[CarlosGJ]

Before you can connect a motor to any kind of gearbox, you need a small gear (called a "pinion") on the end of the motor shaft. While CIMs, etc have a small keyway in the shaft, you have to press-fit to smaller motors. On my team, we first heat the pinion to about 300 degrees in a toaster oven (to expand it slightly), then (while it's still hot) press it onto the shaft with an arbor press. When the pinion cools and tightens on the shaft, it is a very strong connection. While I highly recommend the oven and press method, I have heard that it is possible (albeit difficult and risky) to press fit at room temperature with a hammer. Tip: the bearings and bushings in the motor case are nowhere near strong enough to support the loads involved in press fitting. The motor needs to be resting on the bit of shaft protruding from the bottom, so only the shaft is under stress. Other tip: whatever the method you use, the greatest risk is bending the shaft, resulting in a useless motor. Make sure everything is straight before you start. Also, continuous force is less likely to bend the shaft than impulse, thus the arbor press as opposed to hammer.