Date: Fri, 12 Dec 1997 05:34:37 -0400
Reply-To: mark keller <kelphoto@BRIGHT.NET>
Sender: Vanagon Mailing List <vanagon@gerry.sdsc.edu>
From: mark keller <kelphoto@BRIGHT.NET>
Subject: Head Corrosion prevention Discussion
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Well,
I am still in search of data that explains why some heads are replaced
because of corrosion,and other's have no leaking problems what so ever.
This differance, suggest to me a variable exists.
I reinspected the "non-leaking" heads which my local VW dealer has. The
corrosion and pitting are limited to the contact area of the outside
perimeter seal. The corresion is very consistent both around the seal
contact and "across the seal" contact. An area of corrosion was present
exactly beneath the seal, not just inside the, where coolant and seal meet,
but the complete width of contact area. There were about five pitted
craters, .1mm deep x .2mm across, in random areas. On one head the
corrosion seemed wider, but not any more advanced, in the region from the
water hose nipple, as opposed to the side which a flange is bolted to. This
region is essentially extended from the compression chamber edge closesest
to the narrow end of the head.
There was no corrosion whatsoever on any areas that had coolant flow. Tim
Smith posted about "crevice corrosion" being the actual name given to the
situation where coolant that gets trapped beneath the outer waterjacket
seal, and then quickly becomes depleted in its anti-corrosive properties,
and accelerated corresion occurs at the point of contact, in this case,
beneath the flexiable outer seal.
I still can't come to a conclusion about how the coolant gets beneath the seal.
It would seem that if seal edge is curled up,as indicated by not being
100% backed up, coolant flow is still presents.
Another possibility, If the head contact "flexes at the seal" between cold
and hot tempatures, ie the contact point of the head and the seal is
"rocking" back and forth by virture of expansion, then some coolant could
be present at the region, of shifted contacted, and there is a trapping of
coolant.
Another possibility would be in driving conditions where the motor is
continually in a low operating eniviorment, trips of 6 miles or less and or
not driven on a daily basis, a kind of "coolant caking" may occur. Coolant
would cake,thicken, or otherwise get seperated from his buddies, and
quickly lose anticorrsive strenth, and start muchin on aluminum, aka
crevice corrosion, and then remain at a corrosive state, until the engine
was brought up to operatining temperature and driven for some period of
time that in essence disolved the cake and then replenshied the
anti-corrosive properties by allowing fresh coolant to move into the area.
The tech mentioned that re-torquing the head was a precaution he practiced,
he babbled somthing to the effect that heat cycling, ie hot & cold cycles
of normal operation was a factor in reducing the torque of the head bolt. I
have no input on this. Would sombody email me about it.
My current theory of "hydraulic pressure", generated by the water pump,
which forces coolant beneath the seal before the engine warms up is still a
possibility.
At this point, I would like to suggest that anyone interested in helping in
any way and those having experience to do multivariate regression analysis,
contact me. The idea would be to get a response questionaire to "profile"
listees who have had multiple corrosive failures, and those who have had no
problems. This would move us from theory, to variables which correlate to
corrosive head failures. I'll post this request separately, incase a
interested party in helping doesn't read the techno theroy pieces.
Respectfully,
Mark Keller
91" Carat WE
"Lazarus"
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