RC10T pinion advice

[What's_nitro?]

Yes increasing the pinion size will lower the motor temperature. I know it seems wierd but it is true for brushless motors. The motor spins a little slower since there is a bigger load on it, and so there are fewer current pulses through it and less power is dissipated to the case. OTOH the ESC will heat up more. This is true when gearing up with brush motors OR brushless motors since each pole of the armature remains energized for a longer period of time. Since the armatures of the motor are basically short circuits, powering them for longer periods of time is harder on the FETs of the ESC. The Lehners are very efficient motors so they will run well up to 40000 RPM. I think your getting so much heat because the motor is just taking so much power to run. Probably 70A at WOT with that motor in a 1/10 ST. You may want to try a 4S A123 setup and a higher wind motor like a Basic 3100. It will be more efficient since it draws less current for the same power output as the Basic 4200, so the temps overall (motor, esc, batteries) will be lower

The Lehner motors are rated with an unspecified load. So the 4200kV may be 4300kV unloaded or it may be 5000kV unloaded. I don't know and there is no way to find out unless Lehner publishes the load they are rated with. Be careful using the Warrior controllers with the high RPM because they can only handle 42000 RPM max. They cog like mad if you exceed the limit and that's bad for the motor AND the ESC. Not to mention, your drivetrain. The good news is that A123 cells drop voltage more under high load than other Lithium based batteries. So your guess of 10V under WOT may be high. This is GOOD news since it means you wont hit the RPM limit on the BK- just don't revv it up with the wheels in the air or you might be sorry. Hope this helped a bit.