I've tried an Allen key 7/64 but that didn't work and the others on my Allen key set but none seem to fit this shimano on a Schwinn Ranger.
One of the derailleur cogs broke in half.
Here is an image of a similar bike with the same Shimano drive train.
http://www.target.com/p/schwinn-mens-ranger-26-mountain-bike-red/-/A-15287654
Thank you!