PS PUmp rebuild?

Bernie Benz b.benz at
Mon May 9 19:02:36 EDT 2005

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

DFI if IAB!  Bernie

> From: Phil Rose <pjrose at>
> Date: Mon, 9 May 2005 16:46:01 -0400
> To: Bernie Benz <b.benz at>
> Cc: <PeterBergin at>, 200q20V mailing list <200q20v at>
> 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

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