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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] [xj-s] Bywater on Valve Stem Seals
------- Forwarded Message Follows ------- From: Roger Bywater <101351.1414@compuserve.com> Right, I can see how anyone familiar with pushrod engines might expect stem seals to be an important issue. Overhead cam engines with inverted bucket tappets like the V12 are intrinsically far less likely to lose much oil down the valve guides. If you think about the tappet as being like an umbrella with the valve stem, collets and everything else tucked up under it there is not much opportunity for oil to get onto the stem in the first place. The inlet valve is subject to vacuum so some sort of seal is probably necessary to keep things under control. However this must not be overdone because the valve stems do actually need some lubricant however slight. I recall that an experimental change to put more vacuum on the breather system of the XK 4.2 drastically shortened inlet valve life to a matter of hours and required the valve stems to be tufftrided in order to survive in what had become a boundary lubricant condition. Exhaust valves are not subject to any significant vacuum and at least some of the time there will be slight pressure in the reverse direction up the stem so, apart from that promoted by reciprocatory movement, the opportunity for oil flow downwards is rather limited. Start up smoke may well be due to slight oil leakage down the stems while standing but the amount lost this way cannot be much. Stem wear would surely be a serious problem if hot exhaust gases were actually flowing up the valve guide. The idea that leakage of this sort could have any effect on crankcase pressure seems hard to accept and I would imagine that if the leakage were of such magnitude any seal over the end of the valve guide would have a hard time surviving anyway. Regards, Roger. ------------------------------------ -- Kirbert | If anything is to be accomplished, | some rules must be broken. | - Palm's Postulate
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