Originally Posted by thecrow2k
The Bevel gears are perfectly compatible. Thats why the Evo III boys run the Evo IV front one way gear.
As for the drivetrains. The bearings used in the gearboxes are just flipped. Te motor mount bearings in the Evo III are the gearbox ones in the Evo IV and vice versa. Its only the length that could be the problem. As for spur gears, they're universal.
I do not agree that the are the same. You maybe able to run IV gears in a III, however do not attempt that the other way around. The III for one didn't use a screw to hold in the bevel gear, like the IV does. Also the internal ratio of the IV is differant to the III, although if you swap both front and rear to IV then it will not matter.
The shafts on the two cars are totally differant. III uses a one piece shaft, IV uses a collection of bits and pieces.
Anyway to anwser your original question.
I can't not get an exact measurement for you without romoving the shock tower, but its around 3.5-4mm.
It you need a more accurate measure, it will have to wait untill the weekend when I get some time to work on the car.