If the spring is broken near a winding cone, you might think you can remove and discard the short broken piece of spring from the cone, clean up the end of the long remaining spring, and insert that end into the cone. This is another extrememly risky improvisation. The shortened spring is not going to have the correct weight-bearing characteristics for the door, so you will not be able to balance the door properly. The shortened spring will be proportionately overwound, resulting in extra stress that will increase the expectation of another fracture. And the aging and history of the original spring being broken greatly increases the likelihood of another fracture at other locations.
When ordering springs, be aware that a number of different sizes of springs will make proper replacements, not just the specific size being replaced. The wire size, winding diameter, and length can be traded off to make springs of varied geometry but equivalent torque characteristics. This will also affect the expected lifetime (in cycles) for the spring(s). Since the critical specification for a replacement is the weight it is designed to bear, not the sizes per se, there are likely several stock sizes that replace a given old spring. The spring distributor's inventory may happen to offer only a different size with an equivalent weight-bearing specification. One has to judge whether to trust the advice of the seller in such situations. The seller should have the data to know what substitutions are proper.
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