Ok, for anyone else that encounters this problem:

Some of the early R129 cars have a steering gear box that does not exactly match any pictures in any documentation for the car.

The early cars have an o-ring seal that is not included in the gearbox re-seal kits available for the car.

The seal in question is the o-ring that seals the "cap" that is held in place by a snap-ring on the top of the steering box. This cap provides access to the nut on the top of the pitman shaft. I understand that on newer style steering sectors, this cap screws on...

After careful measurement, I purchased a suitable replacement at McMaster-Carr. They had them in stock for me to pick up at their will-call counter.

I used a Viton o-ring that is good from -40 to +400 degrees F.

McMaster-Carr's part number for a package of 10 is 9263K117

Specifications:

Standard Metric O-Ring
Cross-Section Shape: Round
Width: 2mm
Inside Diameter: 26mm
Outside Diameter: 30mm
Material: Viton


My shop installed it in my gearbox that was on their workbench while I watched. It seemed to be a nice fit. I don't anticipate having any problems with it.

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02 S500
92 500SL
92 400E (Sold)
87 300E (Sold)
83 300D Turbo (Sold)
75 300D (Sold)
74 240D (Sold after 20 years)