My 87 IROC has RPO codes G92, G80, GU5, and J65. This should mean that the car SHOULD have a posi rear with 3.23 gears and disc brakes, correct? Well my speedo is about...17% slower than the car, so I decided to inspect the gears today and change the oil. When I counted the teeth I got 2.77. Confused, I went ahead and wire brushed that little tag that was bolted to the diff cover, and it too says "ratio 2.77." The housing says Borg Warner on it, and I could not find the code, all I found was U25J5 on the bottom, but I think I just needed to wire brush harder because I saw a picture of someone else's code and it's a lot smaller than I expected. The discs are there, the posi is there (did the wheel spin trick, NOTE: make sure your shifter is in neutral or wheels will turn opposite directions and fool you!), but the ratio does not match up to GU5. This car was also a 1AZ, special order, so is it possible this is the originally ordered rear, or has it been swapped, and is slowing down my speedo?

 

I would verify that the SPID label VIN matches your car VIN before thinking the rear was swapped.

 

It does. Where would a 2.77 rear come from?

 

On page 8-9 of the Haynes Repair Manual, the axle codes of 1988 and 1989 include:

GH3-------2.77-------Posi-----Borg-Warner
GW6------3.27-------Posi-----Borg-Warner
GM3-------3.45-------Posi-----Borg-Warner

6HT------2.73--------Posi
6HB, 6HF--3.08-------Posi
6HL------3.23--------Posi

So the Borg-Warner rear ends had a slightly different ratio than the standard rear ends. That still does not confirm that the 2.77 was the original axle. It is possible that the special order substituted the axle, but I would think that the RPO would have indicated what you got Maybe the factory was out of 3.27 axles when your car went down the line, and it got a special ratio

Dave

 

I see that beginning in 88, the standard axle for the L98 is a 2.77 Borg Warner, and G92 got you 3.27. Rear discs and posi were a requirement with the 350 too. It could be that this rear came from a 88-89 350 Camaro, but how can I tell? Is the code the only way? Is it in a different spot on Borg Warner rears? Or do I just need to look harder?

 

I don't think it's the original anymore because like you said, that ratio was available in 88 and 89. The ratios in 87 would be 3.27 or I think 3.45 for BW's. 3.23, what the RPO codes say my car should have had, apparently isn't even a BW unit? So that is even more of an indication of a swap. At least he had the courtesy to keep the discs and posi in there. But yeah, where's the code on this thing, and what does that U25J5 mean?

 

If it's a 9 bolt unit, there should be another tag that has a part number that looks like this...



I thought all of the Iroc's had the BW rear like the GTA's got but could be mistaken.

I personally wouldn't bother with the code on the axle tubes, the BW rear that the above tag came off of didn't even have codes on the axle tubes.

 

Unfortunately, no.

JamesC

 

The first thing that should have been a tip was the 9 retainer bolts instead of 10 for the rear cover - please don't take this wrong.The code on the tube is most probably the manufacture plant/date.I agree with the rear being swapped possibly due to bad posi unit or gear problem.At least you can solve the speedo problem by changing out the speedo drive gears in the trans to the correct ones - that is if you intend on keeping the same rear gear set.

 

Yeah! That's the tag I found. My P/N is 10040660, Ratio 2.77, and Serial No. 14397. And then there's the U25J5 on the bottom next to the logo. That's about all the information I'm gonna get off this rear I guess? Can you decipher any of that to tell where this rear came from? Thanks for the warning on the tubes, I woulda scraped a hole with that wire brush looking for em today.

You're right coolram, I didn't know at the time that the original axle should have been a 10 bolt, for some reason I thought it was going to be a 9 because if G92. But I guess it's pretty cool because I wound up with a better rear, higher gears (which works for me since I drive a LOT and could use the extra boost in mileage), and didn't lose discs or posi. Still curious where this came from though. If anyone knows how to decipher those numbers please share. Thanks everyone so far for their help!

 

Looks like the rear came out of an '86 model, either Iroc, or Trans Am

 

But there's no 2.77 on the powertrain list for that year. Only 88 and 89.

 

Where are you reading that? The 2.77 was the standard ratio with the BW rear in 1986, the 3.27 was optional.

 

The tech data on this site. The article on the 1988 models says it was the first year for them. Unless it's slightly inaccurate. And actually come to think of it I went down to the Chevy dealer to get the right speedo gears today, and the chart with all the rear, tire, and speedo gear combinations didn't include 2.77 in 1987, but 1989 did. Are you sure you don't mean 2.73? That was there.

 

The tech data on this site has a lot of errors, that'd be one of them. Parts books are just about the same, dangerous to use for anything concrete. I can remember trying to get the right diff gasket for my 85 back in 1987... that was fun.

1985 was the first year the BW 9 bolt rear was used on Trans Ams. TPI cars came with the 2.77 standard, 3.27 was optional, just like 1986. I dont know beyond that, but I have a factory invoice for my 86 and it has the optional 3.27 rear. 2.73 is the 10-bolt.