Motorcycle manufacturers have used many different types of bolts and threads over the years. Long before metric bolts became standard on most motorcycles (during the 70s), all manufacturers used the thread type commonly used in their country or region. In addition, during the conversion from British Standard threads to metric, it was not uncommon to find both types on the same machine (again in the 70s).

For the manufacturers and their designers, selection of a bolt is based on its use/location and the amount of load it will be expected to tolerate without fracturing in service. This selection of bolts will also be governed to a certain extent by the price--there is no point in fitting a high tensile bolt to hold a fender in place, for example.

For the classic bike owner who performs service and repairs on their bikes, the ability to identify bolts is essential.

Types and Classification

All bolts are classified according to:

• Size (shank diameter, length, thread type & pitch)
• Material
• Grade
• Head type (Allen, hexagon etc.)

Size

To replace any bolt, it is first necessary to determine the exact size. There are four sizes to be measured on any bolt: the diameter (at the shank), the shank length, the thread pitch, and the overall length.

The diameter is measured at the shank (not over the threads).

The shank length is measured from beneath the head to the point where the threads begin.

The thread pitch relates to the measured distance between the threads.

On standard bolts the thread pitch is expressed as 24 T.P.I. (for example). In this case there are 24 threads per inch of threaded bolt.

The overall length is also measured from beneath the head of the bolt.

Material

The materials a bolt is made from will greatly affect its strength. For the majority of motorcycle applications, the bolts are made from steel (medium and low carbon variants) and stainless steel.