I have a 1978 240 with the 4-speed manual trans. It has a bearing rumble in the trans, and I was lucky enough to find a replacement trans on Craigslist for $85.
But I was curious about what really went wrong with the old trans, and so I took it apart, and found that the countershaft bearings are toasted.
Question now is: Where should I go to buy replacement bearings and gaskets for this transmission? I suppose I could go to the dealer, but is there a better option?
This trans is apparently the older 123 manual with the iron case and non-integral bell housing. I gather that the later ones have an aluminum integral housing.
Does anyone know of a part number or series number, etc., that identifies this particular trans?
Thanks!

There should be a number cast on the transmission that starts with the numbers 115. As for the bearings, I would take them into a bearing dealer and see if they can get them.

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a few mb diesels