12 bolt question
 

Looking through factory specs, the 70 TA had 4 ratio choices on a automatic transmission. Is this pretty variable, or did one get the majority of the "default" factory ratio?

I am trying to see my ratio is in original documentation, but must be doing this wrong. Where is the best place to look? Rear end tag?

It is totally locked. I wll be evaluating once I know what I have got.

12 bolt
 

I'm learning too. Step one I guess you'll be look at your rear end if you have it still. Should be a 12 bolt. There is a code stamped on rear passanger axel. If automatic with ac max ratio is 331 gear. The trans am got a 355 with no ac. I think performance package was 373 but not sure if automatic got that. All in theory should be posi. But over the last year I have seen one with option 307 gear and an automatic ac car with a 331 open ,so thing happen. You need to give more info of options. 2nd thing is order your phs docs but not sure if gear ratio show up. 3rd hunt for billed sheet probably under rear seats or gas tank. Funny a guty had door panels and tried to sell ne a set and tried to convince me to remove door panels to look for billed sheet. Don't do that. There is alot of knowledge hear and hope we can help you. If you got a good housing build it the way you want it and have fun. There is pic of my code for my ra3 auto formula, I didn't have posi and my phs docs say 307 or 331 . I choose 307 from 1970 chart and I.was wrong. Took me another year to correct that error. I would cross reference 71 chart with 70 chart. But it should be simple since you have ta. Hope I didn't bore or confuse you. Im.still learning too. But also learning not to overthink or over restore a car. About a year ago a ta sold for over 100k with all repop Interior from.ames. Some guys in group were upset but that goes to show you you can buy restore fast then enjoy,sell ect without breaking the bank. My 2 cents. I'm never number matching again it's a long detailed process and some never finish.

The gear you want is the gear that matches your driving habits.

Have PHS. Need a little help finding that piece of data on it. Trying to find time to get back on the mechanics of it. Too many cars broke this summer, got me buried.

Doesn't it mention the ratio straight after the engine and 4-speed info? The few I just looked at for 1970 GM did. That's the PHS order form, not the buildsheet.

Also, that tells you what it came with, not what it has now. Even if you found stamped info on axle tube, it may not tell you that the diff has been swapped at some point or the internals, it's extra info though that help complete a story for the car.

You may have to pull the rear cover, give it a fluid change if not done in recent years, count the teeth and confirm with the cast part numbers on the gears. That will confirm what's in it, up to you to decide if that's ideal based on how you drive, or if originality matters, if you can even confirm that detail.

If the current speedo is accurate then checking the current speedo gears and current rear tyre diameter will give you a good ratio figure also, more accurate than trying to jack up and count wheel turns. But also a lot of work to confirm something a backwards way.

Basically, from the factory, if it has AC it had a 3.31 gear.
If no AC it would be 3.31 with TH400 or 3.55 with M20.

These would not show on PHS.

M21 could be ordered and would show on PHS.
If it shows Performance Axle it could have the 3.73 gear without AC.


:)

Found a chart I think I have posted before from the Firebird Suppliment:


https://forums.maxperformanceinc.com...1&d=1694257401




But this is another chart which shows different choices, which I think , may be closer to what I wrote before in post.
(shows Trans Am and other model choices)


https://forums.maxperformanceinc.com...1&d=1694257594


:)

3.31 then. I am down to counting teeth. No ability to turn anything. 100% frozen up. Have drive shaft off, busted brake drums loose . But those were also seized. Car was driven like they stole it, so it's probably a scrap pile inside.

The 1st chart is original from 70I think and it shows m40 auto transmission got only 307 but posi safety track could be purchased. When I was trying to figure this stuff out I swore I has 307 standard. Then a group member should me second chart. Way more detail but it's a 71 chart. Factory was confused back then and cause us to misread charts. Learn from my mistakes and ask group first. I found 2 simular cars with 331 open foe me but most likely your stamp should match a 331 posi unit if auto with ac. If your rear end is beet up just rebuilt it but before you do anything see if it's correct housing and stamped axle. If frozen up I think gears no good , I'd probably get all new stuff and wouldn't worried about what is actually in there unless you want to reuse it gear teeth might be wore out too. I'm learning all this stuff too. Hope I helped you out. I..not sure what is best for rebuilding these units but Eaton road race might be good with new axel,. Factory axel are only hardened about .50 of an inch and they wear at the bearing making them weak. I spoke with a local shop., just sharing what was told to me.

Those axle code charts are pretty difficult to decipher.

In actual production, there were only 3 ratios that would have come on a 70 TA automatic.

-The standard ratio would have been 3.55.
-As johnta1 noted, if it had A/C it would have come with 3.31s. I've seen one or maybe two exceptions to this over the years (factory A/C cars with 3.55s).
-If it was factory ordered with the Performance Axle Ratio (which you'll see on the PHS, if it was), that would be 3.73s. Performance Axle wasn't available with A/C.

If yours was originally a 3.31 car, the axle tube should be stamped with a "CO V" code.

I think if the 3.73s were selected with the M21 trans, the HD cooling option was mandatory. That's how my 70 is set up.

Yes, exactly. A mandatory extra cost "option".

it's easy to mis-read a chart;
your car is a '400 Ram Air' car;
that engine was never combined with 3.07;
Both those charts show this.