when you start to heat up the oven, it takes a while for the inside heat coil to heat up. the relay and the heat sensor sense on the heater. when the heater reaches 250'C that doesnt mean the engine case is also 250'C and it is much lower in temp. by the time the oven relay stops the heater, the engine case is likely to be below 100'c and that is the proper temp to release the main bearing. how long? that's depends on your oven