Is this for real? '70 12 bolt

Gary S

Lifetime Gold Member
Apr 14, 1999
Bismarck, North Dakota
The 3.55 was available on the Camaro only on the SS396/L78, and that car required posi. That leads me to believe that a COW rear end is likely from a Firebird. I can't find a complete listing of Firebird drivetrain offerings, but the Trans Am option mandated posi just like the L78, so it isn't likely from a Trans Am. I don't know if the Formula or Esprit options mandated posi, so it could possible be from one of them.


Veteran Member
Gold Member
Mar 1, 2020
I've looked at the charts and see it was a 3:55 ratio non posi, built by Detroit Gear but what does the 'Z' designate? C OW 0210 G Z
Yeah, typo by me. Sorry as said it’s a 3.55 non posi so likely from a firebird .


Jun 14, 2005
Granite Falls, Washington
Mystery solved!
Badazz81z28 had me go look again, it's a 2 not a Z...
My eyes aren't so good and overtime made it worse I guess!
So it was made on second shift, something I'm well aware of!!


Veteran Member
Oct 3, 2013
SK, Canada
I hadn’t thought about it in years but I’m imagining trying to keep a right rear tire on a non-posi ‘70 350 4spd. :bowtie:
Last edited:


Jun 14, 2005
Granite Falls, Washington
I have storie's from that pic. If there's interest I'll single finger type some posts from work this weekend!
At work I have 8 hours of boredom, you guys help me out.
And Good group of folks


Veteran Member
Sep 6, 2013
Cincinnati, Ohio


Jan 10, 2005
Calgary, Alberta
I'm going to piggy-back on some of the replies here;
Yup, craigslist add for a COZ coded (originally contained a 3.73:1 posi) Firebird/Camaro 12 bolt housing is legit.

From what I have read, that's one of the most desirable setups to have, and if you have an original 3.73: posi car, and you want to built it right - that's the way to go.
I understand the comments on being able to buy new but if it's a desirable car and you put an aftermarket rear end in it versus an original rear end you will see a difference in the selling price.

I would think that no one would be so anal as to insist on all original parts inside - that seems to be in the same vein as suggesting a car is worth more if it's correct or numbers engine uses OE cast pistons, and an OE timing gear set instead of newer better replacements - it's good for a chuckle when you want to make fun of people who are trying to get their car numbers correct, but it's not entirely accurate.

To anyone who might appreciate seeing this, here are the codes used on the 1970 Firebirds with twelve bolt differentials (I'm going on memory here as to which gear set was 12 bolt versus 10 bolt, so please correct me if I make a typo):
COS = 3.07 open
COT = 3.07 posi
COU = 3.31 open
COV = 3.31 posi
COW = 3.55 open
COX = 3.55 posi
COZ = 3.73 posi

Also, I do not know when a Camaro came with a posi, but as far as Firebirds are concerned, that item only came standard on the Trans Am - it was optional on the Formula.

I have recently learned that Camaro's and Firebirds could share differential housings, but there were some Camaro specific one - those had leaf spring perches for a four left spring - Firebirds only used a three leaf spring. I do not know if the Camaro housings had different codes.