The info trader4 gave you is good.
The BORG (Home Depot) where I am (Portland, Or.) has a good, free handout,
in the same area where they stock the garage door extension springs, about
how to weigh the door.
I'd never replace just one sping of a pair. Thats an invitation to problems
of just the type you are describing. In addition, replacing only one
guarantees that you'll be doing the job again in a year when the other one
I understand that your frugal friend didn't want to buy the second spring.
He's awfully free with your donated time, labor and expertise. Me, I'd have
simply told him that if he didn't buy two and replace both he was on his
As to identifying the weight capacity of the old spring, there is usually a
painted area on the very end of the spring, on the loop where it hooks onto
the fixed post at the inside most point of the garage door frame, and on the
end of the loop where it hooks into the traveling pulley. Frequently worn
and faded, but still "seeable".
An alternative is to literally count the coils in the spring. Get some
little 'dots' made from a peel and stick mailing label. Count every ten or
twenty coils and mark with a 'dot'. When you get to the end of the spring,
count the 'dots' and multiply by whatever number ou have in each segment,
giving the total number of coils. Measure the diameter of the coils and go
get a "X" diameter coil by "Y" coils spring.
Of course you could always take the unbroken one of the old spring pair with
you to the vendor and get a replacement pair matched by diameter and length.
As to your 3 questions:
1. In an extension spring system, a pair of 150 # springs lift a 150 pound
door. To lift a 300# door in an extension spring system, you need a pair
of 300# springs.
2. If its a 150# door, two 150# springs are what you need. Two 150 #
springs will not be too strong. But you need to weigh that door or
definitively identify the capacity of the old springs.
3. No, you can't "adjust" two unequal capacity springs. And you can't
"adjust" one brand new and one old spring of the same theoretical capacity.
The old sringis worn and suffering metal fatgue, even though it has not yet
broken. It doesnt have the same elasticity as a new spring of the same
nominal capacity. They won't work well together.
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