I think on the transmission the drain is 13 mm and fill is 14? at least the fill was loose with a 13 mm, but it worked. You do not need to buy a wrench set just use a bolt, with 2 nuts tightened together on it. You put the head in the drain plug and use a wrench on the inner bolt to loosen, and then use the wrench on the outer to tighten. Mine is a gas W126 with manual tranny. Just put in a new clutch. What a job that was!!!!

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Currently
1991 Mazda Miata 174000
1991 Subaru 189,000 Winter beater

got rid of:
1984 VW Vanagon 195,000
1976 350SE Euro 185,000 KM
1973 280SE 4.5 (boy do I regret that)
1980 280SE European W126 4 speed manual (miss it)
1989 BMW 735iL 119000 (good riddance)