Good morning fellow cultist! I'm sure some of y'all are aware of my bastardized 67 cutlass gasser build, so this should come as no surprise.

So I took my car to a ring and pinion shop with the idea of changing my gears. I did not realize that olds 12 bolt was different from a chevy 12 bolt, thus making a $700 quote into a $1700-2000 quote. To add insult to injury, what I thought were 3.08s, turns out it's a 2.78!

So in my desire for "cheap speed" has lead me to a few options.

1: just biting the bullet and getting 3.42s and a posi put on for 2k
2: putting in a chevy 10 bolt with 3.72s and a posi for $250 from a 70 chevelle and swapping gears a little later on.
3: remain slow.

I have looked for better gear olds rear ends, but they're few and far between, and geared a lot worse.

What do you think I should do? And if I go the 10 bolt route, I'm gonna take a guess and say it's not a simple drop in (cause why wouldn't it be?).

Like always, I appreciate any info!

As an update, got some skinnies for the front, and I'm about to order some wilwood brakes for the front.

 

The Olds axle isn't a "12 bolt", despite the number of bolts on the cover. Since that Type O axle was only used in 1966-1970 Cutlass models, there is a limited market and few vendors sell parts for it. The Type O uses an 8.5" ring gear, so the 1971-72 Oldsmobile "corporate" 8.5" axle is comparable strength and much less expensive since it is the same as a Chevy 10 bolt.

You might want to think that through. Brake performance is as much about tire friction and contact patch as it is about the actual brake hardware. Spending a premium for Wilwood brakes that are handicapped by "skinnies" is kind of worthless.

 

Well when the current calipers are single pistons PoS, Its going to be an improvement no matter what.

 

As measured by the "butt G-meter"...

 

The most scientific way possible. Worst case, they should at least cancel eachother out so it doesn't get even worse.

 

10 bolt 3.42 will hold up just fine.

 

Physics doesn't work that way. At some point the tire becomes the limiting factor. You could weld the rotor to the spindle and the tire would still limit braking.

 

Supposedly, the O axle is a stronger rear (stock) than the Chevy 12 bolt. This is due to the larger ring gear, bigger axles and bolt in axle bearings. While I’m not a rear end expert, that’s what I have been told. As Joe mentioned, the limited production run means very little aftermarket support. There are a few gearsets available, I don’t know if limited slip carriers are available, or parts to rebuild a factory part.

The 8.5 rearend is pretty durable. I have a friend with a turbo LS that ran the factory 10 bolt in his 71 Camaro well into the 9s with a spool, aftermarket gears and axles.

 

The A-body Type O only uses an 8.5" ring gear. The Chevy 12 bolt uses an 8.875" ring gear.
The ten bolts holding the Type O ring gear to the carrier are larger than the 12 bolts on the Chevy, however. When you do the math, they actually have more torque carrying capacity, despite smaller number and bolt circle. Unfortunately, that's only one aspect of rear axle strength. Gear tooth strength, load contact angle, case stiffness and strength, bearing sizes, etc, all enter into this. There are many links in that "strength" chain. As an example, larger bolts and bearings don't help if the case is too flexible to hold parts in the correct alignment under load. This, by the way, is one of the big problems with the aluminum W27 housing. Aluminum has only 1/3 the stiffness of iron or steel. Casting the center section out of aluminum without significantly increasing section thicknesses or load paths results in a much wimpier housing.

 

It's not the piston count, it's the clamping force, which is total piston area. They're not 4x better because they have 4 pistons.

 

It is actually a very easy swap; you just need a 10 bolt from an A-Body. I have done it many times over the years with no issues. You may need a different length driveshaft and/or u-joint, depending upon what you currently have.

 

If you're looking at biting the 2K bullet look into Quick Performance out of Iowa. Just did a custom 9in Ford for my 1966 442 with Wilwood 12.3 in 4 piston disc brakes for about $2500. Gives you all sorts of gear options super heavy duty (mines rated for 1000hp) can go way higher. Totally a bolt in deal and stop worrying about breaking a rear end. Jmo

 

contact Brian Trick on facebook, he can lead you in the right dirrection for a rear axle. And since youve switched to narrow front tires, I'd upgrade the rear brakes first

 

For another option, I have a 12-bolt that was under my 66 W30, originally came in a 67 chevelle. Tubes welded to the center section with nickel rod, Moroso posi, Mark Williams axles, c-clip eliminators, steel left side bearing cap. Currently 4.88 pro gears (carrier accepts 4.10 and up). I am tired of tripping over it and would even change gears for the right deal/buyer.

 

Post public WARNING , WARNING ASSUMING , I will be ASSUMING A LOT.

The o/p said this car is a gasser, So I am ASSUMING ladder bar rear end set up. A mile in the sky lift. and tires not fitting the wheel wells.
So, go find a 10 bolt out of a truck, or a corp 14 bolt with 3.73 or 4.10 gears. or a dana from under a dodge/ford.
or from an s-10 with v6 and tow package that has the 8.5 rears and 373/410 gearing.
A junkyard crawl and a tape measure. can get you a rear with gears you want posi even for under 200 bones.

 

Id be interested?

 

Well you're right about the tires and the lift, but I'm trying to keep the 4 link set up because I don't have the fan skills to switch over to a leave spring mounted rear (which would make life 10 times easier). Unfortunately I really enjoy driving the car more than racing it, so I'm trying to go with slightly less crazy gearing since I'll be sticking with a non OD trans.

 

Alright, so it looks like in going with a 10 bolt out of a 1970 chevelle with a 3.73 gear. Anyone got a link for the U-joint needed so I can connect my existing drive shaft? (67 cutlass with type O 12 bolt.

 

Anyone know if a 72 chevelle rear end will bolt right up (minus the U joint)? To my understanding they're just an inch wide, but everything lines up

 

Pretty sure it will. Your rims may need more back spacing, especially with wide tires. I am looking at $1000 for parts and doing it myself in my 70S. Worth it for 3.90 gears to me.