78Cutlass4Speed

I'm looking to put a posi unit in a rear end I'm going to install in a 1979 Cutlass project car. NOT changing the ring and pinion, just the carrier. It has 2.73 gears. I know I need a 2 series carrier. Looking at an Auburn Gear unit. Is this an easy swap? I've never dug into a rear end before. Is it just a matter of bolting the original ring gear to the new carrier and installing? Is it more involved?
Thanks

joe_padavano

It is NOT simply bolting the new parts in. You need to properly set the various parameters of the rear axle - pinion depth, bearing preload, backlash, and gear pattern. Here's a start at the process.

Fun71

In my younger years I successfully installed a limited slip carrier in place of an open carrier on multiple occasions. The only "measurement" that I performed was to verify the backlash was within tolerance. This involved adding/subtracting carrier bearing shims from either side to move the carrier laterally and achieve the correct backlash.

joe_padavano

You can "rebuild" an engine by simply putting new rings and bearings in with no machine work. You might even get lucky...

78Cutlass4Speed

Doesn't sound like a job I want to attempt. Will let my local garage do it for me.
Thanks.

Fun71

I do not see how changing the carrier would impact the pinion depth and/or the gear pattern. The center line of the carrier rotation is still in the same location relative to the pinion and the it's the same ring gear, which has not changed diameter. All I see is a potential change in the backlash (carrier side to side location).

If I am missing something obvious please point it out to me.

JohnnyBs68S

If all I was doing was replacing the carrier w/ a posi unit, I would also replace all bearings and seals in the rear end, which includes the pinion bearings. Once you do that, you'll need to establish the correct pinion depth WRT the center-line of the carrier bearings in order to have a correct gear mesh pattern. This involves shimming the main pinion bearing as it is pressed onto the pinion shaft. I'm not sure I'd trust new bearings to be exactly like the old bearings not to at least check this. Re-using the old shim stack would be a good starting point, and as Joe says, you might get lucky. If not, at least you know what to do in order to not ruin the gears by setting it up w/ the wrong mesh pattern.

If it is decided to not touch the pinion or its bearings (crush sleeve, etc), then pinion depth would remain optimal and the only thing left to do is to shim the carrier bearings for the correct backlash, which should give you a good mesh pattern. Once that is done, set the preload by adding identical shims on both carrier bearings.

JohnnyBs68S

When I upgraded my rear to a 3.90/posi, instead of pressing on the pinion main bearing first to measure the pinion depth, I used a scrap piece of alum. "L" bracket to bridge across the carrier bearing saddles in the housing, and with the new pinion bearing sitting in its race, measured the distance between the face of the pinion bearing to the bottom of the "bridge" (I made multiple measurements and averaged them to remove some uncertainty). From that, I could calculate the required pinion bearing shim stack to achieve the desired pinion depth (also had to measure the thickness of the pinion and know the diameter of the carrier bearings to find its center-line). I was very careful with these measurements and calculations (repeated multiple times) and was lucky enough to get the pinion bearing shim stack right on the first try and avoided having to pull that bearing off the pinion to adjust the shims.

Of course, it did take me 2 tries to get the pinion bearing preload just right. Torquing that pinion nut w/ the crush sleeve installed to achieve this was a bit tedious.

joe_padavano

That's essentially how the CSM tells you to do it with the special Kent Moore tools. And FYI, I've certainly messed up my share of crush sleeves setting pinion preload.

monzaz

I have done thousands of rears and i can assure you that if the rear was running correctly with the old bearings and you just changed bearings with all the same parts it will run fine again. AS LONG as you press on all the parts correctly. ( I have seen people not seal the bearing all the way to the mating surfaces on the carrier and pinion etc. )We have done tons of re-bearing job with out a gear swap and have never run into a bearing that was that out of wack from another. Use a Timkin or Koyo you will be fine
I AM NOT saying not could never happen, as it is going to be a VERY LOW percentage. 30 years and counting....

dirt and debris will be your worst enemy in the saddles and behind bearing lands and the under the ring gear.

Good luck on what ever you choose to do...
Jim
J D