Ring keys and combination wrenches are the simplest and most commonly used tools to tighten bolts and nuts. They are available to purchase separately or as a set and they often consist of 6 or 8 keys (common sizes).

A wrench and ring key have a key piece at both ends, in successive sizes such as 10 and 11 mm. Combination ring/wrench keys are also available. In this case, the key width is equal on both sides.

It’s important to know have the right ring key and wrench sizes

Nuts and bolts usually have six sides, and the different key sizes are made to fit exactly on the nut or the head of the bolt. It won’t work if the key is too small. A key that is slightly too big can sometimes still get the job done, but you run the risk of damaging the bolt or nut. And you won’t be the first person to hurt your knuckles when the key suddenly slips out…

Metric sizes and inch sizes

Here the sizes are indicated in millimetres, while in America the sizes are indicated in inches. The expression that is generally used is ‘UK sizes,’ though the United Kingdom has now also switched to the metric system. One inch is equal to 2.54 cm and the various keys have dimensions termed as 1/8 inch, ½ inch, ¾ inch etc.

Although these widths are somewhat similar to the European sizes, there are still some minor differences and sometimes it only differs by tenths of millimetres. A ½ inch bolt has a 12.7 mm wide head. For this you can use a 13 mm wrench, but if you want to make sure to prevent any damage, you must really have inch keys for inch bolts and nuts.

Chart with the correct key sizes

Sizes on both bolts and nuts are indicated by a figure on the keys. The figure on the bolt or nut shows the width of the screw thread, which is actually a bit weird because when choosing a key it’s important to know the size of the bolt or the outside of the nut. But the combination of screw thread/head is standardized so that the system works. This table lists which metric ring-or wrench fits onto which bolt or nut.

Nut or bolt

M3

M4

M5

M6

M7

M8

M10

M12

M14

M16

M18

M20

M22

M24

M27

M30

Key

5,5

7

8

10

11

13

17

19

22

24

27

30

32

36

41

46

Other keys

Apart from the wrenches and ring keys, there are a number of other tools needed to tighten bolts and nuts. The allround English key and perhaps keys for more specific purposes like allen wrenches or plug wrenches. In our blog article entitles The right key for each job you’ll find an overview of tips for purchase and usage.