PS PUmp rebuild?

[Bernie Benz]

Phil, there are big differences between the physical requirements of
elastomers in dynamic vs static sealing applications.  Both yours and
David's quoted experiences confirm mine, the condition of 250K O-rings
removed from these cylinder heads, leakers or not, that the O-ring material
is elastic enough to perform well as a static seal, hence the failure had to
be caused by locally thinned areas within the circumference of the ring due
to the inconsistant shear stresses imposed on the O-ring during
installation.

DFI if IAB!  Bernie

> From: Phil Rose <pjrose at frontiernet.net>
> Date: Mon, 9 May 2005 16:46:01 -0400
> To: Bernie Benz <b.benz at charter.net>
> Cc: <PeterBergin at aol.com>, 200q20V mailing list <200q20v at audifans.com>
> Subject: Re: PS PUmp rebuild?
> 
> At 11:28 AM -0700 5/9/05, Bernie Benz wrote:
>> No!  Is best to isolate the leak to a specific cylinder head before pulling
>> the pump, as you may not have to pull the pump
> 
> Good advise.
> 
>> or at least will know just
>> which one needs to be opened and the O-ring replaced.
> 
> Bad advice! Once it's decided you need to remove the pump to replace
> one, yopu ought to replace _all_ those cylinder-cap o-rings. If
> you're in the midst of a pit-stop and thus a few minutes extra time
> is critical, then perhaps just replace the one that leaks--but
> ordinarily it's a penny-wise pound-foolish way to go. These o-rings
> most likely share a common "history"--meaning age and/or poor
> technique during installation. There's no reason to expect that the
> other o-rings are not near the failure point.
>> These O-rings don't
>> wear out, just fail because of poor installation.
> That's not necessarily fact--just your considered opinion. My opinion
> is: O-rings, like any synthetic polymer material, are subject to
> age-related embrittlement (this derives from the inevitable changes
> due to a phenomenon called "free-volume relaxation"-- sometimes
> called "enthalpy relaxation"); and that changes the rubber's modulus
> and its ability to recover from compressive deformation.  Couple that
> with constant thermal expansion/contraction and the material can have
> poorer and poorer sealing ability over time. This scenario doesn't
> even account for possible effects from contact with the hydraulic
> fluid.
> 
>> I agree with Bernie that there can easily be damage (from excessive
>> stretching) caused by inadequate lubrication during installation.
>> But if one of those 8 o-rings suffers from poor installation, what
>> are the chances for the other seven?
> 
> Phil
> 
> 
> -- 
> 
> Phil Rose
> Rochester, NY
> mailto:pjrose at frontiernet.net