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Old 09-26-2001, 06:07 PM   #1
dale illig

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Question 12 bolt vs 10 bolt rear ends

Hello fellow olds friends.I have a 68 442 with a beefy 425 ci toro eng.I currently have a 12 bolt open rear,and am thinking about switching to a posi.I havent had much luck locating a complete 12 bolt posi in my area,but have found a few 10 bolt posi units.I dont know much about differential units.What would be the best thing I can do? Should I try to find the guts and convert my 12 bolt to a posi,or just grab a 10 bolt from the boneyard? Will the 10 bolt hold up to the big block torque?Any opinions are apreciated thanks Dale
 
Old 09-26-2001, 06:52 PM   #2
Kevin

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Post

First off, do you have a real 12-bolt as in a Chevy with an 8-7/8" 12-bolt ring gear, or do you have the so-called 12-bolt Olds Type-O with 12 bolts on the cover but a normal 8-1/2" 10-bolt ring gear as used in '68-'70 Cutli?

What gear ratio do you have? I believe Supercars Unlimited has a limited-slip differential for a Type-O rearend, but your selection of new gears for it is limited to 3.91. There is also something goofy about the higher geared rearends using 28-spline axles and the lower geared rearends using 31-spline axles which makes finding parts an even bigger pain.

If you have a Type-O and aren't concerned about originality, I advise selling it to someone who does care and buy yourself an 8.5" corporate rearend like what was used in '71-'72 Cutli. Considering the 8.5" was the ONLY rearend in '71-'72 A-bodies, I'd say it can handle your 425. Beware that the other brands of GM used an 8.5" corporate, but it used C-clips instead of bolt-on bearing retainers to hold in the axles. Anything you could ever want is available for the corporate rearend, and we all know that aftermarket parts are much stronger than GM's "build it as cheap as you can without breaking" philosophy. I've already decided to go that route to replace the 2.56 Type-O in my '69 Cutlass.

And for the requisite smart-ass comment: Okay, now everybody please bash me for this post. It's almost as bad as "cheaping out" and putting a TH700-R4 in an Olds because of parts availability, huh?

One other thing I forgot to mention the first time I posted this -- I believe the other brands of GM (Chevy anyway) use a different style of U-joint than Olds, so if you put a Chevy rearend in an Olds, you either need the Chevy's driveshaft or a hybrid U-joint.

[This message has been edited by Kevin (edited 09-26-2001).]
 
Old 09-26-2001, 07:41 PM   #3
Oldsman71

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Post

First, checkout if it is a true 12 bolt or 12 bolt with 10 bolt carrier.

I recommend with the 425, to use a 12 bolt and use a moroso or auburn posi unit. These units will cost from $300-$500, depending on what carrier you need. Strange offers posi units, which are better but will cost on the high end. With these, they are the whole carrier so you have a little better selection for gears.

------------------
Scot "Oldsman71"
71 442 w/425 773 w/o nitrous, 927 w/ nitrous.
96 Cutlass Supreme SL 2-dr w/3.1 170hp
99 Alero GL w/2.4 w/ dynomax exhaust, mantapart intake system, and polish and ported throttle body = 165+ hp
http://home.talkcity.com/GasolineAll...n71/index.html
 
Old 09-26-2001, 08:57 PM   #4
rocket man

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Post

I've got a Type-O rear-end, It was rodded pretty hard for 80,000 miles, then it's held up to me for over 10,000 miles. It has seen slicks, clutch dumps, 2 motors, 1 transmissions, a few clutches, and 1 driveshaft, but the rear-end is still ok. I just changed the fluid, gears look good and the posi still works great.

 
Old 09-27-2001, 05:47 AM   #5
dale illig

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Post

Kevin,oldsman,rocketman--I know the rear end has 308 gears,I found that out by checking the numbers on the upper right hand side of the housing.Is there any way to check exactly what it is (a true 12 bolt) or O type without removing the cover? If not,can you tell me what to look for when I do remove the cover.Thanks Dale ---OK,I just checked my book again,it says the letters SC mean the rear is an O type with 40.13 teeth Does this help? now what?
 
Old 09-27-2001, 06:05 AM   #6
dale illig

Posts: n/a
Question

Should I grab one of those 10 bolt posi rears? Is there something I should check for first, ratio ect. Are the stamped with Id letters just like the one I have now? thanks.Dale
 
Old 09-27-2001, 06:12 AM   #7
rocket man

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Post

A Type-O has a 12 bolt cover and a ten bolt carrier.
 
Old 09-27-2001, 08:09 AM   #8
Fox

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Post

My suggestion would depend completely on how "beefy" your 425 engine is. If you have around 400 HP or less then I say grab any 10 bolt that has a 8.2" gear and it should handle the torque pretty well. A lot of older cars with big block engines only used 10 bolt 8.2" diffs and held up as long as they needed to. If you're making some serious HP/TQ then maybe you should consider after market.

------------------
Nobody likes a bastard.
(1970 LeMans with 403)
 
Old 09-27-2001, 08:59 PM   #9
dale illig

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Post

The 425 in my olds is mild in comparison to some of you guys.Its .30 over,10.5-1 comp.with a warm cam,nothing radical.It sounds like the 10 bolt rear is as strong as the type O rear,or at least they sound equal.I do have one advantage,time to look around.Patiance usually prevails.thanks all..dale
 
Old 09-28-2001, 07:24 AM   #10
64cutty

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Post

the 71-72 is good to 600hp with 30 spline axles, carrier and welded axle tubes. Don't weld them yourself, you'll just warp the housing and make stress risers. let a 4X4 shop do it. Brian T(507olds) runs this I belive, I plan to follow suit.
 
 


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