Pinion choice is governed by the target head speed you want to run.

Check this calulator: [link=http://www.readyheli.com/Online_Headspeed_Calculator_s/81.htm]Head Speed Calculator[/link]

You need to choose a target head speed, then gear it appropriately. Brushless motors run most efficiently and provide the most power the closer they're tun to 100%. This also benifits the ESC as well.

For the 430 motor which is a 3550kv motor:

2400 head speed = 11T pinion
2800 head speed = 12T pinion
3000 head speed = 13T pinion

Running a 13T pinion with a 2400 head speed put too much strain on the motor, thus the increased heat.