Date:         Mon, 31 Jan 2000 20:53:48 -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...
Content-Type: text/plain; charset="us-ascii"; format=flowed

At 02:45 PM 1/31/2000 -0800, Mike Miller wrote:
>Please keep this on the list.  I don't understand hydraulic lifters after
>years of thinking that I did.  I hoping to learn something from these
>exchanges.

The "barrel type" hydraulic lifter as used in the Vanagon has four basic
parts:  A cylinder, a springloaded piston, a check valve, and a pushrod
socket.  It has the obvious major function of transmitting motion from the
cam to the pushrod; it also supplies oil to the hollow pushrod to lubricate
and/or cool the valve stem and valve train.  It is designed to (fairly)
rapidly elongate if there is any slack in the valve train, and to very
slowly shorten if the valves aren't closing fully.

Here's how:  The cylinder is the outer body of the lifter.  It's solid at
the bottom, open at the top (toward the pushrod) and has a hole and
machined ring in the side that lets pressurised oil from the gallery into
the inside of the lifter at all travel positions.

The piston is is inside, sitting atop a spring.  It has two peculiarities:
there is a ball check valve in the top which allows oil to get into the
bottom of the lifter but not out; and it is very precisely fitted to the
cylinder so that oil can *very slowly* leak out around the sides.

Resting atop the piston is the pushrod seat, and there's a circlip to keep
the whole assembly together.  The seat has a hole in the middle which
matches the hole in the end of the pushrod, and pressurised oil constantly
flows through this into the head, and then back through the pushrod tubes
to the sump.

If you leave a lifter alone the spring will extend and shove the piston up,
in turn shoving the pushrod seat up until it hits the circlip and has to
stop.  If the lifter is immersed in oil at this time, oil will be sucked
through the check valve into the chamber below the piston, and be trapped
there.  If you now try to push the piston down again, it acts roughly like
a solid bar; the oil trapped under the piston prevents it from
descending.  If you continue pushing strongly, the piston will very very
slowly descend as oil leaks out around the sides.  When you're assembling
lifters in the shop you use a cutoff piece of pushrod and a vise to slowly
collapse the assembly far enough to get the circlip in.

In the engine, again assuming that the gallery is full of oil, the lifter
will expand and suck in oil until it takes up all the clearance btw lifter,
pushrod, rocker, and valve.  It will push against the valve with the
strength of the internal spring, and the valve will push back with the
strength of the valve spring.  The valve spring should be strong enough to
overpower the lifter spring, and the valve will remain closed.

When you operate the engine, if any slack develops in the valve train, the
lifter expands to take it up.  Or, if the clearances change so that the
valve is not quite closing, the valve spring pressure will slowly collapse
the lifter until once again the valve closes completely.

So you see that in the ordinary way of installing a lifter, you place it in
the engine in the fully extended state, then you take up the slack in the
sysem and then continue to close the adjustment a specified amount.  This
forces the valve open, but it gradually closes again and leaves the system
able to adjust itself either longer or shorter as needed.  After that you
go away and forget about it forever.  It takes care of itself.  Whenever
you stop the engine at least one of the valves will be open which will
gradually collapse that lifter, but it will pump up again as soon as you
run the engine.  Because of the horizontal position of our lifters they
have trouble purging air bubbles, which is why it's important to use an oil
filter that prevents oil from draining back out of the galleries -- you
want that oil available instantly to fill whatever lifter collapsed overnight.

That's how it 'sposed to work.  If a lifter gets dirty, of course, it will
stick and not adjust itself properly -- but some Vgon engines with new
everything in the valve train are not working properly.  You have to adjust
them so the lifters are really fully extended, or they kill the
compression.  Nobody so far has jumped up with a precise explanation, but
the fact is demonstrable.  Come to Providence if you want to see it
happen.  I have my theories about this, but they're not proved (or
disproved) yet.

Does that help?
david
David Beierl - Providence, RI
  http://pws.prserv.net/synergy/Vanagon/
'84 Westy "Dutiful Passage"
'85 GL "Poor Relation"