Diff mount: new part different than old, new=thinner HELP

J

jjs2800cs

Well-Known Member
Messages
286
Reaction score
105
Help

1970 Euro 2800CS, am in process of rebuilding drive shaft etc. Are replacing flex disc, center bearing, repacking CV joints, replacing motor mounts, and so forth.

Also replacing the differential rubber mount that attaches to the rear floor by 4 bolts and to the diff by one long bolt. Problem is the new one is about 1/2 thinner than the old. Part number by all the parts web sites and Mobile Tradition is 33171126951. Yet the new one is not the same as the old. I am 99% positive the mount removed is original. See photo link at end of post.

By installing the new mount will this not pull the rear of the diff closer to the frame, thus lowering the attachment point where diff attaches to the driveshaft? Would this also not change the running angle of the driveshaft? I have not yet fit the mount to see if there are any clearance issues.

What gives, why the difference? I could make a 1/2 spacer to retain the original geometry. Did BMW change this to change the geometry? Does it matter? What to do?

Help
jjs2800CS

Photos at the following link.
http://jjsandsms1.jalbum.net/BMW 2800CS Differential Mount/
 
Looks like you ended up with the same part number as me when I looked it up on realoem. Do you still have the packaging the part came in to make sure you were handed the right part? From your picture, it doesn't even look like the bolt holes will line up, or is that the photo playing games with my eyes?
 
I have had the wrong parts in the right part bag before, especially when it comes to rubber parts, so that may be a concern here.
 
hello

Hello my friend , i think that if the holes matches these not more than 1/4 inch will not reflect on the car behaviour or harm the other parts , you can mount it ! The new part which you bought is coming out for 2800 CS also ! My be it is replacing the old one (yours) ! IF the holes are not matching which i doubt , it is obvoius different part !
 
That doesn't look like the right part however the most important dimension (assuming that the bolt holes line up) is the height from the body mounting flange to the point at which the diff touches, if its close (a few MM) then you should be ok.
 
Hi there, few years ago I replaced one and am pretty sure it looked the same as the new one you have but the differences do look fairly big - I probably didnt check the old one in my excitement to install the new one!

Regards,

Rohan
 
First, thanks to all who replied thus far.

1. The 4 bolt holes line up exactly with the old part. The pictures are not clear in that regard.
2. The bag that the new part came in matches the number on the new part and matches the number listed in Mobile Tradition parts diagram.
3. The new part is at least 1/2 thinner than the old part and as I describe in my original post, this would mount the rear of the diff 1/2" higher than the old part.
4. All the web sites list the part number received as the correct part and those who have a photo, show exactly the part I receieved.

So I am still confused as to why there is the significant difference in the parts dimensions. Why would BMW change the part?

I am leaning toward making a 1/2" thick spacer and use the new part. This would keep all the dimensions the same as the original unless someone can convince me the new part is really correct because BMW changed it for a reason.

Thanks

Jeff
 
update:Diff mount: new different than old, new=thinner HELP

The mystery of the differential part may be partially solved.

I had forgotten that I have a set of the original BMW part number micro-film from the late 1970's. The part number listed for the rubber mount on the micro-film is 33171103842.

If one searches for that number on several of the parts suppliers web sites, the part number that comes up is the one I received from AutoHaus (and what Mobile Tradition depicts) which is 33171126951. BMW must have changed the part number years ago. In fact if one does a search on Mobile Tradition, the old part number is not even recognized. The new number is the one listed for the 2800CS (and many later BMW's) applications.

This does still not address why the new part number is dimensionally different? Any one got any theories?

I have decided to make a 1/2" spacer to fill the gap.

Hope this helps anyone who might need to replace this item in the future.


Jeff
 
Just a thought- the higher the diff is mounted the less the camber at the rear wheels.
 
Another thought - perhaps the newer version 33.17.1.126.951 was a redesign for the later models (E12, E24, etc.) that share it. If I recall correctly, there is a kind of sheet metal box structure that the mount secures to. The depth of that box might be different on the later models to compensate for the change in the mount dimensions. Then came the usual supersession process as BMW reduced the number of different parts to maintain...

Charlie P
 
eh?

sfdon said:
Just a thought- the higher the diff is mounted the less the camber at the rear wheels.
Click to expand...

How does that matter when a car has IRS and wheel camber is controlled by the positioning of the trailing arm mounts?
 
the mount diff

MEYLE Germany is still producing it number 3003317104 , not expensive !
Walloth & Nesch also have it not expensive -number 33/3/12
BMW also have it - number realoem !
Unfortunately all of them are in EUROPE , the US fans can suggest some other supplier there !
 
We installed the diff today with the new rubber mount and a 1/2" spacer to make up for the thinner mount now being supplied. The Merle part referenced is exactly what I got from Autohaus.com in Arizona. Seeing the diff installed, I believe using the homemade 1/2" spacer was the correct way to go. Otherwise the rear of the diff would be up higher by that amount. If anyone is interested I could post some pictures of the spacer and the installation.

I am still waiting for a logical explanation as to why BMW changed the geometry of the mount for our E3/E9 cars. Apparently on the later applications for the same part number (of which there are many) the geometry is correct, but why apply it to our cars where it is not?

Thanks,

Jeff
 
Re: eh?

Honolulu said:
sfdon said:
Just a thought- the higher the diff is mounted the less the camber at the rear wheels.
Click to expand...

How does that matter when a car has IRS and wheel camber is controlled by the positioning of the trailing arm mounts?
Click to expand...

Because trailing arm movement is not planar, it is in an arc, and raising (or lowering) the diff pulls (or pushes) the half-shafts, connected to the trailing arms via the axle hubs.
 
You must log in or register to reply here.
Back
Top