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  #1  
Old 09-03-2022, 06:33 PM
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3 piece drive shaft - which side is front?

I had the 3 piece shaft for the 91 SDL shipped to driveshaft pros in garden grove CA and supplied all bearings/boots etc.



They rebuilt it/balanced it etc and shipped back.


Now I'm staring at it here and I can't decide which side is the front.


Front and rear shafts are the same length with same bumps for alignment.



The only clue I can find is in the manual where it indicates the centering sleeves protrude different lengths.


Mine look closer to 19mm and 24mm so would that make the shorter end the front?

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3 piece drive shaft - which side is front?-shaft.png  
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W126 350SDL
X204 GLK 250 Sans Bluetec
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  #2  
Old 09-03-2022, 07:34 PM
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My 83 SD has a carrier bearing, and the short shaft is toward the front.
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  #3  
Old 09-03-2022, 08:01 PM
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Quote:
Originally Posted by Proctor750 View Post
I had the 3 piece shaft for the 91 SDL shipped to driveshaft pros in garden grove CA and supplied all bearings/boots etc.
They rebuilt it/balanced it etc and shipped back.
Now I'm staring at it here and I can't decide which side is the front.
Front and rear shafts are the same length with same bumps for alignment.
The only clue I can find is in the manual where it indicates the centering sleeves protrude different lengths.
Mine look closer to 19mm and 24mm so would that make the shorter end the front?
1) Drivelines, being mostly cylindrical, are not usually thought to have "sides". However, they do have "ends"; a forward end, and an aft end.
2) Beginning at the u-joint in the center of the shaft, the one-piece section is the aft end, and the two-piece, collapsible section, containing the center bearing, is the forward end.
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  #4  
Old 09-03-2022, 08:16 PM
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When staring at a straight line from a perpendicular angle with two end points then each point can be considered "sides" of that line.

In other words imagine crushing a car until it was only about the thickness of a driveshaft diameter. It still has sides


Anyways - I think there is a misunderstanding. I have a 3 piece driveshaft, not a 2 piece.



This means you have two identical shafts (forward and aft for the sticklers) and a very small center shaft which connects them using TWO center bearings.


Back to the puzzle - which SHAFT of the two which are equal length is forward and which is aft?


-I-----[]---[]-----I-
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190D SOLD
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Build date: December 1985
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W126 350SDL
X204 GLK 250 Sans Bluetec
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  #5  
Old 09-03-2022, 09:49 PM
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A 126 FSM of 1997 publication date does not cover a three-piece driveline.
A 114/115 FSM does have illustrations of such a shaft.
If the rubber bonded center bearings are offset in the u-shaped mounting brackets, the bearing offsets are towards the rear.
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  #6  
Old 09-04-2022, 12:11 AM
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Unfortunately both center bearings are identical part numbers as are their supports, their boots etc. There is literally no difference to be found other than slight difference in centering sleeve lengths on each end.



I will call the shop Tuesday and see if they remember how they had the shaft oriented in the their lathe when they put the stickers on..




Perhaps a photo will help.


I am going to see if I can find a PN stamped somewhere on either long shaft as they do have different PN.

Edit: I couldn't find any markings anywhere other than the mercedes logo on each yoke and a 00 stamped on one yoke and a B0 stamped on the other...
BUT - This fella on youtube seems to agree with my theory on the factory manual wanting the shorter centering sleeve going to the trans: https://youtu.be/7xqeine6rs8?t=198
Yes he had the wrong shaft but the concept remains the same - shorter centering sleeve goes into trans.

I suppose I will just try that since it appears to work out for him in the end. Weird they did a 3 piece on the non L models...
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3 piece drive shaft - which side is front?-shaft2.jpg  
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190D SOLD
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W126 350SDL
X204 GLK 250 Sans Bluetec

Last edited by Proctor750; 09-04-2022 at 12:34 AM.
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  #7  
Old 09-04-2022, 12:23 AM
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Quote:
Originally Posted by Frank Reiner View Post
If the rubber bonded center bearings are offset in the u-shaped mounting brackets, the bearing offsets are towards the rear.
There is no mention in the above of numbers.


Quote:
Originally Posted by Proctor750 View Post
Unfortunately both center bearings are identical part numbers as are their supports, their boots etc. There is literally no difference to be found other than slight difference in centering sleeve lengths on each end.

Perhaps a photo will help.
Photos do help. Visible in the one that you posted is the above mentioned offset of the bearings in their brackets.
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  #8  
Old 09-04-2022, 12:57 AM
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Quote:
Originally Posted by Frank Reiner View Post
There is no mention in the above of numbers.



Photos do help. Visible in the one that you posted is the above mentioned offset of the bearings in their brackets.

Ah I see - so you are saying that since the bearings are not centrally seated within their brackets, the side (or direction) the offset is towards is front?

Edit: You say the offset is towards the rear...

