Abstract: An arrangement for easily and rapidly replacing a spark plug in a bore of a cylinder head includes a conductive cylindrical body that screws into the bore. It has a tapered internal passage that receives a replaceable plug with a tapered metal outer surface. A resilient sealing ring retained by the plug seals gas tight against the inner passage. An annular groove on the outer surface of the plug receives groove engaging elements or balls mounted in the body to lock the plug securely in position. A sliding locking sleeve surrounds the groove engaging elements and prevents the elements from moving out of the groove in a locked position. The sleeve may be manually reciprocated to an unlocked position in which the elements are free to move out of the groove so that the plug may be easily replaced. A spring biases the sleeve to the locked position.