Hi, can you identify what year a diff was manufactured from the serial number?

C3 differential year of manufacture?

 

Assembled August 10, 1979 at the Warren MI plant on the first shift with an Eaton posi and 3:55 ratio.

 

Winner, winner, chicken dinner!

 

Was '79 a year of soft yokes? The yokes had about 49 thou of clearance and the ends had started to mushroom slightly. I'm going to rebuild the whole thing, wish me luck, my first diff rebuild.

 

Good luck
Yes 79 original axles are not good, if the face dimension is still good and you won't know until your posi is done, you can harden and temper them. Otherwise replace them but be aware of the quality of some new and rebuilt axles today. You're a long way away from the usa to find you have a part that isn't right and needs to be returned.

 

According to GTR1999 they are, and if yours are starting to mushroom then that proves it. I'm going to be starting on my spare diff soon, so I'll wish us both 'Good Luck'

 

@GTR1999
Gary, can I assume the spec you are referring to, if one has correctly followed yours & Tom's advice on the posi tuning, is too much gap from the face to the cross shaft? So if one has seemingly good axles, measurement from snap ring to face, you really can't prove they are 'good' until the posi is tuned and the gap measured.
Also, I have sourced NOS RG bolts, not the flange head style. Do these require washers? (I would assume so)

 

That is correct. The face dimension is only a starting point of reference. I actually have some original prints I was given year ago on the axles, I don't recall off hand if the face dimension - as I call it- was on there. What I do see with rebuilt axles is this dimension is never consistent, it may be very low, mid way or even too much. I don't understand how guys are setting them up correctly given some of what I have seen.

Now you are correct in the fact a tuned posi- one done correctly, is much different than a common spring loaded one that is setup based on the 001-008 spider lash given. That is a mile long dimension and one reason a spring pack is used.

I have seen a lot of rebuilt diff's and spoke with a lot of people who called me over the years to ask why their fresh stock rebuild from the their favorite place has 020-050 axle end play in them, one guy years ago told me his had 100" endplay and he didn't even install it. End play is caused by what I call the big 3- posi setup, axle face dimension, cross shaft hole wear. You are going to rebuild your diff you should check on all of these at the time.

For the RG bolts. My advice is to spend the $35-$40 and buy new ARP 230-3001 ring gear bolts and use them over NOS GM. The original GM bolts used from 63-about late 76 were good, as you may have, but they also used hardened lock washers. By the mid 70's with QC low, bolts started to back out and lock up the diff. It happened through all the year ranges but was most common from about 74-79 with the 77-79 flange heads leading the way. Get the ARP, wash them and RG hole with Acetone, blow them off, use fresh LOCTITE #271 or 272 red- put a line down the hole and a drop on the bolt and install them. Countersink the flange holes in the case first.

 

Thanks Gary, I have been using your papers to guide me in the rebuild I had replacement yokes arrive from a supplier here, eyeballing them it appears they are about the extra length that will be needed to bring the clearance within "spec". The rest of the internals look good but the other reason for pulling it apart is that its noisy at open road speeds. I still have the original to the car '70 diff its a 3.08 and is dead quiet but too high a ratio to work with the 200R4. I was stepping into the unknown when I bought the '79 diff but it was the ratio I wanted. FWIW its the only C3 diff I have seen for sale here in the last 8 years.

 

The axles may look the same in length, that is not the issue. They should slide into the spider without any issue - free and smooth- that's one problem, the other is the face dimension.