The middle intake valve guide will need to be milled on all 660 3 valve heads if you run a cam with 0.400" lift or higher (stage 2 or stage 3). It's very easy to mill the guide down 0.050" whenever the bike is apart for new springs and whatever else you will be doing. Remember to place the valve seal on the side of a grinding stone and take 0.050" off the seal before you install it on the middle intake guide. Good luck.I could have sworn I read you had to mod the center intake guide and seal before installing a cam and springs. Recieved an email back from hot cams saying you only do that on Grizzles and Rinos. Any thoughts?