Automatic chain-driven openers ought to have a slack along the chain for optimum operation. With continuous use though, this slack is bound to widen. Alternatively, the gear and sprocket assemblage may wear out, making the chain trip off the sprockets. It’s possible to replace the chain on your automatic garage door opener with an easy fine-tuning mechanism.
What will you need?
A stepladder
A flat head Screwdriver
A 7/16 inch wrench

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Disconnect the Garage Door from the Opener
To achieve this, simply pull down on the emergency release handle to free the trolley from the door opener carriage.
Press the remote or wall control button
This ensures the opener concludes the close sequence and brings an end to the spinning of the sprocket situated on the motor unit. Spinning of the sprocket is normally clockwise on most units but varies with the model. Manually re-position the carriage and chain to bring the trolley assembly about 10 inches from the garage door opener rail’s door end.

Disconnect the Power Cord from the Ceiling Outlet
Place the stepladder under the motor unit then unplug the cord from the ceiling outlet. Establish the chain sprocket’s location on the motor head then lever open the cover on the sprocket by use of a flat head screwdriver.
Locate the threaded anchor
The threaded anchor has a bolt-like appearance and passes across the opener carriage before connecting to one of the chain’s end. Get close to the garage door end of the track to locate it.

Loosen the Nuts Opposite the Door
Each side of the anchor should have one or two nuts. Unfasten the nuts in five complete turns with the 7/116 – inch wrench. This slackens the chain further which makes it more convenient for you when placing it back on the sprockets. Give the nuts opposite the opener motor a single-clockwise half-turn by hand.
Ensure the opener rail is free of twists
Observe the entire length of the opener rail and make sure the chain has no twirls. Sections of the chain missing openings within the links are expected to lie parallel to the floor. If you notice any twists on the chain, rotate its side to remove them.

Place the Chain Around the Sprocket
Position the chain about the front sprocket on the rail. Situate the ladder at the opener motor and position the chain around the sprocket on the motor unit. Then move back the ladder below the carriage assembly.
Grip the chain a foot from the position it connects to the anchor
Pull down the chain in the garage door’s direction and then hold it in place. Turn the nuts opposite the door clockwise till they come into contact with the bracket then let go of the chain. While your finger is pressed down on the chain, tighten or relax it by making adjustment to the nuts. Ensure you don’t over-tighten the chain as it creates undue strain on the motor and the chain assembly.

Re-install the cover on the sprocket
If the option is available, plug the cord into the ceiling outlet. Lift the garage door until the door clicks in position with the carriage. To check the opener operation, press the wall control button. Tune the limit switches, reverse mechanism and weight sensors properly.
You can totally rely on these steps for detailed guidance on how to place back a chain on an automatic garage door opener.