My car has a stock Old 12 bolt rear end with a jetaway trans, and a 350, The car was built in Michigan. What is the gear ratio that is mostlikely in it?

 

Best leave this to the book worms with GM tech books.

BUT in my opinion AS ALWAYS... Open the rear and you will have your true answers... This is not a difficult thing to do. ...Nothing is going to fall apart when you remove a rear differential inspection cover. ONLY fluid will come out.

This is how the oil gets changed in the old car and truck rears. If you have not changed this fluid since you have owned it...NOW is the time. You have a reason.

I would say nothing over 3.08... But remember sometimes it depends on where the cars are sold also . topographical area received steeper gears as mountainous areas and snow northern area with positraction options etc.
And again if someone ordered the car it could have most anything as a ratio also.
I know this is not the answer you wanted.... But... will see what the tech books say should be the factory optional ratio

Axle tube stamping could help also if the housing is not all rusted up...check picture where to looks 'OLDSMOBILE'

Jim
J D

 

Thanks! yes this does help a lot Ill check and see if i can see any stamps

 

The gear ratios available in the O-Type were fairly limited, and most of them were available only as performance options. On a regular Cutlass (non-442 or W-machine) the 2.56 or 2.78 were standard equipment, no cost ratios.

2.56
2.78
3.08
3.23
3.42
3.91
4.33
4.66

 

You didn't specify the year and model in this thread either. If the car is a Cutlass, the base rear axle with a 350 and Jetaway would have been 2.78. If the car is an F-85, base axle would have been 2.56. As noted, ratios up to 4.66 were optionally available, so the base ratio is meaningless. You can search for the axle code, count the turns, or buy a gasket and gear oil and remove the back cover.

 

Ok, I've removed the back cover on my rear out of the 71' 442. I gave a quick look on the ring gear for some stamped digits but the only thing I saw was a written 6 with possibly a grease pencil on the ring. What should I be looking for?

 

5.0 was also available, but may have been a "dealer installed option".

 

You should be looking for a stamp like this.


11 is the pinion gear tooth count, 41 is the ring gear. Doing the math, that set is 41/11 = 3.73:1

 

Upon further inspection and looking through posts here's what I found. Sorry I've edited this 5 times already.



Looks like 40A13 GM KN 342 2



There's the grease pencil 6 I was talking about.



This looks like 123787 343



Looks like 13A40 GM LD What do you guys think of that gear? Gouges a concern? It does click when you move it back and forth and spins with no effort. The fluid looks like a metallic paint.



SD 0004 on the front passengers side of the tube. Looks like this is the first non original part I've found. Guessing Chevy rear. 343 or 342?

 

40/13 = 3.08:1

 

Not a good sign .

Can you show us photos of the gear teeth wearing surfaces ?
Maybe take a magnet and run it around the bottom of the case . Or better yet , swirl it around in the fluid if you still have it . If a lot of those flakes stick to the magnet , you have a problem .
Was the rear noisy ?

 

It hasn't been on the road for at least 30 years and my friend that I bought it off of always complained it chattered around corners. He bought the car in the late 70's and pretty much never beat on it. I thing whoever owned it before him pounded the snot out of it. The fluid looks like metallic paint and the rear is very lose and sloppy. As of now I think it's a Chevy rear so I'm thinking the previous, previous owner blew the rear to pieces and replaced it with this one. Then old habits die hard. So far it's the only piece I've found that wasn't an original.

 

Chattering around corners is the telltale sign of a posi without the fluid additive and the clutches are grabbing.

 

SD also means limited slip 3.08

 

I remember him special ordering whale sperm back in the day to combat that problem. Not sure if it ever worked. Personally I thought it was snake oil.

Is it specific for Chevy or all GM rears?

I'll post more picks later in the day.

 

A 3.08 posi is a good non OD ratio. It may have been swapped for the better ratio and posi, probably a 2.56 open from the factory. As said the wrong fluid will cause clutch grabbing issues. I would pull the axles and pull the carrier, the clutches are probably wasted. That way, you can look at most of the bearings as well.

 

These are some more pics. Cannot seem to get the pinion gear without dismantling the rear. Not ready for that yet. Each picture of the ring gear is taken at 1/4 turn. The last picture you can see a metal swirl at the 9-11 O'clock positions. These where the best of the bunch.

 

The REAL stuff absolutely does work. I once had a Chevy 12 bolt that around turns made such loud snapping and popping noises that it sounded like there was metal breaking inside. One bottle of the GM anti-spin additive and all of the noise went away.

 

It looks like some pretty heavy wear on the ring / pinion gears .
Still , it is a Posi . And it's probably worth re -building .
You will probably need a new or good used ring and pinion , as well as all bearings and clutches etc .

 

Am I correct in thinking this is a Chevy rear? Didn't the Olds rear use springs?

 

Those gears visually do not look bad at all. BUT you really should put a dial indicator on the ring gear tooth and see what it is. .006-.010 is pretty good for used gears.

This is a BOLT in axle 725 casting Limited SLIP traction lock OLDS rear.
3.08 gears we all know that from your 13 / 40 tooth count.
The clutches are steel and probably where you got most of your metallic material. ONLY truer way to know what is causing the metal in the oil is checking all the bearings , backlash of the ring and pinion and feeling the ring teeth and pinion teeth for indents in the faces.

Pictures sometimes give a false reading with shadows of the gear mesh rub marks etc.

DO not write it off until you check everything.
If the gears or rear is grinding and making all kinds of noise straight and turns you certainly have issues.

