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Basic Knowledge of Fasteners

An average of 500 fasteners are used per car in a car500types/50kg/4000pieces, according to the latest data, in the automotive industry 23%of maintenance issues are caused by loose fasteners,12%of new vehicles have incorrect fastener tightness.

Bolted connections are easy to assemble, easy to disassemble, and convenient for maintenance. They can increase preload to prevent loosening and will not cause material phase changes at the connection. The disadvantage is that it increases the number and weight of fastener parts and requires pre-tightening and anti-loosening in situations with high stress. Also, there is a risk of corrosion at the gaps of bolted connections, leading to connection failure.

The commonly used fasteners mainly include

  • Hexagon Head Bolts: Widely Used

  • Hexagon Flange Bolts: Increase contact and stress area, protecting the surface of the fastened component. This can reduce the size of the head.

  • Non-metallic Insert Hexagon Flange Lock Nuts (Nylon Lock Nuts)

  • Metal Lock Nuts: Thread deformation causing interference.

  • Self-tapping Screws: Often used for connecting thin metal sheets (steel plates, saw plates, etc.). During connection, threads are first formed in the base hole of the component to be connected, and then the self-tapping screw is screwed into the threaded base hole of the component. Since the thread surface of the self-tapping screw has a high hardness (≥HRC45), it can create internal threads in the threaded base hole of the connected component, forming a connection.

  • Internal Hexagon Cylinder Head Bolts: Used when space is limited, can also be used as countersunk bolts to maintain a flat surface.

  • Internal Hexagon Socket Set Screws: Aesthetic and anti-disassembly.

  • Grounding Bolts: Some thread deformation, function of scraping paint during tightening.

  • Conical Spring Washers

  • Flat Washers

  • Spring Washers

Marking of Fasteners

Hexagon Flange Bolt M6×14

M6 refers to the nominal diameter of the thread d (major diameter of the thread).

14 refers to the nominal length of the bolt L.


Hexagon Head Bolt M10×1.25×110

1.25 refers to the thread pitch. Fine thread must be indicated. If omitted, it represents coarse thread.

Performance Grades of Fasteners

Bolt performance grades are divided into more than 10 levels, including 3.6, 4.6, 4.8, 5.6, 6.8, 8.8, 9.8, 10.9, 12.9, etc. Among them, bolts of grade 8.8 and above are made of low-carbon alloy steel or medium-carbon steel and undergo heat treatment (quenching, tempering), collectively known as high-strength bolts, while the rest are known as ordinary bolts. The bolt performance grade label consists of two numerical parts, representing the nominal tensile strength value of the bolt material and the ratio of yield strength. The number before the decimal point represents 1/100 of the material's ultimate tensile strength, and the number after the decimal point represents 10 times the ratio of yield strength to ultimate tensile strength.

For example, for a performance grade 10.9 high-strength bolt:

  • The nominal tensile strength of the bolt material is 1000 MPa.

  • The yield ratio of the bolt material is 0.9.

  • The nominal yield strength of the bolt material is 900 MPa (1000 × 0.9 = 900 MPa).

The meaning of bolt performance grades is an internationally accepted standard. Bolts of the same performance grade, regardless of their material and origin, have the same performance. In design, only the performance grade needs to be chosen.

Nut performance grades are divided into 7 levels, from 4 to 12. The number roughly represents 1/100 of the minimum stress the nut can withstand.

The performance grades of bolts and nuts should be used in combination. For example, an 8.8 grade bolt should be paired with an 8 grade nut.

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916 Wode Blvd, Changqing District, Jinan, Shandong, China, 250300