Hi everyone. I own a 1970 Olds W31. I had it repainted three weeks ago, which used up all my extra money for a new Transmission. So, for this year I just want to regear it so I can enjoy some highway driving as well. It has the 12 bolt aluminum W27cover with the 3:91 currently. My question is. What gears can I fit in that rear end? And what will be the best gears if there's more then one option? I was thinking a 3:23 or 3:08! I called quick performance and they have nothing.

 

I think if you have the proper carrier you can use almost any gearset in there.

If you only have the 3.91:1 carrier, then you are limited to 3.42-5.00 ratios on the "588" id'd carrier.

I am not the expert on this, please check others also.

 

It may be too much of a slug with 3:08s since the W-31 motors like to wind up. I get the full W-31 fun factor with the original 3:91s. 3:23s could be okay - I have one aggressively cammed 455 4 speed and one 455 automatic with 3:23s and they're still pretty responsive.
Terry

 

With your carrier you could use 3.42. With another carrier 3.23, 3.08, 2.78, or 2.56.
Numerically higher mean more RPMs for a certain MPH, more wear, lower MPG; but faster acceleration.
Numerically lower are just the opposite. Any of them will work. The choice is yours. Since I don't race I prefer the numerically lower.
Be sure that whoever does the installation, is knowledgeable. A bad installation can ruin all the fun.

 

Leave the gears alone and put a taller tire out back on a 15" rim and drive that car. This is the cheapest and easiest solution. If you are set on a gear swap then OEM 3.42 gears are your only option without changing the carrier. Note the aftermarket 3.42s won't work with your existing carrier.

Note if you do swap gears and your 3.91s are in good shape I will buy them from you and have you ship them to Sweet Grass MT (assuming you want to sell them to offset your costs).

 

x2 on this, if you want to improve highway drivability with just a higher gear change then you need a factory GM 3.42 gear to easily just swap into your current carrier....

 

Here's some info I got from Brian Trick many years ago regarding the O-Type 12 bolt rears:

Here is a list of the posi units & what gears they will fit:
#673 = 2.56 & 2.78 ratios, 28 spline
#671 = 3.08 & 3.23 ratios, 28 spline
#588 = 3.42 & anything numerically higher, 28 spline
#672 = 3.42 & anything numerically higher, 31 spline
***The 31 spline axles were offered in late 67-68 OAI cars only.

The biggest problem will be locating a set of gears to fit your carrier as none of the aftermarket companies make them. There are some gear sets made to fit the 3.08-3.23 carrier but even those are hard to find these days. I would suggest you check with Brian Trick (507olds) or Jim (monzaz) for parts as they are our resident experts and may have gear sets on hand.

 

 

Hi everyone, thanks for all the advice. I will have to figure out what to do from here now that finding the gears is going to be a little more work then I origianlly thought. And Oldsmobiledave, as much as I appreciate the offer, I will be reinstalling the 3:91 gears once I can afford the new tranny. (which hopfully is next year.

 

No worries. If you change our mind I'll buy the 3.91 gears as my car came with that gear set but a previous owner swapped out to the 3.42s in the early 1990s. Thanks for posting the picture. Your car looks great. Post more pics please.

 

Dave, u guys could just do a swap & get the gears u both want?

 

Yes that would work if I wanted to relinquish my 3.42s which I do not want to do. Similarly he wants to keep his 3.91s so a swap is out.

 

Here's my olds W-31 from 3 weeks ago. Just got it out of the bodyshop after my full repaint job. This is where my Transmission money went for this year.

 

Beautiful car! As others have stated the gears will be hard to find, though not impossible. Some other members of CO may have some on the shelf - they chime in from time to time. I "lucked" into a set of brand new aftermarket 3.42's about three years ago for the "low" cost of $500. Then late last year a CO member posted a used set (3.42) for $300, IIRC. My aftermarket 3.42 set, along with a NOS 588 carrier is sitting on a shelf ready for me to swap in when I get to that point of my rebuild later this Spring.


Good Luck!

 

You can't use aftermarket 3.42 gears on a 588 carrier. The are designed for the 671 carrier.

 

I've been told that I'll have to machine .010 off the back of the ring gear to get the proper alignment between the pinion centerline and the ring gear. Other than that all dimensions match up. What is your take on this?

 

That is what I have heard too but talk to Brian or Jim to get the final word on any Type O differential question.