I recently had a rebuild on a SBC that had less than 1000 miles on it. When the builder went thru the heads it was determined that (3) exhaust valve guides were borderline in spec (on the loose side). Well there were problems with the rebuild and needed to be tore down after 500 miles and now I was told the (3) guides that were borderline out of spec are now way out of spec and suggested they all be replaced (Aluminum heads) with the appropriate bronze replacement guides. Builder really don't know the cause but mentioned that the combination of the borderline out of spec, high spring pressure and previously breaking in the cam MAY of caused abnormal wear. Wondering if anybody can comment on this and also what is the typical Valve Stem to valve guide clearance ? Thanks.