If I understand your problem correctly, it sounds like you are collapsing the strut tube when you tighten the set screws. If that's the case you need a support piece inside the upper end of the strut (above the compression spring) to prevent the strut tube from collapsing. I use a 1/2" long piece of steel rod, sized to the strut tube ID so the tube does not collapse. Robart provides a nylon plug for this purpose but I prefer the steel plug.