If you want a clean look don't use the u-bolts as you just might over tighten them and crush the frame tube.
Try the Beach buggy clamps as used by RedB66, they are a very clean professional look & alternative to the u-bolts.
Here's the Link below to see RedB66's modified motor mount / beach buggy type clamps:
http://motorbicycling.com/f40/updated-stretch-cruiser-mounts-15926.html
Peace Crazy Horse.
Try the Beach buggy clamps as used by RedB66, they are a very clean professional look & alternative to the u-bolts.
Here's the Link below to see RedB66's modified motor mount / beach buggy type clamps:
http://motorbicycling.com/f40/updated-stretch-cruiser-mounts-15926.html
Peace Crazy Horse.