Knurling valve guides is explained here.

I have had knurling done and had good long-term results (10K+ miles) and I have also had short-term problems (less than 2K miles). Stuart's advice is good but it relies on you finding a shop that does not mind replacing (or, putting bushings into) the guides. The easiest thing for a shop to do is to knurl the guides and get the head out of their shop. As I recall, the guides are a press-fit into the head.