Extension Spring Installation
Secure the garage door in its fully opened position using C-clamps, locking pliers or lumber cut to length. Attach the clamps to the garage door track just under the bottom door roller on each side of the door, or prop the door open with a piece of lumber braced next to the track on each side of the door.
Remove the broken spring by detaching it from its mounting points before releasing it from the safety cable that runs through it. Inspect the garage door pulleys and cables to determine whether they should be replaced also.
Use the unbroken spring that is still attached to the opposite side of the garage door, as a guide for installing the new extension spring. The new spring should be stretched no more than one inch when the door is fully opened, and the two-door pulleys should not touch when the door is completely closed.
Once the spring is properly installed, reattach the safety cable, making sure it does not interfere with the door operation.
Detach and replace the unbroken spring on the opposite side of the door. Use the newly installed spring as a guide this time.
Once both springs have been installed and both safety cables are attached, remove the door support, clamps or pliers and carefully lower the door. Watch the springs, cables, pulleys and anchor points to make sure everything is working properly.