Spark Plug Spanners

Spark plugs are a vital combustion engine component that will ultimately wear out. On older vehicles, the task was straightforward as nearly every spark plug was the same size, or there were two at most. However, things change, and you can find numerous HEX sizes for nearly all spark plugs for many engines.    

First, look in your manual for the kind and model of the pre-gapped plug. This simplifies determining the socket size. It is handy to have a gapping tool to adjust your plugs rather than buy new ones.

Look at the spark plug itself to see what size socket it has. Vernier calipers can be used to measure the HEX size or diameter if you have them.

Hex refers to the socket or diameter of a spark plug and the faces used to loosen or tighten using your spark plug socket.

You can find the spark plug socket size chart here, and it shows the most common plug sizes and the vehicles on which they are used.

Because of how the engine world works, there are no direct conversions of spark plug sizes to mm or metric. Instead, all sizes are listed in imperial, which is the global standard.