; castle AN310; plain check, AN316; light hex, AN340 and AN345; and castellated shear, AN320. The patented self-locking types are assigned part numbers ranging from MS20363 through MS20367. The Boots, the Flexloc, the fiber locknut, the elastic stop nut, and the self-locking nut belong to this group. Part number AN350 is assigned to the wing nut.

Letters and digits following the part number indicate such items as material, size, threads per inch, and whether the thread is right or left hand. The letter “B” following the part number indicates the nut material to be brass, a “D” indicates 2017-T aluminum alloy, a “DD” indicates 2024-T aluminum alloy, a “C” indicates stainless steel, and a dash in place of a letter indicates cadmium-plated carbon steel.

The digit (or two digits) following the dash or the material code letter is the dash number of the nut, and it indicates the size of the shank and threads per inch of the bolt on which the nut will fit. The dash number corresponds to the first figure appearing in the part number coding of general purpose bolts. A dash and the number 3, for example, indicates that the nut will fit an AN3 bolt (10-32); a dash and the number 4 means it will fit an AN4 bolt (1/4-28); a dash and the number 5, an AN5 bolt (5/16-24); and so on.

The code numbers for self-locking nuts end in three or four digit numbers. The last two digits refer to threads per inch, and the one or two preceding digits stand for the nut size in 16ths of an inch.

Some other common nuts and their code numbers are:

Code Number AN310D5R:

AN310 = aircraft castle nut

D = 2024-T aluminum alloy

5 = 5⁄16 inch diameter

R = right-hand thread (usually 24 threads per inch)


Code Number AN320-10:

AN320 = aircraft castellated shear nut, cadmiumplated carbon steel

10 = 5⁄8 inch diameter, 18 threads per inch (this nut is usually right-hand thread)


Code Number AN350B1032:

AN350 = aircraft wing nut

B = brass

10 = number 10 bolt

32 = threads per inch