What is the significance of an "O" rear axle?

Old December 15th, 2018, 07:42 PM
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What is the significance of an "O" rear axle?

I see a lot of chatter here on the forum about "O" rear axle assemblies, so I was wondering what is it about them that makes them so popular?

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Old December 15th, 2018, 08:08 PM
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Originally Posted by 72455 View Post
I see a lot of chatter here on the forum about "O" rear axle assemblies, so I was wondering what is it about them that makes them so popular?

Dave
First introduced in 66 and was carried thru 70. Some 66 & 67's had the 8.2 B/P 10 bolt instead of the 12 bolt O Type. All 68-70 Olds A-Body cars had the 12 bolt O Type rear axle. 66-69 had sealed axle bearings & 70 used the roller type axle bearings. They are not rare except for the lower geared posi units. To the purist,the O-type are desirable if their car came with one. They are hard to find parts for,especially gear sets and posi units. The 3.42/3.91 gears sets & a modified Phord 8.8 posi after market unit is about it for replacements. They have 12 bolt covers but have only 10 bolts holding the ring gear. Not really desirable for racing with slicks as they will not take a lot of continues punishment. Some will argue that but there's other rear axles that are much more desirable.

Last edited by 66-3X2 442; December 15th, 2018 at 08:11 PM.
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Old December 16th, 2018, 06:58 AM
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To expand a little, the Type O is simply one of a number of different, unique rear axle assemblies used by various GM divisions in the 1960s. Most people are only familiar with the Chevy 10 bolt and 12 bolt axles and think that these are the only axles used by GM. That is not true. As noted above, Olds introduced the 8.5" Type O axle in May 1966 on the Cutlass line and used it through the 1970 model year. The Type O was ONLY used by Olds and only used on US-built cars (cars built on the Oshawa assembly line got the Chevy 12 bolt axle). To add to the confusion, Olds also used a 9.3" Type O axle on the full size cars from 1966-1970. This is completely different from the A-body axle and nothing interchanges.

The significance of the Type O is only that it was used exclusively by Olds and was the only axle used for the 1968-70 model years. Because of this, popularity is limited and aftermarket parts are nearly nonexistent. The A-body Type O has twelve bolts on the cover, so people assume it's a "12 bolt" axle, but the 8.5" ring gear only has ten bolts holding it to the carrier. The Chevy 12 bolt uses an 8.875" ring gear held to the carrier with twelve bolts. On big advantage of the Type O over the Chevy axles is that the axle shafts are retained into the housing with bolt-on plates that capture the pressed-on outer wheel bearings. This means that if you snap an axle shaft while racing, the rear wheel won't come off. This is a big step up over the Chevy axles that use C-clips to retain the axle shafts, and breaking a C-clip or snapping an axle can cause the rear tire and axle shaft to depart the car at the drag strip.

By the way, I'll add that despite only using ten bolts on the ring gear, the bolts on a Type O are larger in diameter than those used on a Chevy 12 bolt and can actually carry more torque before failure. Unfortunately, this is only one potential failure mode. The larger ring gear diameter on the Chevy axle does reduce contact pressure on the gear teeth, so in that case loads on the gear are lower.
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Old December 16th, 2018, 07:19 AM
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I vote sticky
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Old December 16th, 2018, 07:35 AM
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I'll add your 72 came w a 10 bolt " corporate" axle that has aftermarket gears and posi availible, as well as being plenty stout for most applications.

the 71-72 10 bolt has the same dimensions as the 68-70 a body o type and is often found as a replacement under those cars
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Old December 16th, 2018, 09:29 AM
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Originally Posted by Jamesbo View Post
I vote sticky
In that case, shouldn't someone mention that the A-body Type O also used two different axle splines depending on the gear ratio? IIRC, 28 spline axles were the most common and the higher gear ratios (3.91+?) used a 32 spline axle. Not sure if this was true for the larger 9.3" Type O, I suspect all those had a higher axle spline count.
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Old December 16th, 2018, 10:06 AM
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Originally Posted by JohnnyBs68S View Post
In that case, shouldn't someone mention that the A-body Type O also used two different axle splines depending on the gear ratio? IIRC, 28 spline axles were the most common and the higher gear ratios (3.91+?) used a 32 spline axle. Not sure if this was true for the larger 9.3" Type O, I suspect all those had a higher axle spline count.
Only the 67/68 3.42 & lower geared O Types got the 31 spline axles. All the rest were 28 spline.
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Old December 17th, 2018, 07:18 AM
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exception

