The Differential Swap        Email your tech stories to TechTalk@liv2drv.net

 

The morning air was wrought with the determined welcoming of a new day.  All mechanical minds and willing hands had gathered for the much talked about “Diff swap”.

 

The object of our suffering was a tired 3.25 Limited Slip Differential that had been complaining of internal pain for quite some time with loud and obnoxious grinding noises complete with external bleeding of the viscous type.

 

The scene is set with an 87 535is riding high on a set of ramps and four hung over, tentative, yet enthusiastic hack mechanics gathered with coffee in hand, pouring over the manual through swollen eyes. We had done our home work, had our Bentley manual in left hand and armed with all manner of Metric tools in the right.

 

The replacement is a used but healthy 3.73 Limited Slip Differential from a 328is.  This superb diff was purchased from the worlds largest auction house.  The e-Bay unit had been procured for a mere sum of $274 dollars plus $80 and change in shipping tariffs.

 

With a sigh and groan Kent took the lead and slowly lowered his meager frame (compared to the girth fat ones surrounding him) to the creeper and wheeled under the car hovering above.

 

"Well it looks like we are going to need a metric Allen preferably in a socket", Kent muttered.  "Damn it", Dave gasped, "I don’t have any Metric Allen wrenches especially set in sockets".  "All right, who did not do their home work, no body mentioned that we were going to need any Allen paraphernalia".

 

Dave looks back at the e-mail he received from his fellow BMW e28 faction, sure enough in the first paragraph “You disconnect the axles at the diff,(6) Allen bolts per side” , "Ok" Dave exclaims "it is really not relevant who missed this vital piece of information", knowing full well it was he who had overlooked this little tidbit.

 

Two of the Fat Ones jumped in Eric’s (340 HP) Porsche 928 and galloped to the nearest Auto Parts store to purchase afore mentioned Allen socket wrenches.

 

Meanwhile our fearless leader Kent is making head way on the (4) 19 mm bolts holding on for dear life to the sick Diff hanging under the 535.

Ok the Porsche and Fat Ones reappear with Allen sockets in hand and a few sorted stories of sliding corners and kicking some sushi tuner ass on the way back.

 

Now we can really get started with the dismantling of the bracketry and flanges that hold us from lowering the old diff.  "Oh crap", mutters Kent. "What?" We all reply with a concerned tone.  "This *&%#@! Allen screw is stripped out."

 

(Ok now, hold up right here, we found after an hour of wrestling with a pair of vise grips that the Allen orifices need to be cleaned out so that you can securely sink the head of the wrench all the way in the awaiting cavity

of the Allen head.)

 

Now that the (12) 8 mm Allen head screws have been released from captivity and the captive axles have been securely hung by wire as not to apply pressure on the CV joints we can tackle the (4) 10 mm nuts holding the drive shaft. (The (4) 10 mm nuts are self locking use once only, you will need new nuts when reassembling)

 

Whew, all that is holding us back from gathering up our old authentic diff from the underneath is (1) 19 mm bolt at the end of the rear diff cover.

 

With the bolt undone we very slowly and methodically lower the diff from it’s cavern, "careful now", Kent says as the Diff slides down like an old ship being lowered into the harbor.

 

Well there it is in all its glory, the old Diff that has ruddered this 18 year old 535 to many an adventure.  With a moment of silence and a fond farewell we move the new 3.73 diff onto the all aluminum Craftsman floor jack and move it into place for its debut in a 5 series.

 

Keeping in mind that the flanges and rear diff cover on a 3 series and 5 series are not compatible, and that all differences had been taken into consideration by the e-Bay seller we felt fairly comfortable hoisting the 3.73 into the awaiting womb of the 535, WRONG!

 

After no less than a half an hour of carefully and painstakingly aligning the new diff into place (make sure to align the drive shaft first) we find that the rear diff cover is too long to bolt in properly.

 

Damn *&%#@! e-Bay seller did not change the rear diff cover as promised, leaving us with a diff that is mostly in place and secure in its cocoon only to be either evacuated, drilled or worse, swap covers with the old one!

 

Wouldn’t you know it, the vote is to swap covers with old one, and of course this meant that we had to drain the precious Red Line blood from the new diff in order to swap covers.

 

Dave sighs, everyone consoles him by saying “You know it is only right way to do it right”, to which Dave replies, "Ok let’s get it done."  Upon relenting, draining the new diff and dismantling the old diff to gain it’s much needed cover we realize we have no way to put the fluid back in the unit.

 

After trying several procedures all of which had merit but to no avail, we simply filled a water bottle full of the precious Red Line and squirted it into the top drain plug of the 3.73.  Now we were not able to fill it completely, however it would get us to the nearest Oil Can Henry’s.

 

We were now able to reverse the dismantling procedure and button everything up tidy and torque fasteners to the Bentley specifications.

 

Now before we started our little adventure we documented our 3.25 diff’s numbers, such as 40 mph 1st gear, 60 2nd, 100 3rd and so on.

 

So of course the new 3.73 has to be compared and analyzed, after driving for roughly a half hour we concluded that the shift points were all approximately 20 mph lower than that of the 3.25, keeping the car in it’s optimum power band at all times, which is exactly the desired out come.

 

Was it worth it?  Hell yes.  Would I do it again?  Only with my faithful friends by my side, with their resourcefulness thoughts and playful humor this task was not only rewarding it was fun.

 

After all, if you cannot share your passions with others, they are merely thoughts waiting to be explored.

 

A special thanks to Jeffrey Francis, (AKA Scooter/Photographer), Eric, (AKA Fat Bastard/Muscle/Brains), Kent, (AKA Freak Show/Intestinal Fortitude/Mechanical Aptitude) and Mike Alness for his advise and trailblazing differential swaps.