Hi Guys, I have read and the posts on rear diffs and still can't figure out what I have. I could really use some help. I have a 1970 olds cutlass supreme, I found a torn up RPO tag under the seat and it tells me I have 2.56 ratio rear diff, I want to put 3.42 gears in it but can't figure out how many splines I have. From what I have read in the posts they made a 28 and 31 spline axles I think I would have a open carrier and need a posi unit. I also need to know if I have a 8.5 gear. The other number I found on the paper is a axle number 339144. Can anyone set me straight? I counted the bolts on the cover and have a 12 bolt
Thank you, Mike

 

I think the '70 "12 bolt" type O rear end is the hard to find parts for type used '67-70 or so. In that type, you won't find the 31 spline axles except in the performance ratios- 3.42 and up. So, you would have to get
The posi carrier, which will have 31 spline receptacles ??
The matching 31 tooth axles
And the gear set and install kit

If may be easier/ cheaper to find and install the entire assembly. You could more easiy swap to the next less performance ratio, which I think is 3.23.

I am by no means the expert on these so hopefully others will correct any misinformation that I set forth.

PS the Olds "12 bolt" rear end has 12 cover bolts and ten ring gear bolts. It is still a "12-bolt" heavy duty unit compared to the "10-bolt" units of the era.

 

Like octania said you have type O rear end. As far as I know there is no aftermarket parts for it. Try contacting monzaz (Jim) he may have what you want, the other option is to find a rear out of a 71-72 cutlass and swap it, that's what I did with mine. parts for those are readily available.

 

Thank you, do you have his number?

 

Jim's website http://www.jdrace.com

He is very helpful and he will point you in the right direction.

 

You can also send him a private message from here, that's what I did. My '70 Cutlass S also has the Type O rear end, originally with the standard carrier and 2.56 gear set. Jim set me up with a refurbished anti-slip carrier and 3.08 gear set (my choice), plus all the necessary hardware. The carrier worked with my original axles. Everything went together fine and it works great. Jim will get you straight.

 

Thanks Guys, I really appreciate the info. Mike.

 

What gear would be better with a turbohydromatic 350, 3.23 or 3.42? I know the 350 and 400 had different gear ratios.

 

This comes up a lot, if you are obsessed with green light to red light speed and showing off and dislike anyone getting in front of you the 3.42 is the answer but then you would probably want to go even lower....if you want just decent performance better gas milage and better engine wear and quieter highway drivine the 3.23 will be a little better

 

Would it make much of a difference if it was a th350 or th400?

 

T400 & T350 have almost the same gear steps & very similar 1st gear and both are 1:1 top gear. Before picking a gear ratio you also need to know what converter you are going to run & most importantly how tall a rear tire.

 

I dont think that is as important as much as how the engine is built concerning power band range etc. Thats just me i have had some cars in the past wth a th350 that were pretty quick and some th400 that were not

 

I have a 1970 Olds cutlass, i have a TCI 350 in it and built a 455. I called TCI and they told me to run a 2600 to 2800 stall converter. I did some home work and found this would not work with a 2.68 rear diff, I called TCI back and got some one that seems to know what there talking about. He said that if i were to do that the car would feel like the trans is slipping,[loose] That's when I came to the forum. I called a local diff builder and he said they only make 2 ratios for that year..3.23 and 3.42 but I would have to put in a posi carrier which is what i want to do. I just want to get the most out of this car that I can. I want to retain the 14 inch tires that came on the car. What do you guys think I should do.
Thanks Guys, Mike

 

Im not sure as far as built a 455 or ''built'' 455 and you mention you want the most out of the car so there are some questions you need to answer yourself as far as what you want...with that higher stall sounds like you have the car built for speed so 3.42 sounds right , but are you going to drive extended highway miles? That might change things i know it would for me

 

Another thing to take in consideration is overall there is not a great difference between those gears except at the extremes of top end vs low end

 

Ok, I heard some of the cutlass came with 3.42, is that true? I do some highway driving with this car so would the 3.23 be better?Or would i not notice a dodifference?

 

Just to give you an idea, I have 3.42 gears, 295/50/15 tires 350/350. Motor spins about 3000 rpm at 70mph.

 

Do you do a lot of highway driving?

 

At 70mph with L60 15 tires i was over 4000 rpm according to my 45 year old tach which could be off, but the engine sure was loud and screaming

 

Maybe I should go with the 3.23. I don't really need my runs that high, do you know if GM put 3.23s in the cutlass at the factory?

 

3.23 were the standard ratio for many 442s from 1970-1972. Cutlass was more likely to get 2.78 & 3.08 ratios & many got the crappy 2.56 gears.


The best all around compromise for acceleration & driving is the 3.23 gears. Put them in with a posi & enjoy. Just keep in mind that 3.23 gears with a 28" tall 275 60 15 is going to be a dog off the line unless you have lots of TQ & similarly 3.23 gears with a short 245 60 14" tire is going to scream on the highway over 60 mph. You really need to pick your gear ratio with the tire height in mind.

