What Are The Standards For Bolts In The Aerospace Industry?

Aerospace bolts are mainly used to transfer shear and tensile loads and widely applied in the places which bear large concentrated loads or parts that need to be disassembled on the airplane. What are the standards for bolts used in the aerospace industry? Firstly, we introduce the standards of bolt threads.

What Are Standards Of Bolt Threads In The Aerospace Industry?


American National Coarse

American National Fine

American Standard United Coarse

American Standard United Fine

Differences of these standards

1. The threads of coarse series and fine series with the same size have the same nominal thread diameter, but the number of threads per inch is different

2. Tolerance distribution is different
There are different specifications for threads with a diameter of 1 inch:

NF regulation: 14 teeth per inch, represented by 1-14NF

UNF regulation:12 teeth per inch, represented by 1-l2UNF

2. Fit Quality

The grade of thread indicates the fit tightness of the thread pair. It has five grades.
Grade 1

Loose fit
The nut can be easily turned with fingers.
Grade 2 
Free fit
Specially used for aerospace screw
Grade 3 
medium fit
mostly used for aerospace bolts.
Grade of 4-5
Tight fit
A wrench is required to tighten the nut.

The higher the grade number is, the tighter the thread pair fits.

3. Identification symbol

Identification: diameter, length, material, etc
The measurement unit of diameter is 1/16in, and the measurement unit of bolt rod length is 1/8in.  The manufacturing materials of bolts are indicated by English letters or symbols:

“C” means stainless steel;

“DD” means 2024 aluminum alloy;

“-” refers to nickel alloy steel.

General Standard Bolt In The Aerospace Industry

Hexagonal head bolt

The hexagonal head bolt is one of the general standard aviation bolts, which widely used in aircraft structure assembly and can withstand tensile and shear loads. The bolt is manufactured according to the AN model specification standard and the bolt thread is fine thread.

Material: chrome plated nickel steel, stainless steel and 2024 aluminum alloy

Hexagonal head bolt

Model specification code

Examples of AN series bolt

AN 3 DD 14A

Standard bolt of USAF Navy specification

Bolt diameter is 3/16in

Indicates that the bolt is made of 2024 aluminum alloy

Indicates that the length of bolt rod is 1.5in
Refers to that there is no safety hole at the screw tail

AN 6 H 34 A 

Standard bolt of USAF Navy specification


Bolt diameter is 6/16in

Indicates that the bolt is used to fix the propeller blade on the flange

Means the length of bolt rod is 3.5in

Indicates that there is no safety hole at the screw tail

Engine Bolts Drilled In The Head

Model specification code 
AN73 to AN81 MS20073 MS20074

Its head is slightly thick and has a through hole of fuse. It has the same tensile and shear strength as the ordinary bolt.

The threads of aviation engine bolts are:
Fine thread (without letter “A”)
Coarse thread (the letter “A” is added on the number of thread diameter)

Hexagon Socket Head Bolt

Hexagon Socket Head Bolt

Model specification code
MS20004 ~ MS20024
The last two digits are 04, 05, 06, 07, 08, 09, 10, 12, 14, 16, 18, 20, 22 and 24 respectively, which indicate that the bolt diameter is 4/16 inch to 24/16 inch.

Structure: the head of the bolt is cylindrical and the hexagonal socket holes are made on bolt heads

Material: It is made of high-strength steel. Its strength is higher than that of AN series hexagon bolts, so it can’t be interchanged with AN series hexagon bolts.

Application: Hexagon socket bolts are mainly used for the connection parts bearing tensile and shear combined stresses. The connection between the head and the screw rod is provided with a fillet to avoid sharp corner connection and improve the fatigue life.


1. When installing on the steel parts, the installation hole shall be scored to make the fillet sink into the hole.

2. When installing on aluminum alloy parts, MS20002C washer should be installed under its head.

Slotted Cheese Head bolt

Slotted Cheese Head bolt

Model specification code
The second digits “1”, “2” and “3” in AN2l, AN22, and AN23 represent the threads of “6-40”, “8-36” and “10-32” respectively. The screw diameter from AN24 to AN36 is in 1/16 inch as a unit.

The slotted cheese head bolt has a slotted or cross recessed round head.

Fine thread (NF) is adopted.

Application: It can only bear the shear load and is often used as the pivot pin of the hinge point in the rotating parts. The matched shear nut is closely matched with the bolt thread, and is secured with a cotter pin.


Through the introduction of standards for bolts in the aerospace industry, we can learn what kinds of bolts are suitable for the aviation industry, which are important knowledge.

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