Steering wheel slipping on shaft

[videoguy009]

I recently installed a new steering box and the bottom coupler shaft mount that clamps to the input shaft was spinning on the shaft. I loosened it and re clamped it and it gripped. Now no matter how tight I get the top upper mount clamp on the shaft mount it still spins on the splined part above the coupler. I know they are notched so the bolt can hold the splines in place but could they be rounding out with rust and age. Is it time for a new set of steering box shaft mounts.
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1971 Super Beetle

[locky]

I had the same Problem. I think they do get old and worn out. I am trying to recall, I thnk I vice gripped it tighter and cranked it on. It held after that.
If that doesn't work, you should grab some new hardware. You don't want that thing sliding on you while making a fast turn or it could be deadly.