’86 745T w/ M46, 145,000 miles.

My transmission problems were with second and first gear and everything in-between. Not sure what gave out first, but the snap ring at the synchronizer hub is suspect. Unfortunately it scored up the mainshaft as well.

So I found a “low mileage” M46 (actual mileage unknown to me anyway) supposedly from an ’86 240. The first thing I noticed is that the gearbox case is cast iron, the one I took out of my car is aluminum. Since they are both ’86 models, I expected them to be the same. Am I wrong? Any idea why they are different?

I was planning on removing all the guts of the used transmission and transplanting them into my original case along with new bearings. I know I have to change the bell housing, but should I use the cast iron case instead of the aluminum?

I am also wondering why my transmission failed at 145K. The car has always been driven pretty gently with the majority being highway miles . I did change from using ATF to using Mobil1 10W-30 at about 75,000 miles. Do you think that is what did it in, or bad luck, or just normal wear and tear? I guess just knowing how well these cars were built, I am surprised that the gearbox didn’t see at least 200,000 miles.

I would appreciate any thoughts.
Thanks

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'60 544, '68 220S, '70 145S, '72 144E, '86 745T