Thread types

▶ Once you have determined the diameter of a thread of a nut or bolt, there’s one more consideration to look into – its thread. There are coarse and fine threads. A fine thread has the threads tightly packed together, or the official definition is there are more threads per inch (TPI). This type of fastening is stronger than a coarse thread, so it’s capable of withstanding higher torque settings and is less susceptible to being sheared. It’s commonly found on suspension and seatbelt mountings.

Coarse threads are generally more widely used because there are many more components on a vehicle that are not so safety-critical.

Metric fastenings are generally divided into metric fine and metric coarse categories, whereas American fastenings for instance, are split into two similar categories: UNC (Unified National Coarse), UNF (Unified National Fine) – there is also UNEF (Unified National Extra Fine) and UNS (Unified National Special), but these are rarely used on vehicles.

With a little practice, you can soon tell the difference between a fine and a coarse thread, but there’s one more point to consider – the pitch of the thread. This is the space between each thread, which can be measured with a thread gauge that costs around a fiver.

Metric fastenings, for instance, have three pitches for both coarse and fine threads. The pitch of coarse threads includes 1.25mm, 1.5mm and 1.75mm. The pitch of fine threads starts at 1mm, followed by 1.25mm and then 1.5mm. Consequently, there are three different pitches. UNC and UNF bolts have similar classifications for pitch.