Date: Sat, 8 Jan 2000 04:31:57 -0500
Reply-To: David Beierl <dbeierl@IBM.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: David Beierl <dbeierl@IBM.NET>
Subject: Re: 6 mils of lash...
In-Reply-To: <3876ED05.11AE@warwick.net>
Content-Type: text/plain; charset="us-ascii"; format=flowed
At 02:53 1/8/00 , John wrote:
>David,
>
> How can something with the precise tolerances of a hydraulic lifter
>produce such adverse & varying results ?
>
>Regards, JP
That's the headscratcher. As I said before, the way it looks to me now is
that there's been a goof in the specs and the lifter internal springs are
proving strong enough to open the valves very slightly. But that's a hard
notion to accept. I'm a pretty smart guy, and I can't see how oil pressure
could be affecting the operation of the lifters -- but smart doesn't
necessarily make up for experience. But as it appears to me, the extending
force on the lifters is entirely provided by the internal spring. When the
spring pushes the piston out, it sucks oil into the lifter barrel through
the check ball until the piston is stopped by the pushrod hitting the
rocker arm. All the oil pressure in the gallery does is a) supply oil to
be sucked into the lifter and b) supply oil to flow through the pushrods
into the head/valve-train assembly. When the valve pushes back, the lifter
acts like a solid unit b/c the oil can't escape through the check
ball. The tiny clearance btw piston and barrel allows a slow leakdown
which over time will allow the lifter to shorten if necessary (and causes
the lifter to collapse while the engine is shut off if a valve is pressing
on it. But a clean lifter should pump up rapidly once the motor starts
rotating). Lifters mounted horizontally like ours are subject to getting
trapped air bubbles, because the outlet is on the side instead of the
top. I have verified that a rock-solid, hand-assembled and bled lifter can
become soft, i.e. get a bubble in it, while the engine is sitting. I have
also verified that it will self-bleed and become rock-hard again with a
period of use, even though it looks to me as though it shouldn't.
I would love to get hold of a lifter with a softer spring in it to see what
effect it would have. Or for that matter to get a measurement of the force
available from a fully-compressed lifter spring (i.e. as far as it can
compress inside a working lifter) and see if that force is enough to
overcome the preload on the valve spring and cause the valve to lift
slightly. I'm not in a position to make those measurements as such, but I
am able to remove and replace lifters in my engine as Boston Bob has
provided me with collapsible pushrod tubes. My best gedanken-experiments
convince me that this is the key to the matter, but it awaits the actual
test. And I'm going to have to put the tin back on until springtime, which
makes this sort of thing very difficult.
One possibility that occurs to me is that the valve spring preload is
dependent not only on the rate of the valve spring itself, but also on the
length of the valve and the depth of the seat. But I don't have a feel for
the tolerances and whether this would be a first, second, or third order
effect.
And there is always the chance that there is some dynamic effect that I'm
overlooking, or that I just don't understand. I don't think so, though.
david
David Beierl - Providence, RI
http://pws.prserv.net/synergy/Vanagon/
'84 Westy "Dutiful Passage"
'85 GL "Poor Relation"
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