Each time I rebuild a transmission, I check the flatness of the case where all the valve body passages are routed. (I also do the same for the valve body, but that is not shown in the slides) Typically I will find a couple of spots that are slightly lower than the surrounding passages. As the case has to be cleaned before re-assembly anyway, I like to lap the case with a machinist straight edge and 600 grit wet/dry sandpaper. I will use a plastic scraper to remove the old gasket material before I start. This method has served me well and also "cleans up" the case for the valve body gasket. This is followed up with a through solvent cleaning and hot water wash, then blow dried until no moisture remains.

Does anyone else do this sort of thing?

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Turbo300Mercede
87 300D W124
83 240D W123
80 300 TD Wagon W123, 4 Speed from 79 240D, SLS Rear Suspension