How to Identify Bolts

Written by alibaster smith
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Bolts are used on automobiles to hold various components together. You'll encounter a variety of sizes and types of bolts when working on your vehicle. It is always important to use the correct type of bolt for the component you are trying to secure to the vehicle, because different bolts have different strength tolerances. An incorrect bolt could break prematurely, causing serious injury to you or someone else.

Skill level:
Moderately Easy

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Things you need

  • Socket set
  • Allen wrench set

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  1. 1

    Look at the shape and structure of the bolt. Various types of bolts are on the market. A standard bolt has a hex-shaped head and can have a long or short shaft. Standard bolts have a flat tip on the end of the shaft. A bolt shaft with a pointed end is a lag bolt or lag screw, which is used to fasten wood. Bolts that have a domed head on them are carriage bolts -- also used to fasten wood, but may be used to fasten metal. Socket bolts have a smooth cylindrical head with an indented hex head and require a special type of wrench, called an Allen wrench, to remove.

  2. 2

    Look at the number or markings printed on the head of a bolt. Many bolts use a number or markings printed on the head of the bolt to identify its grade, or strength. If markings are used, they will be one of three types. No markings indicate a grade 2 bolt. This type of fastener is suitable for fastening non-load bearing components. Three markings on the head of the bolt (starting in the middle and angling out to the outside of the head of the bolt) indicate a grade 5 bolt. These bolts are made from medium carbon steel and are tempered. They are used for more demanding applications where bolt strength is an issue. Grade 8 bolts are some of the strongest bolts used on automobiles. They will be signified with an "8" printed on the head, or with six radial lines on the head. These types of bolts are tempered, hardened, alloy steel bolts that are very tough and difficult to break. They are used primarily where load-bearing bolts are required -- like suspension components -- where bolt failure would be catastrophic and cause serious injury to the driver and passengers.

  3. 3

    Check the head size. The easiest way to check the head size, until you become more familiar with various bolt designs, is by trial and error. Fit a socket (for hex head) or Allen wrench (for inverted or depressed hex head) over (or into) the bolt to check the size of the bolt.

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