Thanks for the input! Mine has the original BlackJack, too, but I'm sure it has more miles than yours. One thing that always bothered me about the pedal was the way the spring tension adjustment screw and wing nut would wobble a little. I cured this with another nut on the spring side (opposite of the wing nut on top), sort of the way most other pedals are. It also keeps the tension setting locked down.
Very cool little fix for your loosening wingnut/tension retainer! I will say that fortunately, I have never had that issue with any of mine! On my two "working/gigging" Swivo's, the wingnuts have wax inserts which gives them enough resistance to not back off or lose tension while you're playing, and those were OEM at the time! On many other model Swivo-Matics, Rogers used a thumbscrew/wheel style retainer, and those can loosen as the spring starts to stretch, as there's nothing really keeping them from doing so. Your fix would be ideal there!
Tommyp