Jim
J D

 

What concerns me is how easy the rear spins, there's zero resistance and when I move the ring gear or the axle and it takes about 1/4'' turn before it moves the pinion. I have a 2.78 rear I picked up last year and that takes some effort to move and instantly moves the pinion gear. I assisted a guy years ago in rebuilding a 10 bolt Olds 2.78 posi that was in my then Cutlass and the clutches were grinding something fierce in a snow storm, so I never went near a rear again. Now I'm much older and less intimidated so I'll have to pick up a dial indicator and see what I come up with. I'll have to do the amateur thing and watch the dreaded Youtube to get an idea. I think there's also an illustration in one of my Motors text books from that era.

Thanks to all for your help. Good to know it's an Olds rear. I'll let you guys know how I made out.

 

Sorry to be late. SD is definitely Olds 3.08 limited slip. Looks just like the one in my 67.

 

Get an Oldsmobile Shop Manual .
The re-build procedure is covered in depth there .

 

Measure pinion depth, back lash etc. I would also say it is rebuildable, just inspect everything and replace as necessary. Good luck.

 

I only count ten bolts on the rear cover.

 

Sorry I meant the internals, gear teeth count and posi.

You are right on the case.

 

The Olds rear uses a large S spring instead of plates with four smaller coil springs at each corner.

 

I'm pretty sure I have one, have to dig through my books. Thanks.

 

This where it gets confusing, kind of like the chebby and BOP 8.2 rears. The Chevy 10 bolt replaced the Type O in 71 and since it had unique bolt in axles, many consider it an Olds rear end. Really it is a Chevy design with an Olds tweak. Either way, probably factory in a 442 for 71.

 

Incidentally you can find out the torque spec of your limited slip rear by jacking both rear wheels up, in park or gear, and put a torque wrench on a lug bolt. I found this out quite by accident when I was torquing down lug nuts one day, it will move when enough torque is reached.

 

Should that be jacking up ONE rear wheel?

 

Oh you'll find out that way, too!


Not safely, though

 

OK, I just reared you post and realized my error.
I thought you said to jack up both rear wheel with the trans in NEUTRAL. That wouldn't tell you anything except how much your rear brakes are dragging.

Carry on...

 

 

This info is way wrong. Sorry not being mean just laying down the facts.

1971 1972 Olds had there own rear casting 10 bolt 8.5 round cover with the oiling trough from 12 to 3 on the rear cover
It has the same bolt in axles as the BUICK 10 bolt 8.5
You could find the rears inter changed in the Buick's and old's depending on the plant.... a Buick and Olds NEVER received a CHEVY rear. EVER... chevys did get Pontiac or Buick and olds rears sometimes. The Plant would never degrade parts on a car only upgrade.
Buick had the cover that looked like the Pontiac tempest 10 bolt 8.2 rear BUT is still different as the bolt hole pattern is different.
You can tell a true 8.5 rear as it always has the brake hose like cast and taped on the top of the casting center section of the rear.
ALL Chevy 8.2 rears were a bracket off the rear cover and the 8.2 Buick's and Pontiac were also off the rear cover and the 12 o clock part of the Pontiac Buick 8.2 cover will have a cut in the cover for the bracket to clear.

from left to right OLDS 71 72 10 bolt 8.5 rear BIA (Bolt in axles were used all the way to 1976 77 GM BOP rears)
next is the Buick 10 bolt 8.5 with bolt in axles (Bolt in axles were used all the way to 1976 77 GM BOP rears)
Finally the Pontiac 8.2 rear cover with the 12 o clock notch (10 bolt Pontiac rears were used to 1972 after that they were retired) (same with chevy 10 bolt 8.125 (aka 8.2 chevy) c-clip rear only used to 1972

1973 ALL rears were 8.5 in GM
Chevy 8.5 were all c-clip B O P were bolt in axle rears.
remember... a Chevy could have a bolt in axle rear BUT you will not find a c-clip chevy rear in a BOP car (FROM THE FACTORY PLANT)

NOW being 40 years later...you never know what your car will have...lol. Just ID it and build it.

J D Race
Jim Mitschke

 

You can use Chevy 8.5" gears in the 71 and 72 axles, can you not?

 

10 bolt 8.5 is corporate rear........ THERE IS really NO Chevy and NO BOP in 1973 one rear did it all even trucks started to use the 8.5 and the gears will fit them too all the way to 2017

ALL rears used the same ring pinion BUT they still did carrier over the BOLT IN AXLE carrier 726 and 725 in the BOP and the 410408 and 410409 carriers with c-clip side gears c-clip recessed pockets.

IT is NOT a 10 bolt 8.125 (AKA (chevy 8.2) ) Not the same .

The Chevy 8.2 cover fits the 8.5 bolt pattern exactly (that is interchangeable...and that is it)

Only difference in the 8.5 is the bolt in axles and the c-clip axles which is what normally designated the CHEVY type 8.5 from the BOP 8.5
And eventually NO bolt in axle 8.5 were around when 1978 car were in production even the Pontiac Firebirds were c-clip rear from 1971-1981 all 8.5 10 bolt rears.

in 1975 GM ushered in the metric 7.5 rear.... Yippie for us.
Monza, Vegas, Nova non v8 cars Apollo, Omega ... All G-bodys up till 84
1982-1989 Camaro Firebirds....
7.75 9 bolt had bolt in axles in the Irocs... HOLDENS used these rears... 2004-2006 GTO

 

My 68 Cutlass S has the 12 bolt cover. It is also an open and with highway type gears in the rear. What is the difference between the 12 bolt and the 10 bolt covers? Is mine easily rebuild able?

 

Not to add any more confusion here..... my understanding is that Oldsmobile did use Chevrolet 12 bolt rear ends with C clips in 1968 and 1969 Oldsmobile cutlass models built in Canada. In fact, a number of years ago I parted out a 1969 Olds F85 and still have the 12 Bolt Chevrolet rear end in my possession today w the “eyebrow “ rear cover plate.