This gear kit on amazon.
Garage door chain sprocket broke.
Keep your fingers on the sides of the chain at least three inches away from the sprocket lift up the chain and unwrap it from the sprocket.
The chain wraps around this sprocket as well as the gear and sprocket at the motor.
Pull the chain toward the garage door and hold it in position.
Tighten the bolts on.
Thread the nut s facing the door clockwise until it meets the bracket then release the chain.
Remove the outer nut and lock washer from the trolley threaded shaft and pull the threaded shaft out of the trolley.
Pull the chain off the motor unit sprocket and out of the chain spreader.
A new front sprocket costs about 150 200 include the labor to install it.
A broken garage door chain will immediately impact your garage door s operation and render its automatic motorized capability effectively useless.
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Slide the outer trolley away from the inner trolley.
Slide the pulley toward the garage door until the chain is stretched tight then move it toward the motor until the middle of the chain touches the upper side of the chain arm.
If the sprocket is still on the top of the opener carefully remove the chain from the sprocket.
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As a result the garage door will open a few inches and stop or it will close a few inches and reverse.
Lay the chain on the garage floor while you complete the repair.
The front sprocket is at the end of the opener rail closest to the garage door opening.
Grasp the chain 1 foot from where it connects to the end of the threaded chain anchor.
This was a craftsman model 139 53976srt.
This is a video on how to change the gear sprockets in a craftsman garage door opener.