I'm not a fan of the return springs either. Suggest you don't remove it from the shaft, just pop the end that's engaged in the arm out and release it so it's loose, no longer able to return.

What Futaba servo are you using?

To get 6v compatibility, you wouldn't necessarily need to go with another 645mg. Just something newer than what you're using? This one might be one option?

http://www3.towerhobbies.com/cgi-bin...6&I=LXTRE2&P=K

If you haven't seen it yet, there's a great bunch of guys with a huge Giant Sportster dedicated string here on RCU. Starts way back in '06. There are several issues related to the airframe you should be aware of? You might want to pop in there for some reading, or just ask about them? Taking the trouble to do that could save your plane....

http://www.rcuniverse.com/forum/m_42..._1/key_/tm.htm