Battery is an 8.4v 1800mah, I took out the m120 and threw in a Modify s100 to test it.
What I meant by testing it outside was to see if it works, THEN I put the motor into the mechbox WITHOUT putting it into the pistol grip.
When I push it into the mechbox (sans grip), it works fine; when I assemble the grip, it doesn't mesh right...
Looks like I'm missing this piece, so I ordered one...
This