If it was from a garage sale, it's likely an older Blue Box loco. It sounds like you're missing a slotted coupling, part #41017 on this exploded drawing.
It's still used in their RTR line, and is available from Horizon.
http://www.athearn.com/Search/Default.aspx?SearchTerm=f7+parts
Nelson
Ex-Southern 385 Being Hoisted
Athearn has changed its drive line design to one degree or another several times over the last 45 years or so. Try going to HOseeker.org and clicking on the "Diesel Diagrams" combo box, and pick out one of the F7 diagrams. Check it against the F7 you have, and if it's the right one you can pick out the part and match the part name against the parts list on the side of the page. If it's not the right one, pick another diagram until you find the right one..
Good luck...
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Gary M. Collins gmcrailgNOSPAM@gmail.com
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"Common Sense, Ain't!" -- G. M. Collins
http://fhn.site90.net
I'll try to describe this part as best as I can- I bought a used train set at a garage sale with an Athearn HO scale F7 but after opening it up to install a DCC decoder, a plastic part that couples the motor to the one of the wheelsets is missing. It's the black, rounded middle plastic piece (brush motor maybe?) that connects between the motor and the wheelset. It's slightly larger than the other black pieces. What is that piece called so I won't end up ordering the wrong part from Athearn?