if you are asking heatsink for the motor, don't... it holds the heat in the motor...
Assuming good contact between motor and heat sink to allow for the movement of heat via conduction, and that the thermal conductivity of the heat sink is equal to or greater than the motor can, then it is impossible for the heat sink to act as an insulator. Unless there is some other NEW type of Physics that I did not learn at Uni that causes heat not to move from an area of High temperature to an area of Low temperature.
I was just wondering about this because im planning to get a new heatsink soon.
I do not use one, there is no need. My motors run ice cold as it is. Use correct gearing, don't over rev the motor or load it up, and all will be fine.