Shaft size for what motor? The Speed 300 motors have a 2mm shaft and I think the Speed 400 motors have a 2.3mm shaft. I found it's easier to tap the pinion onto the old motor and remove it a couple of times to loosen it up...then when you go to put it on the new motor, it will go on a lot easier. You want to support the other end of the motor shaft on a hard surface when you press the pinion on. You can use either a vise or a hammer to tap the pinion on. Make sure you keep the shaft nice and straight when you are doing this. Try to go slowly...if the pinion starts to go on crooked, remove it and try again. If you open up the inside diameter of the pinion by installing and removing it a couple of times on the old motor, it will be much easier to put on the new motor. If you think it goes on too easily after doing this, you can use a little Loctite to make sure it stays put.