How can I securley re attach the chain sprocket on my craftsman garage door opener Model# 139.53975SRT1?

Answer 1

The important thing to remember is that you need to attach the chain to the trolley - the metal carriage that slides along the rail - so that the trolley matches the position that the opener "thinks" it should be. The best way is to run the opener without the chain attached, so that the sprocket turns in a counter-clockwise direction when looking down from the top of the motor unit. Another trick is to simply run the machine, making sure that the "beam" between the optical safety sensors is blocked. Doing so will either cause the opener to reverse to the "open" position, or continue running to the open position.

Once this is done, wrap the chain/cable assembly around the pulley at the end of the rail by the doorway, and around the sprocket, and reattach to the trolley with the trolley 2 to 4 inches from the "stop bolt" that goes through the rail near the motor. If you don't have a stop bolt, position the trolley about 6 inches or so from the motor unit. Reengage the door, remove the obstruction from between the sensors, and test the system. You may still need to make minor travel adjustments to have the door close and open to the proper positions.

Chain tension is important as well. Once everything is attached and the travel adjustments have been made, close the door, and look at the chain along the left side (if you were facing the door way) of the rail. There should be a slight droop or sag to the chain, but be sure that it is tightened enough so that the chain hangs no lower than 1/4 inch from the bottom edge of the rail.

After that, you're done!

Answer 2

is the sprocket itself off? if so you need to purchase a gear and sprocket kit to fix this.