Some have reported a tic over .500 lift with no issues with retainer to guide clearance. Others have said they've had contact with as little as .450 lift. So it's best to check. IMO, any type of performance build "SHOULD" have some type of positive valve seal on it, which requires cutting down the valve guide anyway. On my slant 6 head I have built, I machined the guides to accept positive valve seals and I have over .750" between the top of the guides and the retainers. Better safe than sorry.
