Abstract: A piston top-dead-center detector includes an adapter capable of being screwed into a sparkplug hole in an internal combustion engine, a length of electrically insulating hollow tubing disposed within a hollow interior in the adapter and a conductive rod slidably disposed within the tubing so as to be slidable relative to the adapter. With the adapter installed in a sparkplug hole, the end of the rod outside of the internal combustion engine is coupled to the positive terminal of the engine coil through a lamp. The engine is rotated so that the corresponding piston in the sparkplug hole in which the adapter is installed engages the end of the rod within the engine and pushes the rod within the insulating tubing in the adapter to an extreme position at the top-dead-center position of the piston.