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Bob Koch has requested by seperate email copies or references of the engineering documents I use to support my assertions regarding valve materials, stating "You adhere to several positions that are at variance with textbook and repair manual literature that I have." The report I refer to cracking valve seats being caused by knock does not address valve temperature directly but is an interesting insight into engines of the 1930’s. It is "Problems in the Development of a High-Speed Engine" by Sparrow in the SAE Journal volume 36, No. 2, 1935. It is available for a copying charge from SAE.
I didn’t intend to make this into a large technical dissertation, I just wanted to highlight the danger of knock being a cause of seat cracking and that the lower conductivity of modern aftermarket valves may contribute to the problem since hot exhaust valves promote knock. However, since questions remain I have written up a response that is too large to post on the forum, so anyone interested in my response as well as the original 1930’s engineering paper on valve seat recession can email me and I will fwd. My email is listed in the roster on this site.