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A plug spanner is a box spanner or deep socket that should fit securely over a sparkplug without touching the ceramic insulator. Ideally, use one that has a rubber insert to protect the insulator, and that also grips the plug so it can be lifted out with the spanner.

Most plugs are angled in the cylinder head, so the plug spanner should have a swiveling handle. If it does not, it cannot easily be head square over the plug to unscrew it, and there is a risk of breaking the plug. The handle must be long enough to clear the components around the cylinder head, but short enough to get at the rear of the plug.

Most spanners are 14mm, but some are 10, 12 or 18mm. This is the thread size, not the width across the flats. Before buying, check the size needed for your car.