I opened up the pumkin on a 70 supreme covertable I am bringing back to life and this was staring back at me...3.73 posi with 4 clutch plates either side....kind of a nice way to wrap up a busy week

 

Sweet!

 

Good deal!!

 

Nice! Gear looks like it's new.

 

Cool find. If that is an Olds 12 bolt then it is not 3.73 gears, no such animal. 3.08, 3.23, 3.42, and 3.91 were the 3 something gears available for the O 12 bolt.

 

It is a 12 bolt. I identified the gear by turn the driveshaft and marking the hub. So it probably is the 3.91. So, there is not a true posi with a 3.73 gearing?

 

"I identified the gear by turn the driveshaft and marking the hub."

In the pic I thought the pinion looked large for high-threes ratio, more like 2.7-3.08 ratio.


Another good way, what with the cover being off already, is to read the markings on the OD of the ring gear. Something like "13A43" which is secret code for 13 teeth drive and 43 teeth driven. Divide the one number into the other and there's the ratio, very precisely. Generally at least one tooth count is a prime number, to spread the wear out evenly over its lifetime.

I picked "13" and "43" out of the air, they are not necessarily an available actual ratio. They calculate to 3.31:1

 

When a rearend has a posi it is turned one tire revolution and count what the pinion turns. Open rearend it is two tire revolutions to what the pinion does.

What is the code on the right axle tube?

 

This also happens with a posi when one wheel is on the ground due to the differential action. And if the clutches are very worn a posi will act just like an open rear. Best thing is to look at the stamped numbers on the edge of the ring gear as Chris said.

Yes there is in the Chevy 12 bolt and the 8.5" corporate 10 bolt, but not in the O-Type 12 bolt.

 

A posi rear of any gear is always a nice surprise. Best was is to just read the stamped numbers on the ring gear as stated.

 

very cool.

 

I remember as a kid finding out the car my buddy had that we were building had a posi from the factory. We were under the rear of the car and lo and behold, there's a tag (covered in grease and muck) that said "Use positraction fluid only". It was like we hit the jackpot! Looks like you have too! Nice!

 

Canadian built cars had Chevy 12-bolt rear ends.
These had a 3.31 0r 3.73 gears.
Does the ring gear actually have 12 bolts and does the inspection cover have a big wrinkle in it?
If so, it's a Chevy rear end.
Also, the ends of the axle at the posi will be grooved with
"C" clips holding the axles in place vs. the Olds 12-bolt having a bolted flange
at the outer end of the axle housing where the brake baking plate bolts on.

 

If you look at the pics he posted you will see his diff has an S spring, which I think was never used in the Chevy 12 bolt. The ones I have seen had the Eaton style (two plates with a coil spring at each corner).

 

You forgot the best gear set of all 3.55

 

The pics show a type O rear

 

If you rebuilding or just cleaning it, replace the 200# "S" spring with a 300# one and the clutches will last forever! Contact the guys out in Seattle, WA., Super Cars Unlimited as they have parts for type O rears.

 

Good point. I should've looked closer.
Since the OP said it was a 3.73, I assumed it was a Chevy 12-bolt sinced the "O" type doesn't use a 3.73.

Yep, I did.
Also didn't mention the 4.10.

 

Or the 3.07 or 3.08 or 2.73 or 2.56 but who's counting?

 

Thanks for the imfo.
I was out of town for the weekend and a couple of my buds decided to clean and put the cover back on for me. Nice of them, but I did not get any numbers yet.
Plain cover...12 bolt O type.....housing casting number is 402227....

 

"When a rearend has a posi it is turned one tire revolution and count what the pinion turns. Open rearend it is two tire revolutions to what the pinion does."

That is true if and only if
-the posi does not slip and has both wheels off the ground, and
-the open diff has one wheel on the ground or otherwise locked from rotating.

