[Formulabruce]

That COU is WAY too much money, the COS is what you have as both your 1970 cars have similar build times.
If you are dead set on original, why would you want a Safe-T-Track in that rear?
3.07 gears are chevy Gears, which ALL 12 bolts were in 1970 Firebirds. the Firebird Central rear has a 5 leaf perch, ( came out of a camaro ss) you want a 4 leaf perch. and 2500 bucks for that pitted thing is WAY too much..
What rear is in your Esprit ?? . I can build it anyway you want and make it look like what you want, but shipping would be expensive coast to coast.
That rear is a KNOWN quantity, close date and correct perches and Not beat on.
Your Formula came with a COS most likely as most 400 autos with AC did in 1970.
It can be built into a COT ( Posi, or Safe-t-Track)
Are you trying to keep that Esprit all together? It will run fine for someone with a 10 bolt ....

[Formulas]

putting a posi carrier in a housing is easily reversed as long as the open carrier is retained and presented at point of sale if that day ever happens

[unruhjonny]

Originally Posted by Formulabruce

That COU is WAY too much money, ...
... Your Formula came with a COS most likely as most 400 autos with AC did in 1970.
...

wait, wait, wait;
Is this all because 'nacho has a COS, but wants a COU rear end?

How much is the seller of this rear end asking for it??

@ 'nacho;
What documentation do you have for your car?
Do you have clarity on the differential your car should have?

I say this because have a "spare" COU rear end that I bought from another Canadian member of this board a couple years ago;
(If I have my story straight, it came out of another '70 Formula.)
I was going to eventually build this COU rear housing with a 3.90:1 posi (I have a mint 3-series posi carrier, and 3.90:1 gears I bought from another(?) member of this board - I also have 3.73:1 gears that came with this posi (never installed).

If someone needs this code a rear housing, I am open to a trade - but my location is going to be the stumbling block... but I do travel almost every spring to Oregon...

Also, I already pulled this COU all apart and sold the 3.31:1 ring and pinion (I still have the original open carrier).

[unruhjonny]

Originally Posted by Formulas

putting a posi carrier in a housing is easily reversed as long as the open carrier is retained and presented at point of sale if that day ever happens

Agreed.

In my case, I actually sold a correct to my car COW rear end* BEFORE I realized that the rear end in my car was a COX;
But I had never planned on running an open rear end, and my car is not numbers matching, so I'm not going to sweat it.

* = I sold in 2013 iirc; it was from another Formula likely built within a day of mine, the same car from which the correct code & date engine for my car came from.

[Ramairnacho]

I have no idea of what was in it a 307 or 331 open but I know I didn't have a posi unit it was standard and yes bought the esprit for the 12 bolt and other parts but you guys said I need the cou 331 open in a ram air standard rear end. So I bought the wrong one to be 100% number matching on drive train. Like I said I never done this before but know I have a low production car. I got my engine tranny all number matching down to the water pump starter. Not to excited about nos radiator hoses or belts like other guys not trying to sell it but have it original. PHS DOCS say a standard rear end either cos or cou . I do like to have it 100% but that might be impossible. My 307 cos isn't putted but a few days late my maybe 6 from shipping so ideally about 23 to a month out. I'm exploring my options and learning about date codes which is fun to learn.i guess I got choices . Im.adding rally guages and getting a dash redone I terror so it's not all original anyway. Yep I get confused ,never done this but I am number matching from pan to car and tranny guess it might not matter.

[Ramairnacho]

What's different between posi and safety track?

[johnta1]

Pontiac called their locking rear end Saf-T-Trac(k?)
Chevy was called Posi-Traction.
Everyone usually says Posi for a locking rear end.

[unruhjonny]

Originally Posted by Ramairnacho

What's different between posi and safety track?

they are the same thing.

each division had the luxury of creating their own terminology;
The term "posi" is the generally accepted term for a car that did not have an open differential.

According to the chart I just looked at in my assembly manual, your car should have been built with a 3.31:1 rear end.

This rear end was the 'standard' read for any Formula with the a/c & 'Ram Air III', backed by either and automatic, four speed (wide ratio only because of a/c), or three speed.

[unruhjonny]

Originally Posted by Ramairnacho

.... PHS DOCS say a standard rear end either cos or cou ...

I am not sure what you are quoting;
You are citing two different open differntial (stamped) codes;
COS = 3.07:1
COU = 3.31:1

Maybe explaining it this way will help;
The standard gear set for a '70 Formula was 3.55:1;
If the car had the either the automatic, or a/c it activated a gear change;
If your car would have been built without the RAIII engine, it would have recieved a 3.07:1 gear set;
Because your car was opted with the RAIII, that activated a change from 3.07: to 3.31:1

I would suggest that if you have the COS, just build it with what ever gear set you want, and call it a day.
Should you in the future come across a COU rear end that is either convenient to acquire, or the right price, then consider the change at that time.
You will probably never have anyone crawling under your car to look for the stamped assembly code - this is only for those few who are chasing points - and normally those guys buy cars that are completely original - or at the very least have all the numbers parts in tact before pursuing a restoration.

