Date: Wed, 27 Sep 2000 16:44:36 -0400
Reply-To: "1980 VW Westfalia \"Pokey\"" <pokey@VANAGON.ORG>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: "1980 VW Westfalia \"Pokey\"" <pokey@VANAGON.ORG>
Subject: Re: How long to do a valve job and replace the seats?
Content-Type: text/plain; charset="iso-8859-1"
I guess I am OK then. The engine is strong, compression is at manufacturers
specifications; 130, 130, 125, 130. According to Bentley, the requirements
for a new engine are 131 PSI and there is a 40 PSI range. Pokey does have a
moderate appeitite for oil, but it is more a function of some small leaks
rather than "consumption" per se.
Thanks,
Chris Gronski
Toronto, Ontario,
'80 Westy "Pokey"
'87 Chevrolet Sprint (Ice Racer)
'91 Pontiac Firefly Convertible
----- Original Message -----
From: "John Rodgers" <inua@hiwaay.net>
To: "1980 VW Westfalia \ Pokey\" <pokey@VANAGON.ORG>
Cc: <vanagon@GERRY.VANAGON.COM>
Sent: Wednesday, September 27, 2000 3:55 PM
Subject: Re: How long to do a valve job and replace the seats?
> Chris, since you have no coolant to worry about, there are three things to
be
> concerned about on the top end of the engine. Valve stem/guide wear, valve
> face/seat errosion, and cylinder/ring/ring lands wear.
>
> In the aircraft industry, on the average general aviation airplanes that
run
> flat 4 and 6 cylinder opposed aircooled engine, a differential compression
test
> is done that will tell if the engine needs a top overhaul...ie, valve job,
and
> rings. If oil consumption is high, then it's time, no matter what. But if
oil
> consumption is relatively low or modest a different approach was taken.
>
> The compression test is done static. The engine is rotated to top dead
center on
> the compression stroke. This ensures that both valves are closed. The
propeller
> is held so the incoming air pressure doesn't kick the engine over and
hurts
> someone. Then compressed air is pumped into the cylinder to a
predetermined
> pressure. As I recall, it is 100 PSI. There are two gages that are read.
The
> input pressure, and the pressure that is held by the cylinder. Cylinder
pressure
> down to 70 psi is OK. At 60 PSI it's time to overhaul the cylinder. If on
> examinig the spread of pressures betwen cylinders it is disparate, ie not
fairly
> uniform, it's time to look at over hauling all cylinders. You don't want
an
> engine running with uneven pressures all over the place, hard on the
engine.
> Power would be erratic.
>
> During the compression tests, aircraft mechanics listen to the engine
under
> pressure. If a cylinder is going or gone, there will be a characteristic
hissing
> sound internally. Listen to the exhaust pipe. If it is loud, then exhaust
valve
> is leaking. If soft,(you can't hear it as good through the induction
system) it
> is intake valve leaking. If neither, but you can still hear air loss,
it's
> rings that need replacing.
>
> Now I'm not sure how one would apply the differential compression test to
a flat
> VW engine, but a standard compression test would tell a lot. If you turn
the
> engine over, and the gage won't hold pressure, then your pre-emptive
strike
> approach to the head maintenance certainly would be appropriate.
>
> Good luck
>
> John Rodgers
> 88GL Driver
>
> 1980 VW Westfalia \"Pokey\" wrote:
>
> > I have 187,000 KM / 115,000 miles on my air-cooled and I'm thinking
about
> > pro-actively doing a valve job, replacing the valve seats, and replacing
the
> > valve cover gaskets while I'm at it. How long should this take? Should I
> > just wait until 150,000 miles or is it worth doing now? Pokey is going
away
> > for the winter shortly is it better to wait until spring?
> >
> > Thanks,
> > Chris Gronski
> > Toronto, Ontario,
> > '80 Westy "Pokey"
> > '87 Chevrolet Sprint (Ice Racer)
> > '91 Pontiac Firefly Convertible
>
>
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