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2008 SL55 Front axle valve unit replacement o-ring identification
I'm having a hard time finding/identifying replacement o-rings for the threaded plugs in the front axle value unit of my 2008 SL55 AMG. The o-ring ruptured spectacularly while performing a level calibration in Xentry with a large amount of hydraulic fluid spewing everywhere.
This is the newer generation of the ABC front axle valve block and the o-rings are not part of the four solenoid/valves but instead are threaded plugs with a spring-loaded recess in the tip that appear to work via pressure possibly a bleeder or relief valve. There are two of these threaded plugs next to each other in the side of the valve block. Each plug has two o-rings.
The o-rings appear to be more like d-rings but I'm not sure if they have just deformed over time or were originally this shape. Unfortunately the two plugs use different size o-rings and the o-rings are damaged so measuring them is imprecise.
2008 SL55 AMG Front Axle Valve Unit highlighting two threaded ports where the threaded plugs are inserted. 2008 SL55 AMG Front Axle Valve Unit - Additional pictures for reference 2008 SL55 AMG Front Axle Valve Unit - additional pictures for reference - alternate angle showing threaded ports and Front axle pressure reservoir A 2008 SL55 AMG Front Axle Valve Unit - additional pictures for reference - alternate angle showing threaded ports and Front axle pressure reservoir B 2008 SL55 AMG Front Axle Valve Unit - additional pictures for reference - top 2008 SL55 AMG Front Axle Valve Unit - additional pictures for reference - alternate angle showing both dual solenoid housings 2008 SL55 AMG Front Axle Valve Unit - showing main ports and markings 2008 SL55 AMG Front Axle Valve Unit - larger threaded plug - side profile without o-rings 2008 SL55 AMG Front Axle Valve Unit - larger threaded plug - top profile with stamped identifier 2008 SL55 AMG Front Axle Valve Unit - smaller threaded plug - side profile with o-rings 2008 SL55 AMG Front Axle Valve Unit - smaller threaded plug - top profile with stamped identifier 2008 SL55 AMG Front Axle Valve Unit - larger threaded plug - ruptured o-rings - top 2008 SL55 AMG Front Axle Valve Unit - larger threaded plug - ruptured o-rings - side 2008 SL55 AMG Front Axle Valve Unit - identifiers - MB Part A 221 320 02 58 4713 2008 SL55 AMG Front Axle Valve Unit - identifiers - number on one of the dual solenoid housings 2008 SL55 AMG Front Axle Valve Unit - identifiers - barcode on side of the valve block body
I also have pictures of the Measurements (I only have a cheap Harbor Freight digital caliper so not confident of measurements):
Larger threaded plug - slot closest to threads - ID - 15.64mm | 0.6155in
Larger threaded plug - o-ring closest to threads - height - 2.71mm | 0.107in
Larger threaded plug - o-ring closest to threads - thickness from ID to OD - 1.69mm | 0.665in
Larger threaded plug - slot closest to bolt head - ID - 16.43mm | 0.647in
Larger threaded plug - o-ring closest to bolt head - height - 1.28mm | 0.0505in
Larger threaded plug - o-ring closest to bolt head - thickness from ID to OD - 1.79mm | 0.0705in
I can't help with specific part numbers. Many times o-rings aren't listed as separate parts, so you either have to get creative/resourceful or pony up $$$$ for a complete part.
Thankfully, there are standard engineering formulas for calculating o-ring size to arrive at a desired compression ratio, aka "squeeze". Typical squeeze range is between 8% and 35%.
Since this is a fairly high pressure application, I'd shoot for a high-ish squeeze percentage. Once you calculate your target o-ring size, you'll probably find that your exact desired size isn't readily available, so I'd get 2 o-rings for each slot from available sizes - one slightly below your desired size and one above. Try the larger o-ring first, and if it's just too tight to get installed go with the smaller size.
Be sure to get o-rings made of material that's compatible with your fluid.
Engineers usually design o-ring fittings to use commonly available o-ring sizes, so you should be able to find something that'll work fairly easily and that's not terribly expensive.
Last edited by brucewane; Dec 6, 2024 at 12:07 PM.
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