What you are shooting for, really, is one revolution of the RING GEAR and CARRIER. This requires TWO COMPLETE WHEEL TURNS- whether you get that by turn two wheels once... or one wheel twice... or 1.5 & 0.5 or any other combination, is up to you. Whatever works for you that day.

If you have not seen the differential explained in the 1940's GM video on youtube, go see it now.

So, in my experience with open diffs, the car is on stands and one wheel turns first, then the other for a while, making it hard to keep track, so I end up blocking one still by whatever means is handy then getting my two WHEEL revolutions out of the one moving wheel, providing me the one ring gear revolution. The U-Joints make it easy to get accurate shaft turn count to within 1/4 turn, then add or subtract "a little" from the nearest 1/4 turn until you get to the obvious available actual ratio. I.E., if you count pretty much exactly 2&3/4 turns, that is the 2.73 ratio. Just past 3 but less than 3&1/4 is 3.08, etc.

 

I always count teeth.

 

but, as set forth above, if you have access to count teeth, you have the tooth count stamped into the ring. So much faster, and no chance for any error. You need only recognize the "secret code" of the [pinion count][some letter][ring gear count] format.

All these methods certainly work just fine.

I had a guy laugh at me one day, I was inspecting a manual trans 1958? chassis and lifted one wheel to count turns to get the ratio- "you can't do it like that!" he opines. I did not want to offend him, so remained silent, but I thought to myself, "maybe YOU can't do it like this, but I damn sure can!"

The video explaining how 2 turns of 1 wheel results in one turn of the ring gear:

 

2.56 posi...., was this available 1970?...I am trying to id this without taking the cover off again. 10 turns of the tire equals 25 turns of the driveshaft which would be a 2.56? My buddy thought he had counted 41 teeth on the ring gear.....aaargh

 

yep...should be coded R3 on the tube

 

Yep, 2.56 was the standard no-cost rear and the posi was optional. That's how my '70 Supreme came from the factory.

Yes, that is correct. Unfortunately I am all too familiar with the dreaded 2.56 gears so I know the ring gear is stamped 41A16 for 41 ring gear teeth and 16 pinion gear teeth.

 

2.56 Posi are hard to come by. If you want more gea I'm sure you'll be able to sell the gearset and posi for someone wanting highway gear cruising.

 

2.56 posi is AWESOME....so long as you have a Richmond 5 speed in front of it.

 

What is your plans with the car? If cruising at highway speeds it is wonderful. If you want to go drag racing it is not even close. This is a great gear set for a driver car. Unless you are looking to do burn outs and drag race. Will produce the best fuel economy in most cases too.

 

I just bought a 65 Buick GS and the rear end axle code is R8 0620K, What does that equate to??. thanks
Looks like it might be a chevy.

 

You should not hijack the existing thread. Start your own.

 

All highway

 

That's what I did with my '70 Supreme back in high school. It would smoke the 225-70-15s that were on it through first gear and I raced everything in sight. Even with 2.56 rear gears it was quicker than most everything on the road at the time. Spanked a bunch of IROC-Zs, Corvettes, 280Zs, Supra turbos, and even a '70 Toronado.

 

Anything higher than a 3.08 doesn't count.

 

Sammy, I own a 72 Chevy with 2.73s. It's the Monte Carlo G body / A special, so it's heavier than even the 70 convert Cutlass, and your hi-comp 70 engine will beat my low comp Chevy 350, plus the Olds engine has more low end torque. I enjoy my granddad's Chevy, and it definitely takes off with enough fun for the street, and will hang around at 90 all day long. The 2.56 posi will work fine for you unless you want to drag.

 

You may be able to give a mounted police/trooper a run for his money

 

mounted....on a motorcycle.

 

John, I don't think the '72 Monte Carlo is considered a "G" body.
I believe the '70-'72 MC uses the same frame as the '72 Chevelle
which is still considered an "A" body.
I may be wrong so you may want to confirm this.

 

70-72 Monte uses a 4 door Chevelle frame that has 4" added to it in front of the front wheels. It is not an A body.