Most garage door openers have a track which is made of several segments. My chain/cable assembly on a new Craftsman 3/4 HP opener was not long enough to assemble properly. SOLUTION: Have a buddy hold the motor end down firmly on the garage floor. Use a big hammer and gently tap the other end of the track to seat the segments. The track is now a few millimeters shorter and the chain can now be attached properly. Problem solved.