My first engine was faulty, and, Charles Ro replaced, ASAP. Kudos to them. The 2nd one ran fine but after a day the rear coupler stopped opening. When I removed each engine from the box, there was a small screw rolling around in the box. Turns out, that screw was one of 4 that holds the shell on in the back of the engine. When I put it back in and tightened the two back screws, the coupler began working again. So, if you see a loose screw (be nice) in the box, don't ignore it. 
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Glad it worked out.
