Starter ring gear is heated so it can fit over a flywheel how do you calculate temperature required to raise diameter from 254mm to 254.75mm?

Coefficent of thermal expansion (alpha) for steel is approximately 11.5 X E-06 /degree C.

New size after heating = Diameter * (1 + alpha*delta-t).

A change of 256 degrees C is required to be exact. If you heat it to about 300 degree C it should fit over nicely - don't want it too tight when hot, just drop it over quickly and let it cool slowly. Best to heat in an oven or on a hot plate. A lot of mechanics use blow torches but you run the risk of softening the teeth if you overheat a particular spot too much.