Fwd: servotronic rack rebuild

[Bernie Benz]

Begin forwarded message:

> From: Bernie Benz <b.benz at charter.net>
> Date: March 14, 2006 10:18:50 AM PST
> To: "Miller, Chris" <chris.miller at infofoundry.com>
> Cc: <dennis at seaweedmonster.com>, 200q20V mailing list  
> <200q20v at audifans.com>
> Subject: Re: servotronic rack rebuild
>
> Hi Chris,
>
> Thanks for the link to Dennis’ great photos of the ZF rack internals!
>
> Excepting for the addition of the three restrictor plugs added to  
> the valve
> housing, this Servotronic rack appears to be identical to the later 44
> chassis ZF racks, as used in the ’91 200Q-20V, of which I’ve  
> overhauled
> several.
>
> My further comments, as you requested.
>
> Excepting for the high pressure piston rod seal these racks are  
> bullet proof
> and IMO never need to be totally disassembled as Dennis has done. I  
> only go
> in far enough to remove and replace the rod seal and boot.  
> Apparently rod
> seal failure was Dennis’ reason for his overhaul.
>
> The complicating factor (which I have resolved by redesigning the  
> rod seal)
> is that ZF will not sell separately or include this rod seal  
> assembly in any
> overhaul kit, not even for independent rebuilders. They only sell the
> complete kit for the older 44 racks which employ a different rod seal
> housing design. I’ve talked with the ZF US rebuild facility  
> engineer about
> this, they are very protective of their rebuild business and will  
> not sell
> parts for any of their later racks. Dennis, what are you using for  
> this seal
> replacement?
>
> In these later racks ZF has not replaced the poorly designed and  
> prone to
> failure (read cheap) hard plastic rod sealing element that they’ve  
> always
> used, rather just repackaged the same part into a new swaged together
> housing (pictured). It is the identical plastic part as is included  
> in the
> rebuild kits for the early 44 racks and, IMO not worth installing.
>
> For my last overhaul, I have disassembled the swaged together seal  
> housing
> assembly and replaced the ZF plastic seal element with an  
> elastomeric Quad
> ring backed up with a nylon back up ring. With no leaks for a year  
> now, IM
> optimistic O, with this new rod seal the rack should never again fail.
>
> Dennis, feel free to post this as a reply to your AW post.
>
> Regards, Bernie
>
>
>> From: "Miller, Chris" <chris.miller at infofoundry.com>
>> Date: Tue, 14 Mar 2006 11:01:25 -0500
>> To: "Bernie Benz" <b.benz at charter.net>
>> Subject: servotronic rack rebuild
>>
>> Saw this on Audiworld; thought you'd have comments.
>> http://forums.audiworld.com/a6100/msgs/57661.phtml
>>
>> How's the new A6 working out?  I just bought a cheap high mileage C4
>> wagon needing a few parts...
>>
>>
>> ---
>> Chris Miller
>> Training Development
>> [i] The Information Foundry at Robert Bentley, Inc.
>> Tel: 617.528.4113
>> Cell: 617.875.3792
>> mailto:chris.miller at infofoundry.com
>> http://www.infofoundry.com
>