Liftmaster Gear and Sprocket Installation tips:
Whether or not the gear and sprocket sets come with instructions, here are some tips I highly recommend for installation:
First: Have the garage door in the down position, with the trolley assembly connected to the chain/belt. Unplug the power to the machine powerhead. Then loosen the nut on the chain or belt inner slide, so you can lift the chain or the belt off the sprocket, and have it ready for re-installation. This step ensures that the limit switches will not have to be adjusted too much after installing the new part.

Second: remove about 4 screws from the motor case and remove it. You may have to remove other parts, like possibly lens covers to be able to do this, it just depends on your machine. Once the case is removed, you can reach up inside the machine and remove the ~3-4 screws holding the gear and sprocket top cap on. At that point you can just push the gear set up and out of the machine.

Third: Take the gear and sprocket assembly out of the bag, with any associated bushings  you should replace.

chain drive gear set, showing
the optional and not recommended
wormgear to the right

--CAUTION: My recommendation: unless the wormgear on your machine is visibly damaged, leave the new, included wormgear (shown at middle right in the photo)  in the package. It is very difficult to re-align this gear to properly engage the nylon drive gear if it has been removed. If it is not re-aligned perfectly, it will bind up within a few inches while raising or lowering the door, and then must be disassembled to try-try again.--     if you have any questions about whether or not you NEED to change the worm gear, call www.liftmaster-store.com at 1-888-880-8821.

Fourth: take the tube of included lubricant and spread the lube liberally in the teeth of the new nylon drive gear. --NOTE: lubricating the new drive gear is critical to ensuring a long life to that assembly--  Drop the new, lubricated gearset down into the opening, and get it properly aligned with the worm gear and limit switch. Re-install the screws into the cover, tighten equally. Plug the machine in again.

Fifth: Operate the door and adjust the limit switches as needed so the door opens fully, and closes just to the ground. You are done!!