Kiwi69Cutlass

Hi everyone. I have been a member for a while but just made my first post under the newbies section. I have run into some difficulties with my 69 Cutlass Supreme type O gear set. I started the convoluted process of trying to change my ratio from 3.95 to 3.08 a couple of years back. The long story short is that a mechanic told me what was required and a new gear set was ordered from Yukon gears in the states. These arrived but would not fit and it was clearly outside this mechanics knowledge (and mine which is minimal). It had already cost me a fair bit so the old gears were put back in and I parked the idea. Recently I have shifted to a larger city with more expertise. With lots of toil we have worked out that I have a type O 12 bolt cover with 10 bolt inside. This will be why the new 12 bolt 3.08 gears did not fit. The guy from the diff shop has since found a set of 3.45 gears or similar in the states but I would also need to purchase a new carrier for this and the costs are mounting. Other options have been discussed but I would prefer if at all possible to keep all that I have and source some 3.08 gears that will just fit in. I just cruise in the car so would prefer this gearing for this reason. I do not know if this cutlass came out with 3.95 as standard or not. I would greatly appreciate some of your combined wisdom as to known options available and/or any contacts that could assist me. The car is still running all good so this is not urgent, but would make my use of it so much easier on long journeys. Appreciate your viewing and any assistance in advance. Todd
PS I am in New Zealand and unfortunately we do not have the same access to parts and expertise as you clearly have their where these great cars were born.

svnt442

First what you have are 3.91 gears. Second there is no way to put 3.08 gears on that carrier due to the size of the pinion gear. The ring gear needs to be moved towards the left side of the case for clearance and this is not possible with that carrier.
So you have a couple of choices. You can opt for trying to find a 3.08-3.23 carrier for the o type diff and a set of gears to go with it and ship those down under, source a modified ford 8.8 carrier and some 3.08 gears to go with it, or you can opt to have a complete axle housing (either a corporate 8.5 10 bolt or a chevy 12 bolt) with 3.08 gears shipped down. All of which will cost more than you initially wanted to spend.
It kinda sucks, but if this is what you want to do, those are just about your only choices.
Oh....forgot about this one...you can have a custom ford 9" done up. There are a few companies that make them for the 68-72 GM A-body already so that is also an option. Again, more than you wanted to spend, but still an option.

olds 307 and 403

First contact Monzaz or 507 Olds on here, they build differentials for a living. They may be able to find you a used 3.08 carrier and gears. A 2.56 or 2.78 used gear set will be easier to find. Isn't there a axle spline change on 3.91 gears? 31 vs 28 spline? Only 3.42 and 3.90 gears are available new for the type O rear. Since I have an overdrive transmission, I am swapping out my 2.78 open gears for 3.90 posi gears.

oldsmobiledave

Find an original used set of 3.42 type o gears. Swap them onto your existing carrier. This will be the cheapest solution. Any numerically lower gear ratio requires a carrier swap. Good luck finding 3.42 gears.

monzaz

Actually there is enough meat on the Olds original carriers to machine the ring flange .0120 and make the 3.08 fit the tall carrier. JUST as an option BUT you will need a machine shop to do this work as you need it absolutely true or risk a mess.

THAT IS A option.... It will also mess up whom ever gets that rear end down the road some time...LOL. yikes....

Jim
J D

Magna86

He can also buy a new ring gear set and have the ring gear machined .100 to center it in the rear on the 3.91 carrier. Then no harm done to the original carrier. I did that with a set of 3.90 Richmond gears for my 588 carrier.

lemoldsnut

I can do a full set of gears 3:08 and carrier (open) for a type O out of a low mile car

contact me direct lemoldsnut@aol.com

Fun71

I think you need to determine if the gears are GM or aftermarket. It could be that someone installed a set of aftermarket 3.90 gears that fit on the 3.08-3.23 series carrier. The other option is what everyone was assuming, that you have factory GM gears on a 3.42-up carrier.

I don't think you could get the 3.91 ratio on a Supreme, so I would confirm exactly what you have before buying any parts.

monzaz

I second that... What the carrier casting number is too. Jim