I think you can get the whole kit with all the seals for the gearbox through Fastlane for about $20. I was going to get a kit for my gearbox which has a very slow seep caused by flushing with ATF (I guess ATF back in '85 was a lot milder than today's ATF). It's a slow seep and I've only had to top off once in my nearly 7 months of ownership, so the seal replacement will be a rainy day project.

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280