Differential & M-Clunk

Quick Links: Renovating the diff    Differential Bushing   Prop Shaft CV joint    

As in many enthusiast circles there are some myths floating around. In the E46 M3 community the “M-Clunk” is one. Not that the M-clunk does not exist – it does - but what exactly is the cause.

The M-clunk is audible especially when shifting in low gears – perhaps the most when you are manoeuvring on a parking lot.

From the info I’ve been able to find, there are 3 variants of
reasons for M-clunk:

1: Worn LSD plates in the differential.
2: Worn front bushing for differential (holding the differential in
    the sub frame).
3: Worn CV joint between prop shaft and differential ingoing
    (pinion) axle.

1 and 2 are of same “kind” since they present play in the
driveline itself (rotational play). 2 allows the front of the
differential to move up and down – potentially hitting
something “metallic”.

Please observe that the E46 M3 came with two different
differentials made by GKN.

The 2004 models and earlier have the V1 differential while later models have the V2 version.
The two differentials are alike – same housing, crown/pinion, bearings etc. – except form the LSD unit. This video explains the differences very well:  E46 M3 - Differential V1 vs V2 LSD Unit.

Renovating the Differential (V1)

There are several good videos about renovating the V1 differential – see below.

One thing to note is that an indication of worn LS (Limited Slip) plates is that it
causes play in right outgoing axle/flange (where the inner right CV joint is
mounted). The play is radial i.e. you can move the flange up and down (shown
in videos). One should think that such a play is caused by a worn bearing –
but it’s not. The side axles (left long; right short) is not directly supported by
the side bearings but are fixed (spline/lock ring) to the small side gears (see
principle drawing (not as the M3 diff.)). When the LSD plates are worn the
right side gear can move a little and thus the right axle.

Apart from the differential clunk some experience a noise when in tight turns.
This is most probably the LSD locking up in an unintended way. It’s likely that
this tendency increases with wear, but may be solved with the right oil or oil
plus friction additive.  

Renovating / rebuilding the differential is normally not a DIY job, however it is possible if you are an experienced amateur mechanic and have the right facilities (a hydraulic press is a must). AND, of course, you need to have the needed parts.

A good walk-through of renovating the GKN LSD in the M3 by Racing Diffs: Renovating Diff

Another fine video is made by ShopLifeTV:   Rebuilding and Fixing BMW M3 Differential Clunk.

Video made by SPANNER RASH of getting the diff. out of the car:   E46 M3 Differential Removal Guide

This video continues here:  E46 M3 Differential Input Shaft Seal Guide

If you should consider to change the differential ratio in connection with the rebuilding here is a fine video from Racing Diffs : Changing diff. ratio

What you may get out of a ratio change is described here.

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Actually I started the renovation process myself. No problems in getting flanges off, seals out and pinion out (I have a 20t press). Firstly I cleaned the housing and surface treated it with POR15. When I came to taking the crown wheel off the LSD unit I frankly gave up. The 10 bolt holding the crown wheel to the LSD casing sat so tight that I didn’t have the nerve to unscrew the bolts. If one broke I would be “smoked”. The ratio turned out to be 4.1 already so a previous owner changed the crown/pinion. That meant that the bolts could sit with a lot of strong Loctite – so I didn’t dare to go on.
So I changed the seals and assembled. Below some pictures from the process. The last picture shows the tools I had prepared to press in and out the bearings and seals

Differential bushing

The differential sits in 3 pcs bushings. Two of these are an integral part of
the rear cover and cannot be bought separately from BMW (as far as I know).
The may be changed using aftermarket parts.

The third bushing (No 8 in the picture to the right) may be changed – and is
the one that may cause the M-clunk.

In the pictures to the right the bolt No 9 in one picture is the same as No 2 in
the other. It’s a  M14X1,5x65-10.9 i.e. a high quality 14 mm bolt. It goes into
the differential casing.

This just to give you an idea of the location when you see the video that show
how to replace the front 3rd bushing.

To change the bushing without taking the differential out of the car is doable,
but not an easy job.

See the video: E46 M3 Clunk Fix - Front Diff Bush

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Prop Shaft CV joint

The 3rd potential cause to the M-clunk is the CV joint between the prop shaft
and the differential.

The two pictures on the right show the prop shaft (drive shaft).

In the lower picture No 14 is the CV joint in question.

The Center Bearing (No 7) and the donut (guibo) (No 2) are also shown.

This video explains:  Bmw E46 M3 clunk diagnosis

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