Fasteners like bolts have been an essential part of mechanical engineering for thousands of years and have evolved to meet today’s high-performance demands. In fact, there are so many options to choose from, selecting the right bolt for your needs can be challenging.
HTEC has a huge inventory of bolts which includes:
Hex head cap screws
Square head bolts
You can tell a lot about a bolt just by looking at the head. To learn if a bolt is an American (English or Imperial) bolt or a metric bolt, look for markings on the bolt head. With American bolts, there should be radial markings to help you determine the grade, or strength class, of the bolt.
US Bolts (English/Imperial) Grade Identification
Look for the number of radial markings on the bolt head, then add 2. For example, a Grade 5 bolt has 3 radial markings on the head. A Grade 8 bolt has 6 radial markings, while a Grade 2 bolt will have no radial markings. Here are some examples.
Grade 2- Low or medium carbon steel
Grade 5- Medium carbon steel, quenched and tempered
Grade 8- Medium carbon alloy steel, quenched and tempered
Metric bolts, on the other hand, should have a class number on the head.
Metric Bolt Class Identification
Class 8.8- Medium carbon steel, quenched and tempered
Class 10.9- Alloy steel, quenched and tempered
Class 12.9- Alloy steel, quenched and tempered
COMMON CHALLENGES WHEN MEASURING BOLT LENGTH
Identifying bolt grades and classes is pretty straight forward. There is one particularly challenging aspect of bolt selection and that is measuring the bolt length. People often struggle with how to measure bolt length on various bolt heads. On a hex head, you should measure under the head to the end of the bolt. On a countersunk head or a tapered head, you should measure the overall length. Most mistakes are made when measuring countersunk bolts. Many mistakenly measure from under the head to the bolt end, not accounting for the countersink depth. This results in the bolt being too short.
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