The valve guides in the Chevy head are cast in place, simply part of the cast iron of the head. Machine shops center up in the old guides, drill and ream them out, and install pressed-in replacements. If the head of the valve has a visible amount of wobble when it's pushed from side to side with the valve spring removed, the guide either needs to be sleeved with a thin bronze insert, or replaced. I used to do a lot of the ream & sleeve repairs, but recently, it's become more popular to replace the entire guide to repair a worn one.
Jerry
