I just did both oil cooler lines and the O/F housing-to-block gasket on my '83 300SD about a month ago. I've done the job before (many on customers' cars) but it differs between the W123 and the W126 chassis. On the 126, you have to take the engine mount and the shock off to get the old lines out and new ones in. It won't budge any other way. The plus side is that it gives you an excuse to put new (or at least one if you're cheap) engine mounts in. I changed both of mine since they were flat anyways.

I WANTED to take digital pics and send them to Donnie to put up here on the DIY section but wouldn't you know it that I completely forgot my digital camera when I began tearing the car apart in my workshop!

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Regards,
Aaron