JARGON BUSTER
2/5/8: Imperial grading of fastenings often found on US bolts, which is similar to the metric grading system.
4.6/8.8/10.9/12.9: A metric grading system for the tensile strength of a bolt. Often displayed on the top of the bolt’s head.The higher the number, the greater the bolt can be tightened. 4.6-grade is generally used for attaching light objects such as trim, whereas 8.8 and above are for structural and safety-critical components.
A2: Stainless steel nuts and bolts made from steel with around 18 per cent chromium and 8 per cent nickel to offer a high level of corrosion resistance.
A4: Stainless steel fasteners with the same chromium and nickel as A2, but with 3 per cent molybdenum, so considered better with greater resistance to corrosion, longer lasting and more expensive.
AF: Across flats. ¾ inch AF for instance helps to indicate the size of spanner required to undo a nut or bolt, whereas some fastenings are classed according to the diameter of their thread, which doesn’t help with tightening or undoing them.
BA: British Association. A metric thread system for small screws used in components including speedometers. It’s not the same as the metric method for most nuts and bolts. Whilst the system uses mm, the diameter of a thread is calculated by a factor proportional to a power of the logarithm to the base 10 of the thread pitch. Useful to know if it comes up in a car club quiz.
BSF: British Standard Fine. Used alongside Whitworth as a fine thread. Replaced by imperial.
Imperial: Used on most Fords until the mid ’70s.The diameter of a thread or the inside of a nut is measured in inches or fractions of inches.
Thread pitch: The distance between each thread along a bolt or inside a nut. For metric fastenings, a bolt that has a 1.0 pitch means the threads are 1 mm apart.
TPI: Threads per inch.The number of threads present along a bolt, screw or inside a nut in one inch of measurement. Commonly used on imperial fastenings. Metric fastenings measure their threads according to the distance between each one.
UNC: Unified national coarse. A thread type used for imperial fastenings.
UNEF: Unified national extra fine thread. UNF: Unified national fine. A thread type used for imperial fastenings, which is much finer than UNC, with more threads per inch. UNF is stronger than UNC, so it’s suitable for higher torque settings and less susceptible to being sheared. It has a smaller pitch so can provide finer adjustment and has less risk of slackening. Commonly fitted to suspension and seatbelt mountings UNS: Unified national special thread.
Whitworth: Also known as British Standard Whitworth (BSW). Used before imperial (unified). Measured in inches or fractions of inches.