1977 trans am excessive end play

[DirtyWhite77]

Been working on my rear suspension and noticed that my drum on the right side doesn’t sit as close to the backing plate. Pulled drums off and the back of the flange on my axle shaft on the right is about 1 1/8” away from the wheel cylinder, and the left side is 5/8 away from the wheel cylinder.

I think this is end play, and know that it should be with in a spec. I have a Hanes manual for the car and it lists a high and low spec for the end play, I haven’t gotten my dial indicator on it yet. I’ve never had an axle do this in any of my cars or trucks. I got a couple questions.

Is this necessarily end play? The shafts only moves back and forth in the axle tube a small amount and neither one that much more than the other (to my naked eyes/hands). But the shaft on the right is sticking out of the tube almost 3/4 more than the other side.

If it is the end play or something else what are the chances it needs completely rebuilt?

 I can remove and install the whole axle assembly, the shafts the bearings and all the seals, and I can get everything back in except the ring and pinion but my brother can set that up. So I’m not really worried about spending a ton of money. I’ve replaced axle shafts seals and bearing before but never had to work on or inside anything in the diff. The car will be driven on the street as a daily driver so reliability and safety are paramount.


When I got the car there was no fluid in the trans or the rear end. Trans has a small leak, rear end doesnt.

[MNBob]

It could be normal, but I would check it out.  It doesn't take too long to pull the axles and compare them, maybe one was replaced at some point.  As I recall from 5 years ago, you remove the cover and there is a pin bolt that comes out.  Once that is removed, you push the axles to the center and remove the C-clips with a magnet.  Then the axles can be removed and compared.

You do not have to remove the seals or the ring and pinion especially if there are no leaks.  Think of this as an opportunity for a fluid change.  You are supposed to use standard gear oil (non synthetic) and a tube of limited slip additive.  You can also clean and paint the cover, use a new gasket, and clean out the junk particles in the bottom of the diff (trust me there will be some junk).

[DirtyWhite77]

Okay I’ve already pulled the cover before and installed a new cover and fluid. The cover was rusted and pitted pretty bad so it got a new chrome one. I’ll try to pull the axles today. If they are the same and the clips aren’t worn it would be the diff correct?

[MNBob]

I can't think of anything else.  Any other opinions on this?

[pancho400cid]

Sounds pretty weird.  The drums should seal up to the backing plate without a visible gap.  I was wondering if you had a 2" drum on one side and a 2.5" on the other, but as I dig into it I think 9.5" x 2" is standard on the rear?  A 2.5" would not fit at all - plus you said you measured from the back of the wheel flange and got different numbers, so drum width shouldn't be the issue.

The left and right axles are exactly the same on Firebirds/TA's.  I think the GM part number for the axles is 1255802, and I think the stock length is 30-1/16" from the outside of the flange (wheel mounting surface) to the inboard end of the axle (not the C-clip groove, and NOT measured at an angle).  I don't see how the actual differential could be the issue beyond something being seriously wrong.  Is everything original? (housing? axles?, etc?)  If it's a bunch of combined parts, aftermarket axles, etc.  not original to the car, that may be your issue.

As mentioned, C-clips hold the axles in, but they are captive in a machined recess in the two spider gears on the axles.  To get them out you have to remove the retainer cross-bolt holding the spider shaft (NOTE - the cross-bolt often breaks on removal), then remove the spider shaft, and then you can push each axle in toward the center.  Then you can remove the clips.  See the pic below, which I found on the internet and marked up.

It sounds like one C-clip may have failed, which does happen from time to time.  The axle can pull right out if so, so don't drive it until you figure out what's up obviously.



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[DirtyWhite77]

Thank you guys for all your help. I had never taken shafts out of an axle like this one and y’all told me everything I needed to know clearer than the manual did.

Picked up a set of outside wheel bearings, seals, c clips and fluid. Got the axles out and one is definitely longer that the other by 5/16. Standing them up flange and lug nuts up and measuring on the concrete the length of the radius of the flange away from the center point of the small side of the axle upwards toward the flange I get 30 1/6 to the outside side of the left shaft, and to the inside side of the right shaft. Meaning the outside side (drum mounting surface)of the right shaft is 30 3/8”. The face of the flange on the shafts is also slightly different. The left being machines cleaner more precisely and having some number stamped around the center circle. The right shaft didn’t spend as much time being machined really well. It’s not ugly just not as perfect as the left. In my opinion the right side seems to be aftermarket. The splines and bearing surfaces on both axles looked very good no real signs of wear.

So I need to buy another shaft that 30 1/16” long? I measure the left one correctly right? How strong are factory shafts or ~$90 aftermarket ones? I’d like to get this car to 400 hp on street tires one day and don’t want to have to rebuy axle shafts, but there’s also no need for to spend $400 on some high end chrome moly jobs right now if a ~$90 aftermarket shaft will survive alongside a factory one.

I also found a part number for Richmond gears on the ring. Counting driveshafts rotations for a axle shaft rotation the driveshaft spins just a tad over 4 times for every time of a axle shaft. So I’m guessing 4.10 gears. And I found a patent number for Auburn gear and differentials on the carrier.

I know 4.10 in a truck with 32” tall tires and an overdrive gear is steep for me. A car with 26” tall tires and a 3 speed is not gonna be better. Calculator says I’ll be turning around 3200 rpm at 65 like this... not ecstatic about that but I didn’t buy it to road trip.

Anyone have any info on Auburn gears? Never heard of them. Couldn’t find an actual model number from them. Car does spin both tires, at least in the grass around the house.

[pancho400cid]

If the car was originally a Trans Am, it would have come with a GM "positrac" differential, which is an Eaton design and it had 10 bolts holding the ring gear to the carrier ("aka "10 bolt") and an 8.50" ring gear diameter.  All Firebird/Camaro rear ends were actually built by Chevrolet so it get's confusing looking for parts sometimes because it's a "Chevrolet" rear end even though it's in a Pontiac car. 

Sounds like you have mismatched axles and somebody rebuilt the rear end with Richmond gears and an aftermarket Auburn limited slip differential.  I'm no expert but I think Auburn positracs are a "cone" type and not rebuildable... which some people count as a mark against them.  I have no opinion.  Richmond gears are like anything... some say they are fine... some say not.  In my experience the set-up is more important than the gear brand.

Assuming your car has the correct housing then the 30-1/16" inch axle should be right and the longer one is incorrect.  I didn't quite follow how you were measuring the axles.  I got the pic below from the Dutchman Axles website, and it shows how to measure.



As far as figuring out the gear ratio, the best way is to count the gear teeth since it's opened up anyway.

   Ratio = number of teeth on Ring Gear  divided by number of teeth on pinion gear.

Count the teeth on both gears and divide with a calculator.  A 4.10 or 4.11 may be steeper than needed.  Even a mildly built Pontiac 400 generally has plenty of low end torque and so a ratio more like 3.50 or so is more commonly picked.  You can fill books with that discussion, so I'll leave it at that.

If I had 400 HP on tap, I would want aftermarket axles.  Axles do beak sometimes, and a C-clip axle will come out of the housing when that happens.  I used Dutchman SR axles on my car.

Lastly - Factory axles had 28 splines.  Some aftermarket differentials are available with 31 splines.  You should double-check what you have before ordering anything.

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