Correct pinion?
 

I am building an AE B4.1 kit. I know the RTR typically runs a 81T spur with 21T pinion gear.
The Castle motor I am going to use recommends from 12-18T pinion with a 87T spur gear.
When I look at the Team Associated and A Main Hobbies sites, this smaller pinion is not available. Why would I not want to run a similar setup as the RTR B4.1? I know the motors are a bit different (my kit will have a 4600kv motor while the RTR has a 3800kv).
I am thinking a 21 or 22T pinion with the 81T should be a good place to start...?
If not buying the Team Associated brand pinion, what other manufacturers can I use? I see by looking at other pictures of other makes, the teeth appear different.

The more powerful the motor, the smaller the pinion you need to help control the heat and keep the motor operating in the proper rev range.

That being said, Castle's stuff tends to run cool and that motor should be fine geared like an 8.5 which is typically ran at @21 pinion anyway.

If you do find yourself needing more pinion gears in the future, I've always been pleased with the Robinson brand of gears.