W108 Differential Ring & Pinion change
 

I have a 1968 250SE with a 4 speed manual trans and a 4,08 ratio rear gear. Great running car but not so good on the highway or with keeping up with traffic in first or second (too low a ratio) gears.
I also have a spare 4,08 complete rear assembly, a 3,27 ring and pinion (numbers match) and the correct carrier for it (came from same source). I am a former hot rodder and familiar with setting up U.S. versions of ring and pinions for proper gear position/pattern. I have the FSM for my car. There is nothing that I can find in the manual regarding how to set up the ring and pinion to get the proper positional relationship and wear pattern necessary. I do not want to disassemble the donor rear until I have proper specs.
Right now the only thing I can do is measure rotational resistance with an inch/pound torque wrench.The FSM sindicates this should be "20-25 cmkp". I have never heard of 'cmkp' as a unit of measure.

Does anyone have any info on this?

I am also looking for the ring gear-to-carrier bolts.Does anyone know of a source?

Thanks!

Size of the diff housing
 

Before you start you might want to verify that the ring and carrier for the 3.27 will fit in the 4.08 diff housing. If I'm not mistaken the diff housing for the 4.08 is too small to fit the 3.27 ring and carrier. It might be easier to find a complete 3.27 diff.

Also here is a good link to the Mercedes M100 message board on the set up and turning torque specs for the Benz swing axle differentials.

http://www.m-100.cc/forum/topic.asp?TOPIC_ID=5320

John

cmkp is torque.

cm = centimeters

kp = kilo pond (or something like that) which is the same as kilogram force

does that help enough?

I'm pretty sure the V8 gears will not fit in a 6 cyl axle housing. I think Gael has done some experimenting along those lines-maybe he will chime in.
BTW-you will REALLY notice a reduction in acceleration with the 3.27 gears. I went from 3.69 to 3.27 and noticed a difference.

It's better to use the rear axle from a 280SE 4.5. There is a cooling pipe that may not work in your car but that can be removed and the holes plugged.

The 4.5 diff assembly bolts straight,. it's the same basic diff as is the floor pan above the diff.

Thank you all for your help. I have the proper inch/pound torque wrench and I have taken readings prior to disassembly. I also received a PM for the conversion from cmkp. I'm good to continue at this point.

I will be tryng to put the 3,27 gears into the 4,06 axle housing (I got a complete spare axle assy. for free;The 3,27 ring + pinion and carrier I purchased cheap off of ebay) on the bench.
If I get it apart and it won't work, no big deal. The 250SE still has it's original (4,06 ratio also) axle in it. I'll sell all of the bits I have and look for a 4.5 axle assembly.
It appeared from this post
http://www.peachparts.com/shopforum/vintage-mercedes/253396-how-fit-lsd-w108-4-5-rear-axle-2.html
that the carrier is dimensionally similar enough that I MAY be able to use the V8 carrier. Worse case I will try to use one of the 6 cylinder carriers. If that won't work, I'll try 'plan 3' above and look for a 4.5 unit.

Thanks all!!

Updatum
 

I'm still in the process of gathering all of the bits for the complete teardown and reassembly.

I think this whole deal with trying to measure the ring gear/ bearing and then w/axle resistance (gallons of milk should have been beer or schnapps, btw;forget the sausages. Thanks but no thanks!) through the pinion nut is out to lunch.
A small variation on this reading from bearing drag, incorrect lubricant viscosity at ambient temperature, close tolerance on a new bearing etc, etc....will screw the whole deal. I'm a big fan of reusing bearings that look good and have run in together as a set.

I'm going to use ring gear marking paint, partially assemble the ring gear /pinion with the limited slip (I discovered the tag on the boneyard diff;NICE find BTW!) and (used, good) side bearings preloaded as I would with a Ford, GM or Mopar rear and see what it looks like. If it looks like a good lead-to-middle pattern, I'll heat the assembly on a hot plate (60C should be good)for a while and see if it changes.
I know someone is going to call me a clutz (or worse) for doing it this way, but I honestly think the Germans have overworked this setup into something that doesn't even make any sense.The only real position devices are the crush collar and possibly the carrier side bearings. If the pattern is wrong and the torque reading is right, according to the inch-pound wrench and according to the FSM, what does it matter? It will self-destruct anyway because the contact area is wrong. I know some because I've been playing with this stuff a while, but I'm certainly no expert.
I may seem argumentative here, but I'm really just trying to get a discussion going on this. I appreciate all input and value your feedback.

