Hello all, and greets from Finland.

I own a -91 Sierra 4x4 DOCH 2.0.

The current rear diff has started to make noises and shakes during certain speed.

I gather the bearings are shot.

Questions:

Can I use a 2WD rear diff?

Some peeps in Finnish FB group speak about 2WD ratios being different than 4WD.

Is there a type stamped on the rear diff box? Can it be identified what ratio is inside?

All help is appreciated. Lets keep the old banger Sierra on the road.

Cheers!

-Hessu-

 

You have a 3.92:1 ratio diff
You need to keep this ratio
It will normally be in a 7 inch case but you may have a 7.5 inch case as only the UK market had the same thing across the range
Then there are things such as crown wheels and pinions being different sizes between the two, input shafts being different lengths etc
If you were to put a 2wd open dohc diff in it will work but if you are talking cosworth then everything's bigger so wing mix and match

 

Thanks,

I am in the process of purchasing a rear diff from a Finnish guy here.

Is there anything I can inspect from that rear diff as it is removed from the car already?
Can I feel it by hand, if there is something a miss with bearings etc?

Thanks

 

You need a special tool to open the diff, the rear case is billed on, release the two clips that hold the output shafts in place and there are those funny castle turreted covers you have to undo next
There are two washers with tabs on them that hold these in place but there are things like preloaded and backlash you need to factor into account if you are going to start opening then up so best left to the experts

 

Thank you again,

I will not try to open or dismantle the rear diff assy. Definitely not

I have now agreed to buy a second hand diff box from a Finnish FB Sierra group member here.
Should be a correct one with correct ratio and correct type flanges for the props.

Is there anything out of the ordinary when I remove the old under the car? I haven´t done this type of rear diff removal before. Just Atlas type with my summer banger Capri.

Just jack up the rear diff and open bolts and nuts from prop shafts and last the rear diff assys mount?

Cheers!

 

You will need a 3.92:1 ratio 7" diff. The ratio is normally written on a tag that is located at the top left of the rear cover. The rear cover can easily be removed by taking the torx bolts out. The ratio of the diff (3.92) will also be written on the crown wheel.


When you remove the rear diff from the car, hammer the torx bit into the torx bolts to make sure it goes in deep enough. If you don't do that you will probably round off the bolt head and then it is time to take the angle grinder out.


To test that your new diff is actually an lsd, try to rotate the output flanges in opposite direction. On an open diff it will be very easy, on a lsd it should be difficult (if it is in good condition).

 

Yeah,

I have a Makita monster battery driven impact gun and proper impact torx heads. She puts out quite a hefty Nm torque. If that wont open the torx bolts, I have torch welder in my garage.
I´ll get them open for sure

The new (second hand) rear diff assy is said to be LSD version. I do not doubt the seller, as he is quite a Sierra nutter here.
I´ll check it anyways as you described. Thanks.

Cheers

 

i found that it was much easier to swap diff's if the beam was dropped from the car

much more access to the bolts you need to get to and more room to swing the hammer

the long bolt running the width of the diff is the most difficult to remove, plenty of penetrating fluids should get it moving after a couple of thwacks with a hammer

also, the drive shaft bolts can be easier accessed by removing the cv boot from the driveshaft, once they are cracked they are only finger tight but if you do find that you round the heads on the torx bolts the replacements came in T45 heads rather than the original earlier T40's which were more prone to rounding