I have SpeedPro brand GM 2.5 liter 4 cyl valves- p/n 2529 & 2530 (length is right, valve head is heavy unless modified, and exhaust side is larger diam) I'm not certain but I think the stem are chromed, and have guide liners. They do seem to work excellently. My understanding was that phosphor-bronze is the ticket, manganese-bronze and silicon-bronze are generally inferior. The guy that gave me advice overall for my project does 700 hp river boat race motors that are internationally exported, and uses these guide liners. Those motors see a lot of extended wide open use. I think David Vizard also unequivocally recommends bronze.