Valve guides in an aluminum cylinder head eventually wear out, leading to excessive oil burning. Oil leaks down the space between valve shaft and guide. Furthermore, a worn guide puts excessive side pressure on the rocker arm and camshaft, if it's an overhead cam. This will lead to premature wear on the rocker arm or camshaft. When rebuilding an engine, it is best to replace the valve guides--readily available at major auto parts suppliers--since new guides will afford years of trouble-free service.
Select a drill bit 1/16 of an inch smaller than the outside diameter of the valve guide, and carefully and slowly drill out the guide. Do not bore all the way through, but rather stop about 1/2 inch away from the end, in effect creating an inside shoulder in the guide. . Make sure your drill is slightly smaller than the guide itself because you do not want to chew through any aluminum in the head. Furthermore, use the lowest speed setting on your drill motor, and work very carefully so as to not accidentally drill into the head. The wall thickness of the guide, after drilling, should be about 1/32 of an inch.
Since aluminum expands when heated, place your aluminum head with the drilled-out guides in the oven set to 150 degrees Fahrenheit. The guide holes expand, making it easier to press out the drilled guides. Do not heat past 150 degrees, since head warping may occur at higher temperatures.
Work quickly but carefully, since you can get burned by the heated head. Obtain a deep well socket, slightly larger than the outside diameter of the guide. Then obtain or make a solid steel rod (called a drift pin or drift) slightly smaller than the the drill bit. Jim Kartalamakis recommends making a special drift pin with a smaller diameter section for an Alfa Romeo engine. This pin should be about 3 to 4 inches longer than the guide. Wear oven mitts, and place the drift pin inside the guide and the deep well socket on the other side. With a large C-clamp, press the guide out with the drift pin into the deep well socket. Having a helper or two will assist you greatly in this step. If the head cools too much, you can always reheat it.
