Differential

[hrlyfxdl]

Is there a way to tell what type rear-end differential you have in your car by looking, without taking it apart? I attached pics. It's supposed to be a BS 3.31 12 bolt posi for a 68 Camaro, but really feels very low geared. Thanks.

[buenymayor]

Count the number of teeth on the ring gear and divide it by the number of teeth on the pinion gear. That will tell you exactly what gear ratio you have.

[cook_dw]

You can see the springs in the center section so its a posi.  Look on the edge of the ring gear for this..  You wont have to count anything.  Just divide the 2 numbers and thats your ratio..

41/11=3.72727272 or 3.73

[Mike S]

This is how I did it back in 'the day' with no documentation on hand;
Type = raise both tires and spin driveshaft. If both tires turn in the same direction then it's a posi. If one rotates opposite the other then it's a non-posi.
Ratio = Put a chalk mark on the drive shaft and count revolutions of tire for one full turn of drive shaft to get an approximate ratio.

Mike

[jdv69z]

Based on the OP pic, Darrell's method is the easiest. Read the numbers right off the ring gear.

[hrlyfxdl]

Everyone, thanks for the information. It's a 3.73 posi in there, according to the code on the axle supposed to be a 3.31 posi. Appreciate your help guys. Thanks.

[bcmiller]

Not uncommon for gears to be swapped out.  My original 3.73 coded housing now has 3.31 gears. 

[hrlyfxdl]

Ok, next question. I've never changed one of these out. I want to take it back to original from the 3.73 that's in there to a 3.31. How difficult is it to do while the axle is still attached to the car? Or is it better to remove? Thanks.

[Mike S]

Changing the gears isn't too difficult if you are mechanically inclined. But there is a procedure to adjust the gear tooth contact interface and depth and if not done right it can lead to a noisy rear and also damage. The challenge is to adjust the pinion depth and proper crush bearing torque to get the pre-load as well as shimming the carrier (if needed). Honestly, if you don't have the tools or the knowledge then I'd let a professional replace it.

Mike

[cook_dw]

Several variables you have to think about...  Are you gonna be laying on your back or will the car be on a lift?  Does the car have a transverse muffler?  If you have never done it before I would suggest having someone help you that has or watch it being done by someone that knows what they are doing..  You can get the backlash correct and still not have the pinion depth correct and itll howl going down the road.  Also need to make sure you get the pinion preload correct or you will cause bearing failure..


EDIT:
Looks like Mike has beaten me to it..

[hrlyfxdl]

Wow, sure glad there are more skilled and knowledgeable guys out there than me who are willing to share. I'll heed your advise and have a pro do it. Thanks so much......you'll be hearing more from me as I work on my car.

[Mike S]

EDIT:
Looks like Mike has beaten me to it..

  Sounds like you've been down that road before too. 
To the OP. It's a lot of work and a big disappointment if you go through the whole effort only to take a test drive and hear either clunks on acceleration or coast, or the dreaded 'stud' noise one can hear when driving on a bridge with a studded grate.

Mike

[cook_dw]

Ive had my hands in many ah rears....    Wait....  What..??..

Differentials!!!

[X33RS]

I use a pinion depth gauge to get the pinion set.  Kind of an expensive tool for someone that doesn't plan to set these up very often, but necessary to do correctly.   The pinion bearing on GM 10 and 12 bolts will need to be pressed on and off several times to change shims so a good press with a bearing splitter/clamp is needed so the bearing can be removed/installed without damage.  Or take an old pinion bearing and slightly hone the inside so it slides on and off.  Once you find the correct shim arrangement press on the new bearing once and done.   From there you'll need a really strong impact gun to get the crush sleeve to start to compress.  Once it starts to compress then it becomes fairly easy to tighten and set pinion bearing pre-load.  I shoot for 30 inch lbs on new bearings.
    With pinion depth and back lash set I then check the pattern with marking compound just to see how things look.  Backlash is tricky because bearing preload also has to be taken into consideration on the carrier as well.
   Just did mine a couple weeks ago with the transverse muffler in place.  Removed the shocks and the rear drops down enough to remove and reinstall everything.  Not my preferred method but do-able since I have a lift.  I prefer the rearend out and up on the work bench though.  I definitely don't care to do them on my back anymore.
 

[cook_dw]

I disagree on the depth gauge that it is necessary to do it correctly..  I have built many before I decided to make my own depth gauge (which I have now misplaced or let someone borrow and never got it back)..  Does it make it easier?  Yes but you can accomplish the job without it..  I also prefer a solid sleeve over the crush because you will get the same bearing preload every time once you get the correct shim setup.. Once the pinion depth is set and btw the depth is on every aftermarket gear set on the face of the pinion gear.  I usually get the backlash set between 0.005 - 0.009"  Sometimes wider or tighter depending the application but for a street car that has new bearings and gears I try to stay on the tight side of the range..