I just recently replaced the gear on my older craftsman garage door opener and have tried everything to get it to run properly. It will run for 6-10 inches and then stop and reverse itself. Is it possble that the motor has gone bad in it? I have replaced several gears on this unit and have never encountered this problem with the motor running like this, I even took the chain off like you suggested to anther person to try and get the motor to reengage itself. Any suggestions? Tony.

For the OP, what type of a spring assist system does your door have?
If torsion type, it sounds like the cable that attaches to the bottom of the door on one side hangs down farther than the one on the other side, or is broken. Working on this mechanism can be trecherous due to the stored energy in the torsion spring when the door is closed. Repairs should probably be refered to an experienced garage door mechanic.
If you have an extension spring type, I'd say one spring is misadjusted or sprung. I'd replace both springs.

Tony, Breaking gears on a garage door opener is an indication that the opener is doing a lot of work to lift or lower the door. The craftsman openers are made by Chamberlain and that has always been their weak link. THe problem however may be caused by a weak or broken spring. Disconnect the door from the opener and lift it by hand stop at 1' off the floor 3' off the floor and 5' off the floor. At every point a balance door should stay put. A weak or brokien spring will cause the door too fall freely to the ground. If the door is somewhat in balance then try adjusting the down force on the opener. Also, take a look at the photobeams at the bottom. Sometimes they are barely aligned and when the door goes down they misalign causing a safety reversal. If you see the LED on the beams flash when the reverse happens then align the beams and check the wires to the opener.