C2 Differential Identification

**Harry Sadlock** · June 27, 2006, 12:57 PM

Re: C2 Differential Identification

Pete, on the underside of the carrier, just forward of the rear cover there should be several letters and numbers stamped into the carrier. They are visible without removing anything. You just need to jack up the back end and crawl under, lots of light and a magnifying glass also helps. There may be surface rust or dirt/grime making them hard to see. Post what you find and we'll help identify the rear.

As an example for a 63 rear CB 4 10 63 would be a 3.36 posi rear built April 10 1963. The cast date of the carrier, the cast number of the carrier and rear cover would also help if you could post those.

Good Luck

Harry

**Alan Drake** · June 27, 2006, 03:02 PM

Re: C2 Differential Identification

Harry while you have your books open; Mine is CE 3 10 64 as expected, 4.11:1 posi, however not sure of other numbers.
On top there are raised numbers 3830309 and along the pasanger side next to fill nut is B14.
Thanks for your time
Alan

**Harry Sadlock** · June 27, 2006, 03:28 PM

Re: C2 Differential Identification

Alan, is the 3830309 on the rear cover? If it is, it might really be 3830303. If it is, it is a casting number for a 63/64 as I recall. It should have the course thread plug/bolt in it. I don't recall that the rear covers were dated for 63/64. The only other marks on the cove were casting numbers like GM 1, GM 2, etc,etc... But the B14 being where you described is probably on the carrier. If so then the B14 is February 1, 1964.

Harry

**Harry Sadlock** · June 27, 2006, 04:16 PM

Re: C2 Differential Identification

Rear Cover, the GM 1, GM 2 and GM 3 castings on cover 3830303 used the fine thread plug/bolt. The GM 4 cover changes to the coarse thread. What I do not know is when (what date in 63 or 64) the GM 4 cast rear cover went into production.

Harry

**Pete Zietzke** · June 27, 2006, 04:35 PM

Re: C2 Differential Identification

Hello Harry,

Thank you so much for quickly responding to my initial email. WOW!! ... I'm very impressed.

Okay, after jacking up the rear end of the car and crawling under there so as to look up at the location you described, I found the following letters/numbers stamped in the diff casing: 2A011668W. Please note here that the 2 appeared more as a 'question mark' because the bottom part of the number is not there ... so I'm assuming the number is a 'two' rather than anything else.

I also noticed an exposed blue-colored gasket sealing material between the diff body and its rear cover. I vaguely remember having this diff professionally rebuilt at some time ago ... long ago ... am not absolutely sure though.

I certainly would appreciate your identification of this diff. Is it possible to determine if it came out of a BB or SB car? Also, what the heck is a 3-Series and a 4-Series Positraction Carrier Unit?

Many Thanx,

Pete.

**John H.** · June 27, 2006, 04:48 PM

Re: C2 Differential Identification

Harry -

The GM1-GM2-GM3-GM4 I.D. is strictly the pattern number that was used to create the sand mold for the casting; multiple patterns were used, and they were numbered for traceability. The finished configuration of the cover (after machining) was unrelated to the pattern number on the casting. The casting date code is a much better discriminator to identify the effective point for running changes.

**John H.** · June 27, 2006, 04:54 PM

Re: C2 Differential Identification

Pete -

The codes appear to identify a 1968 3.70:1 small-block Posi diff, assembled on January 16, 1968 (or the same '69 diff assembled on November 6, 1968), on second shift.

**Harry Sadlock** · June 27, 2006, 04:58 PM

Re: C2 Differential Identification

Pete, what looks like a 2 may be a scratch. Anyway the AO11668W translates to a 3.70 posi rear. The date appears to be November 6, 1968 making it a 1969 model year rear end. Is there an E under the AO, this would indicate a Eaton Posi Rear.

The AO was used from 1965 to 1969 best I can tell.

Harry

**Alan Drake** · June 27, 2006, 05:16 PM

Re: C2 Differential Identification

Yes its 3830303 with a GM 6 just above the spring mount area. The plug appears to be a fine thread

The code CE 3 10 64 is on the front section of the rear end - the section that does not hold the spring, but has drive shaft attached to it. (sorry I do not know the correct terms)

Thanks for the info, Off to car show
Alan

**Harry Sadlock** · June 28, 2006, 08:29 AM

Re: C2 Differential Identification

Pete, I sent two email's, please let me know if they arrived.

Harry

**Pete Zietzke** · June 28, 2006, 11:12 AM

Re: C2 Differential Identification

Hi Harry,

As I have the original diff from my car, I'll not need to bid on those EBay items you wrote me about. Thank you for the tip though.

Yes, I did receive two emails from you. All of the information you provided has been an enormous help to me.

My apologies to you if I've been unresponsive in responding back to you ... I broke one of my teeth yesterday ... big toothache!!! I'm leaving now to visit my dentist.

I'll be back in touch soon ...

Best Regards,

Pete

**Pete Zietzke** · June 28, 2006, 05:27 PM

Re: C2 Differential Identification

Hi John,

Thank you for the information you provided to me.

But the information opens up another question I have ... how does one distinquish between a small-block Posi diff and a big-block Posi diff?

Is the big-block Posi diff more 'rugged' and able to withstand the higher torque produced by the big-block engines? If so, please describe the additional performance features ...

The reason I'm asking this question is because I've intstalled much wider rear tires compared to the original size tires that came with the car. I'm simply wishing to make sure the diff is capable of handling the 'coefficient of friction' difference. When one goes through the math, they'll determine the new tire size gives a 1.87 advantage over the originals which is almost 2X more 'tractive' energy assuming the tires do not spin with respect to the surface they are in contact with.

Best Regards,

Pete

**Joe R.** · June 29, 2006, 12:18 AM

Re: C2 Differential Identification

It is my understanding that there are no internal differences between the small block posi differential and the big block posi differential. I believe that the only difference between small block and big block differentials was that the yokes for the half shafts used conventional U-bolts for small blocks and stronger metal caps secured with bolts for big blocks. In other words, the only thing that changed was the yokes.

If I am wrong about this I would like to hear from others on the board what additional differences there were.

**Pete Zietzke** · June 29, 2006, 11:22 AM

Re: C2 Differential Identification

Thank you for the information provided.

There seems to be yet another difference??? I'm looking at a ZIP Products 63-65 Corvette Restoration Catalog ... in their ZIP Products Rebuilt Differential section. I see a reference for a 4-series carrier and a 3-series carrier. The written descriptions imply that the 4-series carrier is an "extreme performance" diff. Also, the Positraction Unit for a 4-series diff is different than the Posi for the 3-series diff.

Do you or other board members here have additional information as to better understand the difference between the 3-series and 4-series carriers?

C2 Differential Identification

C2 Differential Identification

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