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The threaded adjustment on the SSL is not there to adjust the pre-load it is there to adjust the ride height so that the rebound spring can be set accurately to just kiss the hub at the static ride height.
I have installed myself the SSL kit on my 2005 PLUS 4.
The threaded adjustement allows to adjust the rebound spring free. When the rebound spring is free, the suspended wheight is balanced only by the main spring... which is pre-loaded by the adjustment rings. There is only one balance position for a just free rebound spring. I you screw or unscrew the adjustement rings the gap of the rebound spring decreases until contact or increases because you compress or de-compress the main spring (pre-load).
With my first initial adjustment after assembly, the rebound spring was compressed and I have to unscrew the adjustement rings until getting a free rebound spring after 3 trials. Only the spacers allows to adjust the ride height. With the spacer on the top, the ride height is determined by the free length of the rebound spring (65 mm).
