Measuring Fasteners

The most common question people ask us is how to measure fasteners, so we decided to create this individual section. It’s been designed to keep things as simple as possible however if anything is unclear or you need any other information concerning sizing please use .

Measuring fasteners

No special tools are required to measure metric screw diameters and threads - a steel or good plastic rule should be adequate. Click on images for larger versions.

Length:

Is measured from underneath the head to the end of the thread, except Countersunk screws.

Diameter:

Simply measure the distance from one tip of the thread to the opposite thread across the middle of the screw - the value will be slightly less than the actual thread diameter for example an 8mm diameter thread might have a value of 7.85mm when measure with a vernier

Threadpitch:

The distance between each thread tip. There generally isn't a need to check the pitch on screws from a Japanese bike up to and including M8 Diameter except older Kawasakis. M10 and M12 are almost always 1.25 pitch but it is always wise to check. Just line the mm scale on the ruler with the thread, if they align the thread is 1mm, if they don't align just measure the distance occupied by ten threads - either 12.5 (1.25 mm pitch) or 15 (1.5 mm pitch). The image shows 1.75mm.

AF:

‘Across flats’, the distance from one side of the Hexagon to the other, this is also the spanner size. No need to specify this measurement on hexagon headed fasteners up to 13mm AF