Over time and repeated guide removal and refitting, the hole in the cylinder head can become too loose to retain the guide. At this point it becomes necessary to fit oversize valve guides.
A standard valve guide in an air cooled classic British engine is already 0.001 inch larger (interference fit) than the standard machined hole in the cylinder head to keep it nice and tight throughout the heat range from startup to heavy use. Oversized guides are available in sizes listed in excess of the standard guide size.
If replacement is being considered, some careful measuring needs to be carried out and the head machined accordingly. Once the guide is fitted to the head the bore should be reamed to the standard size to maintain the correct stem to guide clearance. Valve stems are normally sized accordingly with a little more clearance given to the exhaust valve stem. After fitting a new guide the valve seat may require to be recut. This whole process is generally beyond the average home workshop and best carried out by a specialist auto machinist.