COPO Eaton Posi

**big gear head** · #1 11-09-2017, 05:34 PM

Got a quick question for you. I've been building rear ends for 36 years now and I've seen most of the Chevy 12 bolt stuff out there. As far as I know the COPO 30174 Eaton Posi had special heat treated 17-10 tooth gears in it. I've got a guy telling me that he heard that they got 16 tooth axle gears and that the case was machined different for these gears. I've learned over the years to never say never, so I'm asking the guys who have seen more of these than I have. I've never seen or heard of any 16 tooth Eaton gears. Am I wrong?

**Hotrodpaul** · #2 11-09-2017, 08:12 PM

I thought that the COPO posi unit rear used larger non posi 12 bolt spider gears, special larger tooth side gears, a special machined 30174 case and heat treated 4:10 ring and pinion. Very hard to find especially in good condition.

**m22mike** · #3 11-10-2017, 02:12 AM

BE KQ specific, and the carrier is machined for larger thrust washers/spider gears.
Springs are wound in the opposite direction of the green 12 bolt spring. Side gears are 16 teeth vs 18 standard 12 bolt and are thicker than the 18 tooth. BE spider gears are larger and still have 10 teeth like the smaller 10 tooth standard 12 bolt.
There are some comparison photos in some book I looked at once.
I had posted on this before and can not find it..

Mike

**big gear head** · #4 11-10-2017, 12:10 PM

Thank you very much Mike. That is very helpful. Do you know how many clutch plates were used in these units? Was it 18 or 22?

**m22mike** · #5 11-10-2017, 12:45 PM

I think it is the same clutch pack. And on the carrier unit it will be the same casting # but the pn# is different , and of course that is not on the carrier.

3917971 is the part number for the r&p set.
The numbers that are stamped into the actual gears are 3916226 and 3916228.
The 4.56 set is stamped 3916230 and 3916231.
The 4.88 set is stamped 3916232 and 3916233.
The assembled COPO carrier part number is 3916234.
The COPO carrier casting is 30174, just like the standard 4-series unit.

**big gear head** · #6 11-10-2017, 02:30 PM

Thanks again. I may have a set of those 4.56 gears. I need to check the numbers.

novadude · #7 11-10-2017, 07:09 PM

Interesting...I never knew that the COPO cars had a special Differential. How does all of this compare to the currently available Eaton Posi? Are they using the standard 12-bolt design in the new piece, or the COPO set-up?

**m22mike** · #8 11-10-2017, 08:48 PM

Not like the COPO units at all, some very hard to acquire pieces.

Mike

**big gear head** · #9 11-10-2017, 10:14 PM

The new Eaton Posi has several changes. The original Eaton Posi had 18-10 gears, 18 disc steel clutches and 200 pound preload springs. The new Eaton Posi has 17-10 gears, 14 disc carbon fiber clutches and 400 pound preload springs. I think the new gears are powdered metal. The earlier Moroso bruit strength was a heavy duty Eaton Posi with forged 17-10 gears, 22 disc steel clutches and 800 pound preload springs. The case is still the same and all of the parts interchange. The COPO is apparently a little different from all of these.

bcmiller · #10 06-05-2022, 01:26 AM

Old thread but I came across a set of the 4.88 gears stamped 3916232 and 3916233 dated 11/72.

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