I am unclear on what gear set you intend to run in your car.
Many people here seem to these days advocate for a non-over driven car to have something akin to 3.08:1 - should you have your heart set on runing a 3.31:1 gear set in the name of correctness, then if you find a COU coded rear at a later time, you can quite literally swap everything over.

[71GP76TA]

Originally Posted by unruhjonny

You might be one of few.
Appearance should come second to original/correct.

A rust pitted rear end under a car with a clean California undercarriage? I'd hold out for one that wasn't pitted and run the wrong one in the meantime.

[71GP76TA]

Originally Posted by Formulajones

Restorers that want correct deal with that all the time. A little primer filler and prep work takes care of the pitting without much effort.

As far as filling in the pits with filler primer. Over time it will look like crap once you start driving it.... If I was a New England rust belter.. I probably wouldn't care as that would be the norm.

[Formulabruce]

I sense what you REALLY want is a COV rear, which your car did Not come with or it would be an option on the PHS. at $42.13 cents and sale code 461. .
The 3.07 gears you have are better cruising gears for highway. You Have those Gears and they can be put on a 3 series posi carrier and installed in in your rear, IF you must have a posi.
I just did 2 of these 12 bolts last summer.
You can make that nice esprit Rear into anything you want really.. Including a date change ( gasp.. gasp)

[Formulajones]

Originally Posted by 71GP76TA

As far as filling in the pits with filler primer. Over time it will look like crap once you start driving it.... If I was a New England rust belter.. I probably wouldn't care as that would be the norm.

I would imagine with crap materials used that would be the case.

Do you want a show car or a driver??

[Ramairnacho]

Originally Posted by Formulajones

I would imagine with crap materials used that would be the case.

Do you want a show car or a driver??

I want a nice driver car is my goal with a good solid number matching drivetrain with out breaking the bank . I put my original rear end in a 72 formula 400 I build fir a daily driver got in trouble with law did some time and 72 was on Craigslist for 17k. I regret loosing my rear end. But had to either buy a house or buy back my old car. New owner wouldn't let me get my rear end. I have no exact idea what it was. As a boy I counted 1 tire rotation to drive shaft rotation and it was close to 3 so I thought I had a 12 bolt open 2:73 and bought a car for the 12 bolt 307 not knowing I needed a 331 open. Lol. I just learned about this date original parts stuff. I got a 256 open 10 bolt in there at moment. Some guys say 10 bolt can be built better than 12 and cheaper. So im.simply trying to fix my error I did earliest in life. I realize it's not a trans am but it's a low production formula and would be nice to have it all number matching. I been looking for a few years and found a rearend that's even correct date coded but slightly pitted. I will be replacing my trunk and floor pans with new repops. Getting a good paint job , want new interior, dash ect. Never done this before, I'd just go get anything that would work at junk yard when I was a boy. Yep im.a rookie and I appreciate everyone's input. Thus car is almost a curse . I say this cause the machine shop as a boy kept my original heads and I got back #13. Dumped 1500 back then into the heads I'm talking 1996 and to find out 2 years ago #13 where never on ra cars almost killed me. I bought correct date coded #12 from Eric B but they cores and im.ok with that. Guess I got a long journey ahead but it will be fun.

[Trevor78]

Unless you find the remains of a build sheet stuffed in the car, then you can't get one.

[poncho-mike]

There are some oddball cars out there. A number of years ago I bought a wrecked 1970 Trans Am. The cowl tag had a fairly early date, mid-February. The motor had a later cast dates, mid-May if I remember correctly. The car was shipped in July.

It got even more strange when I found the block had no VIN stamped into it. It wasn't an SR block, it appeared to be a production block with engine code and the second number.

After talking to some of the guys on this board, I think the car was originally built with a defective engine. The car got moved off to the side for a couple of months before a replacement engine was installed. I have owned two blocks like this, the other one wasa 1971 YE 455HO motor.

Things don't always go to plan in manufacturing. I work for a company that builds industrial and commercial UPSs. When one fails final test, it is pulled off the line for rework. The more extensive the rework, the longer it may sit around. Manufacturing lines are set up to produce quantities in the most efficient manner possible. Major reworks are handled during the slack times when peak demand is lower. In fact, the end of the month and end of the quarter are busy times for the manufacturing line because the data coming from those production deadlines can affect pay and bonuses.

[Formulabruce]

Bottom line for cost , take your nice 3.07 12 bolt and
Run it , Or put a posi in it and run it. It IS a factory Pontiac 12 bolt. How often you going to look at numbers? You plan on selling, or Diving... you car came with an open rear, can't change that. What you already have is a nice set up, posi or not.

[71GP76TA]

Not having the correct coded rear is not going to kill the value of your car like having the wrong engine block.

[Formulajones]

Well, not on a car like this in question anyway. So I wouldn't be concerned about it here. On some certain types of cars it can be a deal breaker though.

[71GP76TA]

Originally Posted by Formulajones

Well, not on a car like this in question anyway. So I wouldn't be concerned about it here. On some certain types of cars it can be a deal breaker though.

If it was a COPO Camaro or a 4.33 geared RAIV Judge.. etc..