I'm looking for anything 3:42 to 3:91 that will fit in our 69 o type differential. Good used or new. Thanks!

 

FYI there are no gears in between the 3.42 and the 3.91 and you will also need the correct carrier to run them.

 

The GM 3:42,3:91,4:33,4:66,& 5:00 gears use the GM #588 or GM #672 posi carrier.
The #588 is for 28-spline axles. The #672 is for 31-spline axles.
The Richmond gears(now unavailable)3:42,3:90,& 4:10,are made for the GM #671 posi carrier,which is the original carrier for the 3:08 & 3:23 gears. These gears will also fit the new/aftermarket carriers that are available.
Gears are the missing piece of the puzzle for a lot of people.

 

That's pretty interesting. So if I'm starting with a SF 3.23 posi and want to go with steeper gears, I'm better off *not* getting the #588 or #672 carrier?

 

Correct. Unless you find a set of GM gears.Then you will need the 588 or 672 carrier.
The big thing is to find the gears first.

 

Sorry, I meant 3:42 or 3:90, 3:73 would have been nice but I don't think that's an olds option.

 

I have a set of used 3.90 gears from a GM 12 bolt. I bought them with the intent to install in my Olds but never got around to it. I don't know much about the gears other than the guy I bought 'em from said they would work. Maybe 507Olds can shed some light on these and if they'd work for you....