[DBONES152]

My 66 GTO currently has a 12 bolt rear end that matches to a 1972 Chevelle. It’s a 410 ratio, and for years I’ve been wanting to take it down to a 355. I’ve been watching e-bay and have found 2 gears that I am interested in. The first is a Richmond, a brand I’ve heard of, and this is the link:

http://www.ebay.com/itm/Richmond-Gea...item5ae2e40b01

If I buy this one, I can add a rebuild kit for about $100.

The other is a complete kit with a Motive gear, a brand I am not familiar with.

http://www.ebay.com/itm/GM-8-875-12-...item3ef3ac743f

I know a retired mechanic who said that he would install the gear reasonably.

My question is 3-fold.

1. Will these gears fit?
2. Which is the better gear and deal?
3. What do you think reasonable is for installing the gear?

Thanks,
Dan

[melwff]

I was under the impression that a 4 series gear set uses a different carrier assembly than a 3 series gear set meaning you would have to change the carrier to go to a 3:55.

[DBONES152]

Thanks melwff. I did a little more research after your post, and there are 4:10 gears available for both the 3 and 4 series carriers. So I will have to figure out what carrier I have in there before I proceed. If I'm lucky it will be a 3 series, whereas I can just replace the gear, if not, it will involve a new carrier also.

[AG]

Just make sure which carrier you have and the gears fit it. They make a 3.73, 4.10 gear that fits both 3 and 4 series carriers.

[A Schab]

The casting #'s for chevy 12 bolt posi units are,2 series ED32088,3 series 30140PM1,and 4 series EDB30174.

[DBONES152]

Originally Posted by A Schab

The casting #'s for chevy 12 bolt posi units are,2 series ED32088,3 series 30140PM1,and 4 series EDB30174.

Thanks A Schab. Where on the differential do I find these casting numbers?

['ol Pinion head]

The EDB xxxxx casting number of oem Eaton posi carrier is on the rh end of the carrier. Or with the posi carrier loose & the carrier setting on a table on its left hand bearing boss, wou will see it on the top.

"174" is the most common 4 series Eaton 12 bolt posi, & will not take a 3.55 gear set, but will take richmond's special thin 3.73 gearset.

"140" is the oem Eaton 3 series carrier & will take GM & aftermarket 3.55 gears.

Most well used Eaton posi carriers will need side gears & spiders replaced, always something to consider.

[Half-Inch Stud]

I've owned 2,3,4 series 12-Bolt Posi's & the Strange 12-Bolt 4-series Spool.
Also, used a 2-3series spacer. I'm sick of 12-bolt rears.

I'm impressed with the Big Car (Impala, Kingswood estate) 12-bolt posi, and the Dana-60.
Big Car 12-Bolt is capable of Highway gears whereas Dana-60 is not.

My point: Strange Engineering will drop-ship a 3.54:1 Dana-60 with Pro-posi&35spline axles, 1350U-Joint, & chrome cover to your garage door for $2100 or so.

[Cliff R]

Good info HIS. I've got that much invested in my 8.5 ten bolt. Next time I blow it up, I'll be shopping for something a LOT stronger that I can just bolt in.....Cliff

[Half-Inch Stud]

Dan Debones152;

Stock GM 3.55 gears on the original posi are probably what you could aim for.

One more idea: The 8.8 Lincoln Continental (Mark 6-7?) Disc-Brake rear may bolt-right-in to the 66 GTO , and be axle-axle width compatible. 8.8 rears look quite like a 10Bolt, disc-brakes usually 3.27 posi, have 5"pattern and...............Half-Inch Studs