F80 M3 Diff Input Seal

The Diff Input Seal is known to leak oil in the F80 M3

This can cause a mess under the vehicle (and your drive), as well as play havoc with Electronic Diff Lock which in turn can sometimes be one of the causes of the 'Rear Axle Differential Lock' warning message on the i-Drive due to fluid loss and/or heat build up:

Surprisingly, the F8x M3 Diff Input (Pinion) Oil Seal is the exact same one which is used in the:
1M: E82
M3: E9x, E46, E36.
M5: F10, E6x, E39, E34, E28.
M6: F1x, F06, E6x, E24.
7er: E65, E32, E23.
8er: E31.
Z4M: E8x.

According the to the BMW Parts Catalog, the Diff Input Seal ( was introduced in November 1987 and fits almost all M-power models since!

It hasn't gone through a single redesign, super-secession, or part number change in over 30 years!

It failed and leaked badly on the E46 M3, E9x M3, E6x M5/M6, and it now looks like the F8x new 'E-diffs' are going the same way.

How do we know?
Well, our very own F80 M3 had a profusely leaking diff at just 32k miles.
Plus, in the past 3 months we've carried out F8x Diff Input Seal changes on a further 4 more vehicles.

The F8x M3/M4 uses a very cool Carbon Fibre Propshaft which takes the power from the Gearbox to the Differential.

BMW have designed a new Propshaft and Differential locking mechanism for this era of vehicle which requires a huge M39 (thread ID) floating nut with an oval shaped retaining piece to sit inside a unique Diff Input Flange.
BMW install this huge M39 threaded nut with what seems like the worlds most strongest permanent Loctite.

The only way to separate this ultra rare and expensive Carbon Fibre Propshaft is to use the BMW Special Tools, which naturally we've invested in:

It's a 2-man operation to separate the M39 Propshaft Nut even when using these Genuine BMW Special Tools. The forces required to undo the nut are nothing like we've ever seen before.

Then, once the Propshaft has been carefully removed and protected, the same effort is required all over again to remove the huge Diff Input Flange Nut with another set of Genuine BMW Special Tools.

Once the Diff Input Flange Nut has been removed (which commonly needs the special pulling device) you can then see the Oil Seal which is leaking.

Another special tool is required to remove the seal without damaging the diff input gear or the diff housing. 

That then allows the new Diff Input Seal to be installed, of course with another Genuine BMW Special Tool.

Then everything can be reassembled in reverse order and a big clean up can take place which brings the diff back to an original (non leaking) look.

We follow the BMW Technical Instructions meticulous and obvious mark the Diff Input Flange Nut to the Diff Input Gear in 3 places before removal, and then re-instate it back to the exact same position - usually confirmed by a 2nd person for quality control.
This is because there is no torque setting on a Diff Input Nut and there never has been on an BMW Diff as that's done at the 1st install/build when the diff is new and it clamps the 'crush sleeve' to allow the worker to achieve the desired preload and not to mention the contact patch between the crown wheel and pinion gears inside the diff which also equates to backlash.

We also install the exact specified 'permanent' Loctite by BMW and install the Prop and M39 assembly within the 5 minutes working time to ensure the Prop thread is fully engaged to the M39 floating nut and only protrudes 4mm< at a maximum.

It goes without saying that we also replace the Diff Cradle Bolts into the Diff and into the Subframe, as well as Exhaust Gaskets and Copper Nuts.

This is a very labour intensive job which has to be carried out with absolute skill, special tools, and in many parts with 2 people.

We use all Genuine BMW Parts, as well as replace the Diff Fluid with the correct MSP/A Final Drive Oil for a package price of £935 including VAT.

To make a booking for this job we require a £100 deposit.

We also offer Diff Output Oil Seal replacements.