Recently, My electric garage door failed to open. It jammed after starting to open. It looked like it was twisting. I was able to open it by disengaging the worm gear and opening it manually. Looking around it while it was closed I noticed that the spring on one side was not extended like the spring on the other. I discovered that the hook bolt was no longer there. It had snapped and disappeared. Note that these pictures are taken retrospectively to show how I did it, not during the process. That is why they move from one side to the other in the pictures. It must be remembered that the springs are stretched a lot when the door is closed and just a bit when the door is open.
This first picture is of the broken J- hook bolt and where it attaches.
Step 1: The Hook
First step. Measure carefully to see that the bolt to be bought is long enough and has enough thread to tighten up to the same length as the other bolt. After obtaining a replacement J-Bolt, open the garage door and attach the spring to the mount with chains or rope as best you can. Chains are better because the rope will stretch. (Voice of experience here). The tighter that you can get it the better, because the spring is under some tension when it is working properly.
Step 2: Stretching the Spring
Close the garage door. This stretches out the spring. Then insert as many thin pieces of wood/fibreboard as is possible in the springs. You want to make it so that the spring is still slightly stretched out when the door is opened in order to fit the J-bolt.
Step 3: Fixing the J-Bolt`
Open the garage door and fix the J-Bolt. See picture. Tighten up to as near as possible the same length as the other side. Close the door and remove the spacers.
That's it. Job done.