Valve Guide Inspection reveals how much play is in the valve.   This spec is one of the tightest clearances in the engine, with less that .0008" of an inch.   If the valve guide measures over twice that, around .0020" of an inch or More, the guide MUST be attended to.  There are a few methods to remedy.  The classic method is to replace the valve guide, which requires re centering the valve seat.  The method is to first remove and replace the guide.  Second is to ream and hone the new valve guide to STD size, generally .0006-.0008" larger than the valve stem.   Once to size, a carbide pilot tool is inserted into the new guide hole.  This pilot tool is what the cutter system is mounted to when machining your new valve seat geometry.   This is a complete seat and guide job, which is an extensive repair in the machine shop.  Valve guides are $20 each to replace, ream, and hone.  Recutting geometry on a valve seat adds another $10 per seat.   This equals to $30 per Seat and Guide repair, when replacing guides.  You CAN then have a stock valve in the casting, but you will most likely have oversize guide bores (.002" OS).   The casting may never be rebuilt again if guide work is required in the future. 

The SECOND way of repairing worn valve guides is to simply oversize ream the existing valve guide, and replace the valves with oversize stems. This oversize reaming service is only $5 per guide, plus the price of new oversize stemmed valves.  Since the machine shop no longer will need to clean and reface your old valves at $5 each, that part of the rebuild budget can be used to offset to cost of this optional method.  (You can also choose Stainless Steel Valves at this time!)  Since the valve guide center is still the same after reaming oversize, there is no major valve seat geometry to reshape, and the valve seat is concentric enough to reface with the piloted seat grinder.   This is the economical way to keep the heads together with minimal repair when your guides are completely shot.  The head may be rebuilt again after this service, as the original valve guides have never been out of the cylinder head and new .002" OS guides could be used in the future.