Abstract: A device for testing the functioning in an internal combustion engine of spark plugs which are recessed within the cylinder head. Connection to the recessed spark plugs can be accomplished by an elongated device shaped to fit within and extend beyond the spark plug recess, preferably with a secure frictional fit within that recess and preferably designed with portions of different diameters so that, by inverting, the unit can be used with cylinder head recesses with a wide range of internal diameters. The device may, if desired, carry a shield member adapted when the device is in use to cover the exposed top of the recess, thereby to prevent foreign material from entering the recess while ignition system testing is carried out. The device may be readily combined with other devices to constitute a kit of virtually unrestricted testing capacity.