How to choose the right spanner for nuts and bolts

WHAT SIZE SPANNER DO I NEED?

“What Size Spanner do I need?” is a common question. The immediate answer is another question: “What type of thread does your nut and/or bolt have?”

In engineering circles a Spanner is often called a wrench and is generally used to grip a nut or bolt while applying torque or put more simply to help you fasten and loosen nuts and bolts. 

Most common are open-ended, ring spanners or combination open/ring spanners and sockets

Most spanners and sockets do have their size stamped on them, but this is usually the distance across the flats of the hexagon and can be confusing, so we will help you to determine the size you need and show why you should not just grab one that seems close enough.

There are many thread types, Metric, UNF, UNC, BSW, BSF, BA to name but a few. Each of these thread types has their own thread specifications and specifies the distance across the flats of the hexagon. It is the distance across the flats of the nut that is most important when choosing a spanner size. As shown on this diagram the dimensions labelled “S” is the ‘across the flats (AF)’ measurement. The jaws of the spanner or socket should fit snuggly with little or no gap.

You should also be aware that the size given to any nut or bolt is relevant to the thread type and size. A metric M6 nut for example means the measurement ‘d’ shown above is approx. 6mm and this ‘M6 – 6 mm’ measurement bears no resemblance to the distance across the flats, or the spanner size required. 

There are many types of threads available for nuts and bolts, Metric, Unified (UNF/UNC and British Standard (BSW/BSF) are just three that we deal with, and they all have different measurements depending on the international standard they are manufactured to. For example, a metric M6 nut and bolt are very close to an imperial 1/4″ UNC/UNF thread. Measuring the thread manually you could be mistaken for thinking they are the same. Also, a 1/4″ BSW/BSF nut may sound similar but is different again in size.

A standard metric M6 nut will be 10 mm across the flats, a 1/4″ UNC is just under 11mm and a 1/4″ BSW just over 11mm. Attempting to use the incorrect (oversize) spanner could cause the spanner to slip, rounding off the edges of the hexagon.

As you can see, it is best to know the thread type of the nut and then choose the spanner size that will then give the best fit. Once you know the thread type and size of the nut, then it is simple to look up the correct spanner size. 

Just to mix things up a little; for nuts and bolts with BSW or BSF threads, you will need a spanner marked with the diameter size of the thread e.g. a 1/4″BSW or BSF nut or bolt would require a spanner marked 1/4″BSW.