Many cast iron cylinder heads came from the factory without any valve guide inserts. Instead they had a boss with substantial mass cast into the design and drilled for the valve to reciprocate through. If the guide is worn, a few different procedures can be taken to restore the proper clearance between the valve stem and the wall of the guide. The best approach is to install an insert into the boss. The head shop machine is used to drill the old guide diameter out, as seen in this photo, preparing it for an insert.