The Boxes that I have are Vtwin 4 years old. Yeah I know I can't afford new boxes yet:(. I have installed them temporarily on the heads , and have not felt anything binding. No sign of the valve spring retainer touching the box. I have not run them, I don't want to fully assemble then find out I have a problem, then tear it all apart again. Did some test cutting on my old old chromed rocker boxes. The material is pretty hard. Is there a best method to cutting away some of the box? The install instructions do not list a best method. The height of the SS valve spring retainer is about 1/16th taller than the ones I had on the bike.