So - when looking at the center bearings in the photo above - the circular portion of the housing which contains rubber that surrounds the bearing is offset from the portion of the housing which is mounted to the chassis.
You are saying that the portion of the center bearing housing which is essentially flush with it's mounting bracket portion faces rear? The only issue with that is it's facing the shorter yoke centering sleeve i.e. to the RIGHT in the photo.
Attached Thumbnails
3 piece drive shaft - which side is front?-shaft-3.jpg  
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Last edited by Proctor750; 09-04-2022 at 01:11 AM.
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  #9  
Old 09-04-2022, 01:36 AM
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Quote:
Originally Posted by Frank Reiner View Post
A 126 FSM of 1997 publication date does not cover a three-piece driveline.
A 114/115 FSM does have illustrations of such a shaft.
If the rubber bonded center bearings are offset in the u-shaped mounting brackets, the bearing offsets are towards the rear.
Quote:
Originally Posted by Proctor750 View Post
Ah I see - so you are saying that since the bearings are not centrally seated within their brackets, the side (or direction) the offset is towards is front?

Edit: You say the offset is towards the rear...

So - when looking at the center bearings in the photo above - the circular portion of the housing which contains rubber that surrounds the bearing is offset from the portion of the housing which is mounted to the chassis.
You are saying that the portion of the center bearing housing which is essentially flush with it's mounting bracket portion faces rear? The only issue with that is it's facing the shorter yoke centering sleeve i.e. to the RIGHT in the photo.
The reference to the 114/115 chassis was to illustrate a possible way of determining orientation.
It is entirely possible that the 126 chassis has the center bearings installed in the opposite direction.
In dealing with unknowns the Brits have a fine, useful expression:
"Suck it, and see!"
Put the shaft in place with the short bushing forward, and check for alignment of the center bearing brackets to the mounting holes.
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  #10  
Old 09-04-2022, 06:31 PM
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Quote:
Originally Posted by Frank Reiner View Post
The reference to the 114/115 chassis was to illustrate a possible way of determining orientation.
It is entirely possible that the 126 chassis has the center bearings installed in the opposite direction.
In dealing with unknowns the Brits have a fine, useful expression:
"Suck it, and see!"
Put the shaft in place with the short bushing forward, and check for alignment of the center bearing brackets to the mounting holes.
Haha Im using that line

Yes I think I will do that. Ill call the shaft shop Tuesday to see if they have any clues just in case but I agree I should "Suck it and see" ha

Im curious what difference it would even make if it was installed backwards successfully. I can't really think of a negative other than the centering sleeves being the wrong lengths.
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W126 350SDL
X204 GLK 250 Sans Bluetec
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  #11  
Old 09-04-2022, 06:57 PM
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Quote:
Originally Posted by Proctor750 View Post
I'm curious what difference it would even make if it was installed backwards successfully. I can't really think of a negative other than the centering sleeves being the wrong lengths.
To ease the checking operation, consider disassembling the shaft into three pieces. Each of the end sections can then be checked both in the forward location and the aft location in the chassis. Much easier to handle.
Re: centering sleeves, if the sleeves do not bottom on the transmission and rear end shafts before the bolt flanges capture the flex discs, there should not be a problem with sleeve protrusion length. The drive line sections should be able to move endwise when the flex discs are all bolted up.
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  #12  
Old 09-10-2022, 12:14 PM
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So everything bolted up fine and the refurbished shaft has definitely eliminated MOST of the vibration under acceleration but not ALL....

SO what else could cause this? Rear shocks are new as well.

Edit: come to think of it I had to tighten the 2 gland nuts with the car on jack stands. Perhaps having them loosened and retorqued on a rack where car is on it's wheels can help.
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W126 350SDL
X204 GLK 250 Sans Bluetec

Last edited by Proctor750; 09-10-2022 at 01:34 PM.
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  #13  
Old 09-14-2022, 10:04 PM
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So the newly balanced shaft with all new bearings and guibo in front (rear one appeared to be close to new so didn't replace) Eliminated the vibration under normal accelleration.
But if I floor it then I still get a pretty wicked drivline vibration. It still feels like it's coming from the rear.
I suppose I could replace the rear guibo as it's the only thing left which isn't brand new but it seems perfect.
Could tightening the 2 large gland nuts with the car off the ground really do this? I must be missing something.

or.... could the torque converter cause this?
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W126 350SDL
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  #14  
Old 09-14-2022, 10:51 PM
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Quote:
Originally Posted by Proctor750 View Post
So the newly balanced shaft with all new bearings and guibo in front (rear one appeared to be close to new so didn't replace) Eliminated the vibration under normal accelleration.
But if I floor it then I still get a pretty wicked drivline vibration. It still feels like it's coming from the rear.
Recall that the center section (aka diff) is mounted in rubber. As torque increases, the upward deflection of the pinion nose increases. At a greater angle between the driveline axis and the pinion axis, there will be greater vibration.
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  #15  
Old 09-15-2022, 11:17 AM
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Quote:
Originally Posted by Frank Reiner View Post
Recall that the center section (aka diff) is mounted in rubber. As torque increases, the upward deflection of the pinion nose increases. At a greater angle between the driveline axis and the pinion axis, there will be greater vibration.
I didnt consider the diff mount, though it is now only under full throttle so perhaps thats enough torque to oscillate the diff

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