Originally Posted by 66-3X2 442 View Post
First introduced in 66 and was carried thru 70. Some 66 & 67's had the 8.2 B/P 10 bolt instead of the 12 bolt O Type. All 68-70 Olds A-Body cars had the 12 bolt O Type rear axle. 66-69 had sealed axle bearings & 70 used the roller type axle bearings. They are not rare except for the lower geared posi units. To the purist,the O-type are desirable if their car came with one. They are hard to find parts for,especially gear sets and posi units. The 3.42/3.91 gears sets & a modified Phord 8.8 posi after market unit is about it for replacements. They have 12 bolt covers but have only 10 bolts holding the ring gear. Not really desirable for racing with slicks as they will not take a lot of continues punishment. Some will argue that but there's other rear axles that are much more desirable.
Your statement that ALL 1968-1969 Olds A body cars had the Type O is incorrect. The Oshawa Canada built 1968 and 1969 Olds A body cars used the Chevy 12 bolt exclusively.
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Old December 17th, 2018, 07:49 AM
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Originally Posted by oldsmobiledave View Post
Your statement that ALL 1968-1969 Olds A body cars had the Type O is incorrect. The Oshawa Canada built 1968 and 1969 Olds A body cars used the Chevy 12 bolt exclusively.
I was talking American cars,not foreign cars. Besides,I said 68-70,not 68/69. Read for for comprehension,it's a wonderful thing. I think I've seen where you have misspoke a time or two or am I wrong?
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Old December 17th, 2018, 08:06 AM
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Originally Posted by 66-3X2 442 View Post
I was talking American cars,not foreign cars. Besides,I said 68-70,not 68/69. Read for for comprehension,it's a wonderful thing. I think I've seen where you have misspoke a time or two or am I wrong?
"All 68-70 Olds A-Body cars had the 12 bolt O Type rear axle". That is a direct quote from your post.

There is no mention of American built in your post and I am pretty sure that 68 and 69 are part of 1968-70 model run that you reference.

Many of the Oshawa built cars were sold into the US market as well as across Canada. Whether or not an Oshawa built car qualifies as a "foreign" car is debatable.

I point out that the 1968-1969 Oshawa built cars are an exception to your statement. This is just facts.
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Old December 17th, 2018, 08:09 AM
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Originally Posted by oldsmobiledave View Post
"All 68-70 Olds A-Body cars had the 12 bolt O Type rear axle". That is a direct quote from your post.

There is no mention of American built in your post and I am pretty sure that 68 and 69 are part of 1968-70 model run that you reference.

Many of the Oshawa built cars were sold into the US market as well as across Canada. Whether or not an Oshawa built car qualifies as a "foreign" car is debatable.

I point out that the 1968-1969 Oshawa built cars are an exception to your statement. This is just facts.
I see you don't have a sense of humor either. Lighten up,this ain't real life.
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Old December 17th, 2018, 08:29 AM
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To get back on track, the only real significance to the Type O is its originality to the car. You can argue the pluses & minuses of the rear all day, but if you are building for performance there are better choices due to the previously mentioned lack of availability of parts.
If its a street car subject to normal driving it should be fine.

Worth a mention is the W27 Type O with an aluminum center section. Really rare and very expensive today.
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Old December 17th, 2018, 08:29 AM
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Just the facts

Originally Posted by 66-3X2 442 View Post
I see you don't have a sense of humor either. Lighten up,this ain't real life.
You seem to have taken my post of facts as an insult when that was never the intention. It was just a statement of fact with no ill intent towards you. Apology if you were insulted.



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Old December 17th, 2018, 09:00 AM
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Originally Posted by oldsmobiledave View Post
You seem to have taken my post of facts as an insult when that was never the intention. It was just a statement of fact with no ill intent towards you. Apology if you were insulted.
No,you can't insult me and no apology needed either. Everything is cool on my end.
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Old December 17th, 2018, 09:06 AM
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Originally Posted by 66-3X2 442 View Post
Only the 67/68 3.42 & lower geared O Types got the 31 spline axles. All the rest were 28 spline.
Thanks for the correction, at least I got the 28 part right.
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