Originally Posted by 1QuikWS6
Just a thought (I'm not totally up on newer BMW engines) but can you simply knurl the existing guides as we did on muscle car V8's?
Yes you can. In fact BMW makes two sets of valves for the M52: 5mm and 6mm. Normally you'd need to replace valves anyway so you'd get the larger ones and bore out the guides slightly. I think that's the way BMW wants you rebuild the head. I talked to several shops and they said that's a really risky thing to do because there a good chance the valve guides are most likley not true. With the tolerances being so tight on the N52, here's a good chance of a significant amount oil getting through. They said its safer just to replace the guides since they're so cheap.