I installed my Staton friction kit on my 20" Dahon, '70 Schwinn heavy duti and Raleigh girlie cruiser. None of the frames needed drilling at the U-bolt, but I did enlarge the rear support mounting holes to .25". That way, I could use grade-8 bolts there.
Please don't drill your frame -- I had to re-drill one mounting bracket on my friction drive setup because my disk brake was in the way -- never touched drill to the frame.. Happy Riding from - Mountainman