I have three B230 cylinder heads laying around, and am wondering if two of them are worth keeping or not. They have a fair amount of corrosion on the mating surface, where the solid metal of the head caps off cooling passages in the block. The aluminum is deeply pitted (1/16", maybe more) and the corroded area extends past the opening in the head gasket for the (nonexistent) water passage. There is also pitting in the thermostat gasket area, and one of them has heavy carbonized oil deposits in the camshaft compartment. How much thickness can be machined off of these heads without causing a problem? Or should I just scrap them out, save the cams tappets and shims? The third head I have looks almost like new, so even if I junk the two suspect ones, I'll still have a spare.