[xj-s] Re: XJ-S Digest V2 #227

["Richard F. Dixon" <rfdixon@chs.cummins.com>]

>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.<

This is absolutely not true. There are many more factors than simply the
pressure on each end of the guide. One you are totally ignoring is the
fact that this is not a static but dynamic system. The motion of the
valve stem and the oil's wetting action on the metal can pump
considerable amounts of oil down the exhaust stems. Here at Cummins
Engine Co. we do a LOT of testing around oil consumption. I have been
heavily involved in developing an instrument that measures oil
consumption in real time. We run pressures in the exhaust manifolds of
our turbocharged engines of around 40 to 50 in. hg. That's about 20 to
25 psi above atmospheric. One of the problems we encounter occasionally
is so called oil 'slobbering'. This is when LIQUID oil dribbles out of
the end of the exhaust pipe while the engine is running. We have found
that this is caused by oil coming past the exhaust guide. Note that our
engines are not assembled loose. Our engines run from 500k to 750k miles
between overhauls. Hell, our warranty is 350k miles! Without getting
into the long and complex discussion of the causes and fixes of oil
consumption, let me just say that good valve stem seals don't really
seal oil, they meter oil. We spend many hours in test cells (@ $200 /hr.
+ fuel cost) to find the right balance between low oil consumption and
valve stem lubrication. The Jag engines use inverted bucket tappets so
there is no side loading on the valve stems. I would expect valve guide
wear to be very low on these engines. About the only materials that I
know that can run dry are chrome plated stems and bronze valve guides.
But they won't last 200k miles like the rest of the Jag engine if run
completely dry. The teflon valve stem seals you guys are talking about
are good seals. But be aware that if they work too well, you run the
real risk of a valve seizing in the valve guide from lack of
lubrication. So listen to the folks who spent time developing this
engine before you fix something that isn't really broken. The nitrile
seals ( if that is what they are and not neoprene) will last about 8 to
10 years in most engines. The teflon will last longer, but not forever.
Any polymer that you immerse in hot oil will age and harden. Ask a
chemist why.

On the subject of valve rotation, the collets that hold the valves
should not grip the valves. They should seat in the taper of the spring
retainers and the split ends butt together so that only the ring in the
retainer and the groove in the valve stem touch. Is that clear? It is
hard to explain. The valves should rotate when the engine is running.
The lobes of the cam are ground with a slight taper to make the tappets
rotate and some of that motion should get transferred to the valve
stems. Obviously this is occurring, because other than the occasional
loose seat, I understand valve wear is not an issue in this engine. It
is indeed a tribute to the designers of the valve gear that these
engines can go the life of the engine without a valve adjustment. I know
of no other mechanical tappet engine that this is possible.

Richard Dixon

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