 

I measured my tire height and it 26 inches, that would mean that thing would be really screaming at 60 mph,

 

Thank You!!
Mike

 

That's TCI's standard answer. Converters stall differently behind different engines. Stall is just a figure of speech any way. The correct way to tell stall speed is to get the car moving in high gear about 25-30 mph and nail it. What ever the tach flashes to is the stall speed. I guarantee you you can call TCI and they will tell you they have it on the shelf. Any body who tells you they have it already is just selling you a converter. The correct way is to give the builder all of your cars specs. Weight,CID,rear axle gear,cam etc. all play into getting the correct converter for the car. A custom built converter is the correct way to go about it and it takes a few days to get it built. TCI is known for hit and miss on converters because of selling generic units. There's a company called PTC that was founded by some guys who left TCI. They are great guys with a great reputation as far as I've heard.
http://www.ptcrace.com/

 

FYI, the '70 O-Type used 28 spline axle shafts; it was the "early" O-Types ('67-'69) that had a HD version with 31 spline axle shafts.

The O-Type uses an 8.5" diameter ring gear but it is unique and NOT interchangeable with the 1971-up corporate 8.5" 10 bot ring gear.

 

Thank you, I will call them as soon as I get home from work. I told the diff builder it has 28 spline axles and he said it wont be a problem, he told me I need to choose between 3.42 or 3.23, Great info, good looking out guys! Thank you all! Mike

 

The 31 spline was not used in 69,only 28 spline. The only way to get the 31 spline axle in 67/68 was to have a 3.42 or lower gear set,all of the other higher ratios used the 28 spline axles. A little known fact is that the O type axle was first used in 66,not 67. Just a little info here for Christmas.

 

Thanks for the "Christmas info"!

As for 3.23 vs 3.42, as said earlier you should use your rear tire size to determine overall RPM with each. I had 3.23s with short 245-60-14s for years and the highway RPM was not bad. Also, it's just a 6% difference between 3.23 and 3.42 so you could change rear tire height to compensate if you don't like whichever one you get.

 

I would like to do that, So if I go with a tire that is 1 inch taller it would set me into the 3.23 if I install the 3.42?

 

OK Guys, I called Performance Converters and gave them all my specs. They want to build 3000-3500, 10 inch converter. This is a 455 bored 0.100 over. I estimate from the forum that it weighs around 3500 lbs or so. The cam is a custom ground comp cam. Duration @ 50 is intake 236/241 exhaust, Gross valve lift is intake .553 and .548 exhaust, I'm running Edelbrock performer rpm heads. Edelbrock performer intake manifold, DUI distributor. The engine is internally balanced and blue printed. These are the specs I gave them. We run the car on the street and it will probably never see the strip, I have decided to go with the 3.42 rear differential. Our compression ratio is 10.5 to 1. My question is, do you think this will be to much converter for the street?

 

The great thing about converters is, if it's too loose or tight,you can send it back and they can change it according to your needs. Were they nice and friendly to you? Everything I've heard about them is they are very good @ what they do. To answer your question about too much converter for the street,there's no way to know until you try it. You can tell tell them and I'm sure you did that it's a street car but they can lean to a little tighter converter if you prefer. Me personally, I do not like a loose converter for the street. I had rather give up a little performance for some sanity. The converter is mostly for down low leaving the line or red light anyway. Once a car is in high gear it's a little harder for the converter to lock up when called on. The thing about converters on the street is,you can't have it both ways but there is a sweet spot and that's what you want.

 

hell, you might as well go all out with 3.91 gears with that set-up

 

Ya, and if it was my car I'd definitely consider taking to the strip. It'd be very interesting to see what it'd pull.

 

Yes, they were very nice and I will do business with them as soon as I get the rear diff done. and if I can get it set up differently than how they have it set up that's a plus. I now know from the forum that a loose converter will feel like the trans is slipping off the light, What would a tight converter feel like? Maybe I will try the strip just to see what she'll turn, I don't know about the 3.91 gears I don;t want to get the rpms to high with the little woman in the car, she would have me by the headers...LOL

 

When you flash the converter from the light,it will lock up. It's the running @ highway speeds that it will feel loose and harder to lock up. If your cruising rpm is going to be between 3000-3500,it will feel like it's slipping. A converter is a torque multiplier and a BBO doesn't need a lot of stall/flash for the street because of it's torque. You need to be very honest with PTC about your needs,they are not mind readers. Do not try to make it a race car if it's more of a street car. Automatics with the right converter are as fast or faster than a stick car. Just look @ the NHRA National Records for stock/super stock classes and the automatics are faster in more classes than not. The reason I'm familiar with this is I'm a multi time NHRA/IHRA/NMCA National Record holder with my cars. I got me a good education @ the drag strip.

 

Ok, Thanks guys, a lot of great info,

 

I run a 10" 3500 stall converter in the T400 in my 87 442 behind a very stout small block Olds hooked to 3.89 gears and running a 27" tall tire with no issues. You will be fine with the 3.42s and that converter.

 

Thank you!