Do you know your classic car nuts and bolts?

Do you know your bolts?

This is a brief guide to some of the technicalities regarding specifications of both metric and imperial size nuts and bolts.

Let’s start with metric threads

All metric nuts, bolts and washers are described as an M something to describe the diameter, for example M8 describes a fastener with an 8-millimetre diameter, and if there is no other dimensional description following this other than the length it describes a standard metric coarse thread – which in the case of M8 is 1.25mm pitch.

If the fastener in question is a metric fine pitch, it will be described with the pitch dimension following the diameter dimension, for example M8 x 1.00 will be M8 x 1-millimetre fine pitch. For clarity, the pitch is the dimension from one thread to the next as shown in the top diagram below. Also the pitch varies with each diameter.

Next, imperial threads

Imperial threads are available in a larger range due to there being American and British versions, these are all non-metric and are described differently.  The American threads – otherwise known as unified – are available in coarse thread referred to as Unified Coarse or UNC, and fine thread which are referred to as UNF. The threads on these are counted numerically to the number of threads there are per inch, therefore referred to as “Threads Per Inch” or TPI, this varies according to whether they are UNC or UNF and the count decreases as the diameter gets larger.

The (older) British threads are known as British Standard and British Association, two different thread forms.

First let’s cover British Standard, which like the American threads are available in coarse threads referred to as British Standard Whitworth or BSW and fine thread which are referred to as British Standard Fine or BSF. The threads on these are counted per inch (TPI) in the same way as the American threads.

The diameter for both Unified and British Standard threads are measured in inches or fraction thereof. See diagram below showing TPI.

The British Association range of threads, known simply as BA, dates from 1882 and is based on the metric measurement system i.e. millimetres for the thread pitch and imperial inches for the lengths. They are numbered from 0BA (largest diameter) to 16BA (smallest diameter)

You will have noticed at the beginning that the question was “Do you know your bolts”? When most people talk about bolts, they are referring to a machine screwed shank with a hexagon (hex) head, however there are full thread and part thread versions. The correct definition is as follows:

HEXAGON HEAD BOLTS have a plain shank from under the head to the beginning of the thread as per this diagram.

HEXAGON SETSCREWS are fully threaded from under the head as I this diagram.

Just to confuse thing further, if you are looking at an American drawing, specification or description these could both be referred to as Cap Screws (not to confused with socket cap screws which have a hexagon Allen Key drive).

Bolts are usually driven in and out by SPANNERS & SOCKETS and it is important that the correct size is used ensure efficient tightening and loosening, and to prevent rounding the corners of the hexagon.

Metric spanner size is referred to in millimetres according to the measurement across the flats of the hexagon. Likewise Unified spanner size is referred to in inch measurements according to the measurement across the flats of the hexagon, these are referred to as A/F spanner size.

BSW spanners are available according to the thread diameter and are stamped accordingly, these will in most cases also fit corresponding UNF bolts (unless they are very old in which case the UNF head may be one size smaller).

Reference to grade and strength of bolts can be found in the news section on the Leyton Fasteners web site referring to bolt and screw head markings.