PS PUmp rebuild?

[Phil Rose]

At 4:02 PM -0700 5/9/05, Bernie Benz wrote:
>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,

The other o-rings can "perform well"? Heck, Bernie, you don't really 
know anything of the sort--you (and we) know merely that the removed 
o-rings don't _look_ obviously defective... Nor do you (nor we) know 
if the remaining, non-leaking seals will continue to perform for 
another 30 days (or 30 months). Anyone with common sense will suspect 
that the remaining o-rings will have a high probability of  either 
also having been installed improperly (if that's what you insist is 
the only possible reason for becoming leakers) or of becoming unable 
to seal effectively because of aging effects.  Now, if someone 
happens to enjoy a bit of gambling (don't you Nevadans have better 
ways to gamble?), then the idea of spending two or three hours 
R&R-ing the pump--for changing a single o-ring-- and  ignoring the 
other seven o-rings; well that's just another fun roll of the dice. 
But I'll stand by what I said: it's bad advice. That mantra of "DFI 
if IAB"  sometimes can be appropriate--but as I've said before, you 
push it to unreasonable extremes.

Phil



>  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


-- 

Phil Rose
Rochester, NY
mailto:pjrose at frontiernet.net