Date: Wed, 27 Sep 2000 14:55:35 -0500
Reply-To: John Rodgers <inua@HIWAAY.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: John Rodgers <inua@HIWAAY.NET>
Subject: Re: How long to do a valve job and replace the seats?
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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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