Bronze valve guides are very nice for extended wear life, but they do have a slight problem with increased thermal expansion compared to iron guides. This requires machining the bronze guide to have slightly more running clearance with the valve stem to prevent valve seizure when the guides run hot. One function of the guide is to transfer heat from the valve stem into the head for cooling the valve. Smaller running clearance promotes better heat transfer.

Here we have an alternative to the bronze valve guide. You can install a thin bronze sleeve in the original iron guide to give the desired bronze bearing surface while avoiding the problem of excess thermal expansion. For engines with cast in place valve guides this is often done to reduce machining cost. For engines with replaceable guides this method can still be used to achieve minimal valve stem running clearance without excess thermal expansion or valve seizure.

See the Goodson Tools web site for details of the tools and methods used for installing valve guide liners.
Also see the Hastings Manufacturing web site for more notes on the use of valve guide liners.