78 Estate Wagon Rear Axle Ratio

[dmfconsult]

Hoping someone might have this information handy, as I'm trying to identify the rear end in my 78 Estate Wagon.  I suspect it's the standard 2.73 but want to be sure...  The stampings on the axle are "2YJ P185D".  I can decode that it is a Pontiac sourced rear end built during dayshift on July 4, but the 2YJ is what is stumping me.  Here's a pic of the stamping:

All I can find online is a reference to RPO codes (which I will get next time I'm at the car's winter storage).  Thanks in advance!

 

Edited November 8, 2017 by dmfconsult (see edit history)

[Frank DuVal]

While you are there at the car, jack up one wheel, and count the number of turns the drive shaft makes for the wheel to go around once. Use jack stands and other safety equipment, of course!

 

If you have a posi, (limited slip in GM speak), you will need both wheels off the ground.

 

Yes, using the stamped numbers or RPO codes is easier than crawling under.... And way cleaner than pulling the pumpkin cover and counting teeth. But counting teeth give the REAL ratio! I doubt anyone changed your third member, but in Corvairs you have to count as so many people changed things around over the years, we do not believe any stamped codes....

Edited November 8, 2017 by Frank DuVal
Spelling error, I need new fingers. (see edit history)

[dmfconsult]

Thanks Frank,

I doubt the rear end's been changed as I know the history of the car.  The RPO codes should tell the story, but as it will be a few days before I can get those, I thought someone with a chassis service manual could chime in (assuming the codes are listed in there).  Everything online seems to be related to the intermediates and chevies, not full size BOP. 

[NTX5467]

The stamp code you've found looks a little different from how I remember they were.  Granted, it will detail the source and production date, but as I recall (at least on Chevy axles) that it's the last places that specify ratio and PosiTrac or not.  In that respect, the stamping you found looks "out of order", comparatively.

 

The RPO is an option/order code, NOT related to the rear axle stampings per se.  The RPO will define ratio, with "G80" for "limited slip" separately.  The stamp code on the axle combines these two things in one code.  AND, those stamp codes suffixes were typically for ONE model year, while RPOs could work for multiple model years.  Similar, in concept, to the engine stamp codes.  As far as I know, the only place to find what you desire is in an appropriate GM parts book, in the front section where the decodes for engine codes and axle codes are listed.

 

The only way to accurately determine what the ratio is a little messy and would require a new gasket and some bottles of rear axle lube.  Typically, on the edge of the ring gear, the ring gear & pinion teeth are stamped.  Like "41-11" format.  In this example, 41 teeth on the ring gear, 11 on the pinion gear.  While in there, you can look to see if it has PosiTrac or not (rather than just spider gears in the case, you'll see clutch plates and the spring(s) holding them in there).

 

NTX5467

Edited November 8, 2017 by NTX5467 (see edit history)

[dmfconsult]

Thanks NTX, I was hoping you'd chime in.  According to what I have, the "2YJ" code will identify the rear end ratio and whether or not it's limited slip on these corporate rear ends, but of course no joy on finding a decode reference (I guess the internet doesn't know everything!).  My parts book only goes to 75, but I checked it anyway in case there was carry over and no reference to YJ.  I'll get the RPO codes to see what I can glean from them before I pull the diff cover off.  

Edited November 8, 2017 by dmfconsult (see edit history)

[JohnD1956]

according to my 78 service manual that would be a 3.08 rear axle code, non posi.  EXCEPT It's not supposed to be stamped into a "B" body axle.   The manual says all the 78 Buicks. except the B body cars, have that code stamped into the forward side of the passengers side rear axle tube.  The "B" body cars just have a tag attached to the rear axle cover, similar to the posi tag, but on the other side of the rear cover.  It shows a Posi tag would be on the passengers side of the cover, and the axle ratio tag would be on the left side of the cover.

[dmfconsult]

Thanks John, super helpful as usual!  I don't recall seeing a tag on the diff cover bolts but I will look again when I'm at the car on Friday (I have looked for a posi tag in the past, but one was definitely not there).  The car was built at the Kansas City plant, not sure if that has anything to do with it or not, but I'm not surprised it would be a 3.08... it does feel peppier off the line than the Electra which has 2.78's.  I'll post an update once I get the RPO codes and check the diff cover for additional tags.  Would it be weird if it had both a tag and a stamping? 

Edited November 8, 2017 by dmfconsult (see edit history)

[JohnD1956]

I thought it was crazy that every other model was stamped but not the B cars. Did you find the stamp on the pass side tube? 

I know my former wagon had a tag on the  drivers side of the cover when I got the car because I assumed it was a posi tag. But it wasn't a posi rear. By the time I found the section I referred to and checked again it must have rusted and fallen off as there were no tags. 

[JohnD1956]

[dmfconsult]

7 hours ago, JohnD1956 said:

 Did you find the stamp on the pass side tube? 

 

Thanks John for the pics. 

 

Yes, the markings are right where they should have been for a non "B" car, the front of the right (passenger) side axle tube.  It took a bit of cleaning, but there they were.   The diff cover has the scallops as well, so I presume it's an 8-1/2" rear end.  It's interesting what you find when you start looking...

Edited November 8, 2017 by dmfconsult (see edit history)

[Ben Bruce aka First Born]

1 hour ago, dmfconsult said:

Thanks John for the pics. 

 

Yes, the markings are right where they should have been for a non "B" car, the front of the right (passenger) side axle tube.  It took a bit of cleaning, but there they were.   The diff cover has the scallops as well, so I presume it's and 8-1/2" rear end.  It's interesting what you find when you start looking...

 

  And know what one is looking for.

 

  Ben

[NTX5467]

In the illustration with the rear axle covers, the "G" axle would be the normal Chevy 10-bolt 8.5" ring gear axle.  The "scalloped" covers would have been on non-Chevy vehicles, typically, except in some specific cases or models, but not on the '77+ B-car Chevy platform.

 

NTX5467 

[dmfconsult]

Unfortunately no joy on the RPO codes.  Whatever sticker they were on, has disintegrated or otherwise disappeared from the car.  Not in the usual spots or anywhere else.  It's weird because both the emissions sticker and manufacturer sticker (GVW, Tire info, origin, etc) are both still there (although the Manufacturers sticker is so faded the data is not legible).