From: http://www.engintecs.com/html/how_its_done.html
As it happens with every head we work on , after cleaning, checking dimensions and straightness , making sure that they are not cracked, inspecting valves and springs, the first thing we do is take care of the Hearth of a head: the Valve guide. While Go-no-Go or Feel-the-play methods are used by others, these are not good enough tests for us. We use a specially-designed Gage Set to check for not only Size but also taper, bellmouth or other irregularities of the guides' ID and we do this by using the valves as a reference, achieving measurements with an accuracy of .0002”. If guides are out of
spec (most of them are, unless the engines are fairly fresh) we replace them using either Cast Iron (as most OE) or Manganese Bronze.
While Cast Iron guides are generally made out of better material than OE , Manganese Bronze ones are better, having excellent high strength characteristics and offering better heat transfer from the guide to the head. Guides are sized through a Honing process which produces a superior Bearing Surface that will retain oil better and last longer .
There are shops that use alternative methods to “repair” guides using Thin-wall guide inserts (Liners). It is a quick, cheap and easy way to do it but they just don't last. Each one of the heads we've seen with liners installed were badly worn out and they did not have half of the OE lifespan. If you want this kind of work done on your engines you've come to the wrong place ! Guides in aluminum heads are “replaceable” so they should be replaced, if needed, during engine service.