Roger60

New to the whole Olds rear end thing.
67 Cutlass. I have a 10 bolt and a 12 bolt. Neither are posi.
I know that the 12 bolt is not like the Chevy 12 and hard to get parts for.
Each diff has at least 150K mi.
Will likely run the 320 hp 330. Also have the 250 hp 330. Likely keep the 2 spd auto trans. Have all of that.
Need advice as to which rear to use as I would like to replace gears and may add rear disc brakes (not that likely, but maybe).
Thanks for any info.

lemoldsnut

Both rearends are plenty durable for that engine set up. You did not say what ratio they are. Posi only really matters if you are going racing. Otherwise a one wheel wonder will work fine. Is there any signs of leakage? if not and the gears look fine than there really is no reason to change the gears unless you want a different ratio. Ratio all depends on what you want to do with the car. If you plan to drive it as a cruiser and just enjoy it as a classic car then 2:73-3:23 is a good range to be in. Will drive well, not wind up on the highway and get half way decent mileage. Check the axle and pinion bearings and seals and change the fluid and I bet it works fine for a long time.

I run 3:08 in the one we drive often. It works fine for us.

Just some input

Larry

Gary M

12 bolt is desirable and used for higher HP. If you go with the 10 you could sell the 12 pretty easy. My 10 had a posi unit installed from West Coast Differentials. I got 3:23 gears and 455. Not sure what mpg but it gets up n goes.

joe_padavano

The "12 bolt" axle that the OP has is NOT a 12-bolt. It is apparently a Type O, which only has TEN bolts holding the 8.5" ring gear to the carrier. The Type O would have been factory installed in a 67 Cutlass.

Gary M

Thanks Joe. I Learn something new on here all the time.

monzaz

If your going for all out resto find out what the car came with and build it...IF you do not care and want a good driver. PLEASE oh please dump the 2 speed trans for the street get a 350th 400th, 2004r overdrive etc.

You will enjoy the car much more with more gears and you will not need such a steep rear differential gear so thwat will give you low end with out sacrificing mileage too.
IF you need evidence this is the way to go look at how may 6 speed 8 speed 9 speed trans mission are in the new cars. THIS IS the way to go.

I build differential for a living and so many people make the steep gear ratio purchase only to be disappointed with the car after the novelty wheres off of great launches and they find out they can not hit the freeway and drive normal with the car. Just some more to chew on...

Your car and will all help where we can.

Jim
J D

oldsmobiledave

Put an 8.5 diff in the car from a 71-72 Olds A body. Install a limited slip carrier & either 3.08 or 3.23 gears depending on your rear tire height. Install T350 trans. Enjoy car.


Cost effective, simple, reliable, durable & affordable. Done.

lemoldsnut

in 67 there was three possible rearends. The B type 10 bolt (the lightest duty) From my experience I have seen these in the lower horsepower and base cars like F85 and Cutlass, In Cutlass Supremes and 442 cars I have seen both The P type 10 bolt Even in a W-30 and the 12 bolt O type rearend. Which Joe has pointed out is a 8.5 inch ring gear with 10 bolts holding it on.

If you want all out performance than go with a 9 inch but for daily driving with a 330 any of them will work just fine.

monzaz

Keep your GM vehicle GM.

For what you have Any one of the rear you have will more than do the job and you still not have mentioned what you want the vehicle to do for you for GOOD suggestions.
Please let us all know what your intensions are for the ride... freeway far away cruise or just hanging around town. Light to light accel fun?
AS stated 3 speed auto trans will cost the same as the 2 speed so REALL I would start there for your car. YOU will be ammazed that an extra low first gear will pep up the stock gears in the rears you have already

WHAT IS the cam shaft in the motor as long as you hang around STOCK cams you will have very good low end torque in any ratio will be fun. If you get too HARRY (long duration cam .230 and high at .050) will cause the vehicle off line sluggish until higher RPMs then you will have to get gears 3.55 - 4.10 and a high stall converter. This will all lead to a ALL or nothing gas pedal usage in driving ... So there has to be a path to make all your parts work together properly.

Jim

DYSKIE

Is 3.7 available for 12 bolt O type and what is part number?

oldsmobiledave

no such beast. 3.42 or 3.91 and no gear ratio in between in Type O

monzaz

You could play with the tire size to tweak the ratio some.
IE 3.90 with large tires will drop to 3.80 ish
3.42 with smaller than 26" tall tires will get steeper lik3e 3.50-3.60 ish... all depends how extreme you go with the tire size.

WHAT RPM range are you looking to run at 65mph and what size tires? rubber diameter or height... 25 26 27 28" etc.

DYSKIE

[QUOTE=monzaz;885799]You could play with the tire size to tweak the ratio some.
IE 3.90 with large tires will drop to 3.80 ish
3.42 with smaller than 26" tall tires will get steeper lik3e 3.50-3.60 ish... all depends how extreme you go with the tire size.

WHAT RPM range are you looking to run at 65mph and what size tires? rubber diameter or height... 25 26 27 28" etc.[/QUOTE

My car already has 4.10 GEAR, 275/60-15 Tire and TH400. I want drop 500 RPM AT 65-70 mi.

oldsmobiledave

[QUOTE=DYSKIE;885828]You need a significant gear change and a taller rear tire to drop 500 rpm and it will kill the performance. Live with the rpm or start looking for an OD trans.

BlackGold

DYSKIE:
You currently have a 28" tire. If you were to swap those for a set of 26.5" tires -- which is about what the original tire diameter was when it left the factory -- then a 3.42 gear would drop your RPMs 400 and a 3.23 gear would drop 600. Take your pick. 245/60R15 is a good 26.5" tire for these cars.

oldsmobiledave

I am confused. Why are you recommending a shorter tire? He has 28" tall tires and going to 26.5" tall tire will add rpm.

lunaboy

the 10 bolt in an early 64- 66 olds rear and most '64-'66 A body 8.2 rears are 1-1/2" narrower than the '67-'72 bop rears, but all the suspension mounts are in the same location. if you are going with a wide tire it could be a factor to use the 10 bolt 8.2 if its the narrower one.

monzaz

Yes this too confuses me also... IF you go from 28" tall to 26.5" tall the 3.42 will turn into a 355 3.60 ratio not 3.23 ratio. Jim

oldsmobiledave

Almost. The 64-65 is narrow. The 66 is the same width as 67-72.

BlackGold

I think the confusion is because I was replying to DYSKIE, not the original poster. DYSKIE says he has 4.10 gears and a 275/60R15 tires. He says he wants to drop 500 RPM at 70 MPH. Switching to a 26.5" tire AND a 3.23 or 3.42 gear would achieve his objective.

I shouldn't have replied to him, since he wasn't the OP. Sorry for the confusion.

lunaboy

early 66 could have the narrow one.

66SportCoupe

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

redoldsman

This is a five year old thread.

66SportCoupe

My bust. It showed up as new thread

oletrux

Old or not, it's still relevant.