Many bolts that were once used on domestic cars and trucks maybe measured in inches, and standards for these bolts were established by the Society of Automotive Engineers (SAE). Special markings on the head of the bolt indicate its tensile strength (resistance to breaking). The SAE grade number, corresponding to the special markings, is an indication of the relative strength of the bolt. Grade 0 bolts (no markings) are usually made of a mild steel and are much weaker than a grade 8, usually made from a mild carbon steel alloy, though a grade 0 or 2 bolt is sufficient for most fasteners.
SAE fasteners are also identified by size. As an example, a 3⁄8 -24 bolt means that the major (greatest) thread diameter is 3⁄8 inch and that there are 24 threads per inch. The head diameter is always 3⁄16 inch larger than the bolt diameter. A 1⁄2 16 bolt would be 1⁄2 inch in diameter and have 16 threads per inch. More threads per inch are called "fine" threads and less threads per inch are "coarse" threads. Generally, the larger the bolt diameter, the coarser the threads. There are actually six different classes of threads, but most bolts are Unified National Coarse (UNC) or Unified National Fine (UNF). The term "Unified" refers to a thread pattern to which US, British and Canadian machine screw threads conform.
| Fig. 5: Fasteners commonly found on automobiles
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| Fig. 6: SAE bolt head markings indicate their relative
strength
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| Fig. 7: SAE standard torque specification chart. Torque
values are based on clean, dry threads. Use this chart only as a guide,
check your vehicle service manual for specific torque values. NOTE:
The torque value required for aluminum components is considerably less.
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