The 645 is SLOW and not the best for torque. I recommend at least 170oz of torque with at least a speed of 13 or faster. If you ever run a decent servo you will never go back. I recommend at least this, which has 172oz torque .13 speed on 6 volts and is coreless. In the listing some of the specifications are missing but you can go to Hitec web site to check them out.
http://www2.towerhobbies.com/cgi-bin...7&I=LXMUS4&P=K