The flywheel ring gear is an important part of a reciprocating internal combustion engine. The two are keyless interference fits. The ring gear is preheated to a certain temperature to increase its diameter, and then heat-mounted on the flywheel. After cooling, the torque is transmitted by the locking force. According to the shape and size of the ring gear, it is very suitable and effective to use a core power frequency inductor for preheating. In this way, it can be directly connected to the industrial frequency power supply without frequency conversion equipment; high power factor, no compensation capacitors and special power distribution; simple structure and convenient operation: low power consumption per unit product, low production cost, and improved heating and assembly Quality, power frequency induction preheating of flywheel ring gear.