- Written by Carole Nash Editor
- Created: 12 March 2008
Once the spark plug is out you need to check the gap between the central electrode and the outer electrode, the one which looks like a small arm that reaches half way across the plug.
First clean the plug with a wire brush – do not use glasspaper or sandpaper as these can leave small grains behind which can go on to damage the engine. If the electrodes are badly eroded (they will be very rounded and might look ‘eaten away’), replace the plug with new – check your manual for the recommended gap, then establish the actual gap by sliding different width feeler gauges into it until you find one which only just fits.
The gap tends to grow with use so it will need to be closed – this can be done by gently tapping the plug on a solid surface, checking the gap frequently. If you close it too far, do not lever it open against the central electrode, as the porcelain can crack – instead, use a plug gapping tool, available at motoring shops, to pull the outer electrode open again.