Thread Pitch and Screw Types

When it comes to hardware there are a number of different fasteners on a British Motorcycle. Being able to determine the thread pitch is critical to ordering the proper part. It also assists us in supplying the proper fastener for the application.

The simplest way to measure thread pitch is with a thread gauge. These are available from us and any company that sells hardware.

The bolt diameter should be measured and matched to the nearest size with matching thread pitch or threads per inch (TPI).

In general, the major diameter of a bolt will be slightly less than the exact diameter listed in the thread identification and drill chart .

Fraction to Decimal Conversion Chart includes Metric equivalent

Further reading:

Origin of modern thread systems – Thread standardization under the Whitworth System

Screw Sizes – the Ultimate Guide

Pitch Conversions:

Sometimes you don’t have the thread gauge you need on hand. For example you have a metric thread pitch gauge and you need to know the thread per inch or TPI.

Example:

1/4-20

20 signifies “20 threads per inch” =.050 pitch ( 1 divided by 20 ) or if you are measuring in metric it should be 1.27mm

Click here for a comprehensive chart

Screw Head Types:

This section will mostly pertain the engine case screws. There are plenty of specialty fasteners out there you won’t encounter on a British machine. We will only mention what you will encounter.

Cheese Head –  Traditional case screw head style supplied in slotted drive

Filister Head – Similar to cheese head but with a rounded top available in slot and posidriv

Round Head – Points cover screw head style – slotted and posidriv

Flat Head – Typically countersunk and found inside the engine  – slotted and posidriv

Cap or Allen Screw  known to some as a socket head – Not stock to British machines but available as an upgrade for case screws