When I took my Unistyle up today, I found it`s slipfitter coulden`t tighten up enough for a 1-1/4 inch arm. I will make an adapter, but why can`t it get tight enough? BTW the Unistyle was made in 1965.

You've just discovered the one odd ball design feature of the Unistyle series of streetlights. Line material Industries made an adapter that fastened to the slipfitter clamp with a screw to allow the use of 1 1/4" pipe. You can see it on mine by enlarging the photo and looking at the slipfitter on the lower right of the picture . https://www.lighting-gallery.net/gallery/displayimage.php?album=5198&pos=142&pid=164031  I don't know how hard it would be to do but I'd guess you could fabricate one some how .

Thanks for the info. I was able to make a new clamp. I did save the old one though, of course.

Excellent !