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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] [xj-s] Valve Stem Seals
David Low wrote: >The Jaguar engines, from the outset were designed to be relatively 'loose', That's a myth. Jaguar valve stem clearances fall well (0.025 to0.10 mm for the XK engine; 0.05 to 0.06 mm for the V12) within the industry standard. To quote a few examples: BMC B series: Intake 0.0381 to 0.0635 Exhaust 0.0508 to 0.0762 Renault 5: Intake 0.020 to 0.068 Exhaust 0.05 to 0.075 Ford 2l Twin Cam:Intake 0.02 to 0.069 Exhaust 0.046to 0.095 Ford V6:Intake 0.020 to 0.063 Exhaust 0.046 to 0.089 Honda Civic:Intake 0.025 to 0.05 Exhaust 0.05 to 0.075 (I don't have my Chilton at hand but I am pretty sure the figures from American manufacturers are quite similar). I'm afraid the only loose items on the jag engines are (unfortunately and respectively) the tappet guides (XK) and the valve seats (V12) >and always burned/used/consumed some oil. That's true. Until 1968 Jag engines had no valve seals at all, and when they did fit some on the intake side, all they could come up with were the crummy ones we all know, carried over to the V12. Bloody pathetic! >If you start reducing the oil feed to stems, especially exhaust valves, you >are going to cause localised overheating. When your engine is running, the pressure gradient between both ends of the exhaust valve stem is such that absolutely no oil can find its way between valve and guide. The only flow there can be is in the other direction, namely blowby gases. The only time oil finds its way down the guide is at a stop. Given time, it will seep down onto the back of the valve and --should the latter be open-- thence into the combustion chamber. When you start the engine again, the oil in the cylinder will burn (blue smoke), that on the back of the valve will cake it up, part of what's between valve stem and guide will be blown back whence it came and the remainder will cook and produce a nice varnish coating on the valve stem. Varnish has no known cooling or lubricating property. I am confident that, in a not so distant future, Kirbert Palm will find a way to divert the flow of pressurized oil so it constantly cools and lubricates the guides without leaking into the cylinders(may I suggest we replace the cotters with some sort of ball bearing so the valve can be rotated via a ratcheting tappet, an oil feed hole half-way down the guide, a helical groove in the stem and a flame proof seal at the bottom of the guide?). In the meantime, if you want to lubricate your exhaust valve stems properly, you will have to stop your engine for a half hour for every few minutes of running. If the exhaust side is not normally lubricated, why would you want the intake side to be, which is subject to much less stress and heat? But assume you would. It would stand to reason that you would want to reduce the oil flow progressively as the engine wears and the clearance increases. It appears that this is the exact opposite of what the original seal achieves, since it loses all effectiveness after a few thousand miles! Let's face it: the sole purpose of intake valve seals is to prevent ANY oil from being sucked down that way. >The original nitrile seals do wear, and are known to be a problem, and they >also get brittle, again overheating?. However we have few problems here in >the UK, and I'm sure the lower ambient temperatures must part of the >reason. Leaky valve guides are rarely perceived as a serious problem as they usually do not have a direct effect on the running of the engine. But they are responsible for increased oil consumption( dirty plugs, combustion chambers and valves),and increased blowby (dirty oil, plugged breathers, leaky gaskets). I rest my case. Cheers. Michel Carpentier
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