As standard production items there were two slightly different cylinder head castings usually identified as the GM and Coscast cylinder heads. There are many differences between the two head designs, some of which are by no more than reputation.
In consideration of the valve guides the two cylinder heads do have slightly different designs with the Coscast head using a thicker valve guide, however in both cases the guides are made from an iron material as is commonly the case with many production engines.
Iron guides need a generous amount of oil to lubricate them and prevent friction as the valves move through them. In race engines, which obviously run at higher speed, and temperatures, as well as using thinner oils, the lubrication supplied to the valve guides can be insufficient.
To prevent any problems we highly recommend fitting Colsibro bronze guides to all performance engines. Colsibro is a much better material to make valve guides from since it needs only minimal lubrication to operate safely even when fitted to high revving race engines.
We supply versions of our Colsibro guides for both the GM and the Coscast cylinder heads.