* The following applies to stock motors but some can also be applied to 19T and modifieds.
Stock motor racing has become one of the most popular and competitive electric racing classes in recent years. Manufactures have responded to this with ever faster motors and new technologies. Although these motors could easily be used straight from the box, a little effort in preparation, can give a significant increase in performance - to give YOU the edge.
Start by dismantling the motor. You'll need to remove the springs and brushes. Make sure you replace the screws that were holding the brushes down. Next, undo the screws holding the end bell on and remove it. Take all the shims and fibre washer off the armature and keep them in a safe place, you may need them later. Take out the armature and any remaining shims.
Many motors are supplied without their comms true. It is very important that the comm is true otherwise you'll never get maximum performance from your motor. If you don't own a lathe, take the armature to your local hobby shop, and they can usually do it for a small fee, or ask a friend nicely. A diamond true is best, but a carbide one is better than not at all! Now that the armature has been prepared you can look at the rest of the motor.
A lot of perfomance is lost by the bushings in the endbell and can. It is against the rules to replace them with ball races so you need to reduce friction here as much as possible. You'll need a bushing break in lube for this, which usually contains graphite powder to smooth the bush faces. Apply a drop to each bush and rebuild the motor, but with an old arm in. Turn the motor over by its output shaft (it helps if you put a pinion on for this) and also push it back and forth. Keep this up for about a minute and unassemble the motor. You need to look at the bushes to see if they look shiny. If they do then you've done a good job, but if they dont you need to repeat the above until the are. Finish off with a blast of motor spray to clean off any remaining lube.
Now you need to rebuild your motor again but only with the fibre washer in. Spin the output shaft to centre the armature in the cans magnetic field. Feel the gap when you pull the output shaft out, and estimate how big it is. Disassemble the motor and shim the output shaft side with as many shims you feel nesessary. Put the motor back together, and centre the armature. You need to check for just a little play on the output shaft side, and add/remove shims accordingly. Do the same for the comm side. When your done there sould be just a little play and it is important there is no binding. You can replace the outer shims with teflon shims (offered by a few companies but I prefer the Trinity ones). These will reduce friction if the shims ever rub against the bushings.
Most stock motors today use laydown type brushes and there and many different compounds and cuts offered by various manufactures. One that I like is the Trinity 4499 brush. It combines good performace with a relatively long life. It also requires no break in and so has good power from the start. For most performance here solder the brush shunts to the same solder tabs as the power wires, to minimise resistance. To increase RPM performane (but lower torque) you can drill a shallow 2mm hole in the face of the brush, this is ideally suited for 1/12 scale racing.
Use the spring tension to tune RPM and torque. Generally you can run a lighter spring on the negative side but it still ok to use identical springs on both sides. For touring cars I use Trinity red (+ve) and green (-ve), but will sometimes run green on both sides. For 1/12 you can use lighter springs for more RPM, less torque.
Finish off with a single drop of light bushing lube on each bush, and your done. Happy racing!