So I recently put together an S4 NA with pretty much stock components, passed smog and registered it yesterday.

Today was basically my maiden voyage in this car. About 5 miles into the drive, while accelerating from a stop, shifted into 2nd at about 3000 rpm. Heard a loud bang from the rear and the car coasted to a stop.

Looking underneath the car, the diff was being supported by the exhaust via the halfshafts. Obviously that's not right.


I'd installed the diff on this one and thought I forgot to tighten down the nuts on the rear diff mounts. Not exactly....



BOTH mounts had sheared off and the diff was basically flopping around between the driveshaft u-joint and half-shaft CV joints. It is less than an ideal setup.

Not sure if there's a moral here, but just saying it can happen. Even to an S4 NA.

 

It helps if you put on the Big *** washers(looks like a shallow cup) that are supposed to go on before the nut!..it looks like you totally overlooked that!

 

^ Guess we found the moral. Remember your cups for your undercarriage!
Strangely, I didn't see them in the FSM assembly drawings when I looked up the torques.

My own fault for assembling this car from parts on the floor and a box of rx7/miata/ bmw bolts.

 

Ya,if you do not put those on,the Diff is essentially being held up by the rubber mounts on the inside of the Ears.
Once they RIP,like yours did..Plop..
I'm surprised that it wasn't worse,so you can thank your lucky stars there!I think the exhaust saved your hide.

 

it does beg the question though, what holds the diff in? does it just sit on the big stopper washer things? (which aren't in the FSM, but are in the parts catalog)