What Thread is it?

2011/02/22

By Alistair Plint

Have you ever looked at a bolt and having measured it with your vernier, and to all extents it measures to a size but the nut just won’t fit?

Well the truth is there are many different bolt threads and the threads are there for a reason, it is never a good idea to use the wrong bolt thread. The drawing room drew that thread for a reason.

So how do you know what thread it is?

First it’s a good idea to buy a good quality thread pitch guage.

In the tables below we give you the thread size acording to the nominal diameter and pitch of your bolt.

Standard Metric Course

A Standard metric course thread, (Metric) as it is commonly known is the most commonly used thread in South African applications. Metric Fine, however is most commonly used in our automotive applications.
A general rule of thumb, is that if its for a car its Metric Fine and if not its Metric, this is a very general rule and you will often find this not to be the case.

In this table we look at the pitches for Metric Course threads…

The basic design profile of the Metric course thread is international standard ISO 68-1. A metric ISO screw thread is designated by the letter M followed by the value of the nominal diameter D and the pitch P, both expressed in millimetres and separated by the multiplication sign, × (e.g., M8×1.25). If the pitch is the normally used “coarse” pitch listed in ISO 261 or ISO 262, it can be omitted (e.g., M8).

So the correct discription for a 12mm Metric Course that is 25mm long would be : M12 x 1.75 x 25mm (Bolt discription here) which tells us the bolt is 12mm with a metric course pitch of 1.75mm and is 25mm long. Being a metric course thread we call it M12 X 25.

The diagram below shows the metric thread design and its angle.

Discussing the Metric thread in finer detail…

So what is the major differance between a Metric Course thread and A Metric fine pitch?

The description for a Metric Fine pitch is MF. Basically the difference in the distance between the threads is known as pitch and the closer the distance the finer the pitch. So in the case of our Metric course example being M12 x 1.75 our pitch was 1.75mm apart from each other. In a Metric Fine example being MF12 X 1.25 The pitch would be 1.25mm apart from each other.

The blades on a thread gauge are marked with a pitch, when holding the thread gauge blade against the thread of the bolt the gauge blade should fill the thread gap (and allow no light through.) This youtube video explanes it perfectly…