Airplane engines generally have temperature probes on all of the available cylinders so that the pilot can effectively monitor and lean the engine according to the hottest cylinder which may not always be the same one.
The Cylinder Head Temperature (CHT) is monitored by a probe that is plugged right into the bottom of the cylinder. The CHT probes screw into little bayonet fittings that are in turn screwed into special bosses on the bottom of each cylinder.
