Re: [xj-s] Valve Stem Seals

["Michel Carpentier" <carpenti@math.jussieu.fr>]

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> From: Kirbert <palmk@mailer.gadcomm.net>
> To: Michel Carpentier <carpenti@math.jussieu.fr>
> Subject: Re: [xj-s] Valve Stem Seals
> Date: vendredi 24 juillet 1998 00:42
.
> 
> The two critical diameters are only one set of factors determining 
> whether the seals will fit the engine.  Are they pretty much the same 
> shape?  Will they snap onto the same groove on the inlet valve guide 
> that the stock seal snaps onto?  And what is done about the fact that 
> the exhaust guides have no such groove?  And are they short enough 
> not to interfere with the spring collar when the valve is opened?

Overall outside diameter is the same as original Jaguar "seals", i.e. they
both just fit inside the inner spring. Teflon seals sit 1.5 mm higher on
top of the guide than OEM (4.5 mm total) .This should not be a problem. I
don't know the exact figures for the V12, but on the XK engine (which uses
the same hardware),the distance between the top of the guide and cotter is
never less than 14.5 mm.
 Teflon seals do not require a groove. Snapped around the bottom section is
a flat steel spring which clamps the seal around the guide. Likewise a
smaller wire springs is wrapped around the thinner top section so you get a
tight fit on the valve stem. As you know, teflon is plastic but not
elastic.

> I can't see leakage past exhaust valves being significant.  It's a 
> small diameter, long sleeve, and close clearance.  Also, the 
> backpressure in the exhaust system shouldn't be THAT high.  Seems to 
> me this leakage would fade to insignificance when compared to piston 
> blowby.

It is not "back"pressure we are dealing with here but direct pressure from
still expanding gasses. In fact this pressure is enough to propel them all
the way to the back of the car and beyond through two restricting mufflers!
Certainly much higher than the atmospheric (or ideally less than
atmospheric) pressure on the other side of the guide. Valve stem clearance
is 0.05 to 0.06 mm. This amounts to a total leak area of about 8 square
millimeters or the size of a 3.2 mm hole. Try drilling a 3.2 mm hole into
one of your exhaust manifolds and see how much gas escapes. OK I admit this
is oversimplifying things a bit, but you should see my point. On an older
engine, with increased wear on the valve stems and guides, this leakage
becomes far from negligible. I have seen exhaust valves from well worn
engines: the stems can be blackened all the way up to the cotter grooves.
And often times, significant valve gear wear occurs long before
ring/cylinder wear. 
  
> Also, are valve stem seals supposed to seal in either direction?  I 
> thought they primarily keep oil out of the intake and exhaust tracts.

 On the intake side the pressure gradient is reversed (low pressure in the
intake ports) and oil tends to be sucked along the valve stems and into the
cylinders. This does not happen on the exhaust side except for what little
oil can seep down under the force of gravity when the engine is not
running. This accounts for some of the blue smoke on start up if you have
let the car sit for a while. As far as I know blowby control is the main
reason why many car manufacturers fit valve seals on the exhaust.

Michel Carpentier

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