I've replaced springs on both doors, sort of nerve wracking, like using a table saw only more so: all hands on deck. I bought a 36" long 1/2 steel rod and cut it in half for the two winding tools.

Word I've read is you get the springs tense enough to hold the door in balance about half way up. On the door I have yet to install the opener, that didn't quite work as then there was too much spring tension for the motor to close the door. As it begins to close, there is virtually no vertical weight bearing down on the cable/spring so the motor has to tighten the spring until some of the weight starts moving down the guide. I had to loosen the spring a bit to find the balance. Perhaps I could tighten it a tad more for better function.

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1986 300SDL, 362K
1984 300D, 138K