We'll see if I'm right or not. Hang on!

I usually just install a 3.69 diff complete from a W108 280SE etc and that,with 14' wheels gives a far more liveable ratio .
When it comes to setting up the dif,yes it is exactly the same process as any US type diff.
Using colored marker is essential to determine if the teeth are contacting properly and when you set up the pinion spacer ,as long as the marks look good,it will be OK. You can buy the nut ,washer and seal as a kit from mercedes.

I re-use the old bearings and it's a really bad diff that needs the side bearings replaced. Mercedes tended to over engineer the diffs .
The LSD carrier,accord to GGR wont fit in your case ,although I have the bits here i have never got around to seeing why not because the same LSD carrier was used in the W112 diff and a few W111's were supplied with the LSD as an option and both series have the 4.10 diff.

Here is a procedure that might be usefull:

3,27 axle checkout, reseal, repair & install

I used a 4.08 LSD carrier into a 3.27 so carriers are interchangeable.

You also have shims behind the race in the left wheel tube to adjust offset of the carrier.

Just strip the center housing inclusing the pinion and see if you can get the carrier and ring in there and close it with the left wheel tube. It may work.

W108 swing axle; Learn somethin' new...
 

After many delays, I finally have most of the swing axle assembly disassembled.
I noticed at the outset that the brake backing plate on the driver's side was rusted through-not sure why. The other side looks like it was replaced at some point prior.
Prior to dis-assembly I discovered the 'special oil' tag indicating it's an LSD!
After I got it all apart I found that the tips of the pinion gear teeth have cracks to the root of some of the teeth. Some have chips missing. The ring and the rest of the pinion look fine. I'm guessing somebody got stuck and beat the heck out of the car to get it unstuck. Who knows.
Anyway, the outer axle bearings mount using a special lock nut. I need to replace the backing plate. To do that the nut hand bearing have to come off
Two questions-
-Does anyone know where I can find the special spanner wrench needed to remove this nut? The only reference I found indicates it is no longer available. I can always make one if needed.
-Does anyone know where I can get a brake backing plate? I'd even buy the whole driver's side axle if it includes a good backing plate. I can always fix the old one if I have to.
Without the part or wrench both options are a PITA.

Thanks for reading this!

It sounds like that car may have been sold new in Germany to someone who needed a good fast car to go to the Ski feilds. Why else would you need a manual trans and LSD?.
The bearing and nuts are availabe as a kit from the Mercedes Classic center . They contain a new nut,washer ,bearing and seal. A backing plate should be very easy to find . I would also replace that ring and pinion. Those Cracks sound like the parts are on their last legs anyway. The get the nut off,simply belt it undone with a hammer and punch,you wont use it again. To fit the new bearing,warm it with a hot air gun (WARM NOT HOT!!) or a hevy duty blow drier.
Check you homokinteic joint too. Before you fit everything back together,check the swing arm bushes and adjust the play so it goes up and down but not back and forth... you'd be amazed how well the cars handling can be when those bushes are in top condition.
:):)
Nice find with the LSD too.

Thank you for the reply and the advice. it's most appreciated!

The damaged pinion (which has the limited slip)is from the spare rear I picked up from a friend. I'm putting a 3,27 gear into the limited slip carrier. I still have the original rear in the car. It has a 4,08

Thanks for the advice about the nut. I was going to do what you describe, but I was afraid I couldn't find a replacement.
I can't seem to find the backing plate, but then again I'm not sure if I'm looking in the correct places. I'll keep looking around.

Thanks Again!

What do you call "backing plate"? The plate that bolts on the wheel tube to enclose the wheel bearing or the bigger dust shield ?

Did you try to test fit the 3.27 crown and pinion in the 4.08 housing and if yes, does it fit OK?