The instructions say to use the brush with the guards on.  I have a 4 1/2" Porter-Cable and it is a very close fit.  The brush sparks on the guard but cuts fine.  I guess the 4" wheel would have been better in my situation.  You'll probably have to take the guard off if you do much with this brush.  I also have a smaller cup brush and don't have any issues like this one.  I gave it Stars because it does what it is supposed to do.