First and foremost, thanks to all the folks on this forum who have patiently replied to my previous posts and email messages as I work my way through the resto on a 83 240d 4sp manual tranny sedan. This car is my daily driver and sees 100+ miles per day of highway driving (very little "town"). Total front suspension, tires, rear shocks and CV Axles have been done within the past year. All seemed well......until I noticed a small wet spot under the differential in the parking lot a few days ago....it would appear I have sprung a leak at the pinion seal, and not really a slow one either; the diff is covered in fluid.

The car has 255k and runs great. The drivetrain has always had clunks with shifting, my mercedes mechanic stated the 240d model stick shift was always a bit noisy, and that I needed to put the diff mount and sub-frame bushings into my summer project plans to lessen the "clunking" -- I have no reason to not take him at his word.

Now for the painful part. In just the past few days, along with the pinion seal leak, it would appear I have a clunking noise in the rear end -- cant tell if it is passenger side spider gears or the CV axle on that side, but there is a point at which it would appear something is slipping inside the differential. The clunk is accompanied by a a lurch in the movement of the car, so something is loose or broken I am assuming. I suppose I will have to take the diff cover off to check for broken gear teeth -- beyond that, I am "diff-illiterate."

So here is my question(s):
1. I have a 79 300D n/a Auto tranny with apparently good differential. Since my driving is primarily highway, should I consider just swapping that in? Should I, while I have it out from under the car and before installing it in my 240, replace the pinion and side seals? I have tried to read through the posts and noticed the 300d likely has a slightly taller gear (lower number) which I would presume would cause me to loose a little initial acceleration, but maybe gain me some mpg on the highway? Do I need the speedo out of the auto tranny 300d? I believe with patience I can safely remove the diff without damaging it since I know how to get the axles out of the wheel hubs.

OR

2. Should I just limp my car to my mercedes mechanic since the diff is so heavy and the R&R (again based upon what I have read here in the forums) is not super easy, and let him swap in the diff out of my parts car and/or locate an exact replacement salvage for me and install it?

Thanks again in advance for any ideas or thoughts. I just hate to start on a complex project unless I can safely and effectively see the repair through to the end.

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Jonathan K.
1983 240D, 4 sp. manual, 245k miles
1986 Dodge Ramcharger, 360 V-8, police pursuit package, propane conversion
Scott's 16hp Riding lawnmower

Don't sweat the petty stuff....and don't pet the sweaty stuff

A 346 would really gut your performance. That is a 6.6% overdrive from the stock 369.

I think you could do it but the lack of acceleration would be very tedious.

I have a 358 for sale in parts from a 280e that would give you a 3% overdrive though and I believe I may have the speedo to match.

Tom W

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

The 300D rear end will be fine on the highway. The speedo would also need to be changed to match the changed rear end ratio.