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10-12 bolt posi

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I’m looking to upgrade from my 2.73 open rear end bop in my 70 Monte. What are some of the most common vehicles that would have a 10 or 12 bolt posi that bolt right in?

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I believe all the 10 bolt GM A-body rears were the same. Widths should be the same from 1968 and up. The Chevrolet 12 bolt rear was the better of the "bigger" rears for A-bodies.

 

If you upgrade to the Chevy 12 bolt, the left upper control arm is different.

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Oldsmobile offered the W27 rear with an aluminum center section. Light weight but I'd say it's not as strong as cast iron. Very hard to find at any cost, the holy grail to Olds fanatics.

 

post-201-0-19118700-1524289620_thumb.jpg

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Also a tidbit, the Oldsmobile "12" bolt is actually has 10 bolt internals. Also there are 8.2 and 8.5 inch 10 bolts. The 8.5 when built is just as strong as a 12 bolt without the bragging rights. And 10-12 bolts that bolt into our Monte's can be found in any 68-72 GM a-bodys and Grand Prix. Additionally any 64-67 a-body will fit but you will need different wheels due to those rears being narrower. Probably confused you more than you were. Lol sorry and good luck!

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If I have a 10-Bolt non-posi rear end can it be converted into a posi rear end?  My plan is to swap to a 700-R4 transmission and I would like to have a posi rear end to go with it.  Is it as simple as dropping in a kit like this and boom you have a posi rear end?  I actually don't know what size my 10-Bolt rear end is and I don't know much about this kind of stuff so any advice would be appreciated.  

https://www.quickperformance.com/GM-85-10-Bolt-Posi-Gears-Bearing-Kit-Package_p_20372.html

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One of my 70 Montes came originally with a bop 2- something open rear and i converted it to a 3.55 posi pretty easily. A club member helped me with the conversion with parts and instructions. All I needed to purchase was the posi unit, new ring gear and some shims. The pinion did not need to be touched.  Maybe someone in the club can help you do the same. I didn't know anything about rears and did the conversions by myself over the weekend. 

Good luck!!

Mike

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Hey MC Rudy,

If all you are changing is the carrier unit it's a fairly easy thing to do. First you need to find out what 10 bolt you have. Order the correct posi unit. I recommend the Detroit trutrac. Great unit and uses regular differential fluid with no additive.

As far as the swap, along with the posi unit I suggest putting new carrier bearings also. Then it's pretty much pulling the old carrier out, swapping the ring gear, installing the new posi unit, reassembling. 

It's a little more involved but that's the basics. As long as you are not changing gear ratios, no need to go crazy with shims and pinion depth and such. 

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48 minutes ago, MC1of80 said:

Hey MC Rudy,

If all you are changing is the carrier unit it's a fairly easy thing to do. First you need to find out what 10 bolt you have. Order the correct posi unit. I recommend the Detroit trutrac. Great unit and uses regular differential fluid with no additive.

As far as the swap, along with the posi unit I suggest putting new carrier bearings also. Then it's pretty much pulling the old carrier out, swapping the ring gear, installing the new posi unit, reassembling. 

It's a little more involved but that's the basics. As long as you are not changing gear ratios, no need to go crazy with shims and pinion depth and such. 

Hey Mike & MC1of80,

Thank you both for the info.  I will look into the Detroit Trutrac. Is there an easy way to figure out if my car has a 28- or 30-spline.  Needless to say, I have a ton of homework to do before jumping into this because I have so much information to figure out.  

Thing I don't know but need to know:  (If there is anything else please let me know)

-> 8.2" or 8.5" 10-bolt 

-> 28- or 30-spline

-> Gear ratio

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image.jpeg.d8e36f3289197dcd51c10f843f6fe534.jpeg

10 hours ago, MC_Rudy said:

Hey Mike & MC1of80,

Thank you both for the info.  I will look into the Detroit Trutrac. Is there an easy way to figure out if my car has a 28- or 30-spline.  Needless to say, I have a ton of homework to do before jumping into this because I have so much information to figure out.  

Thing I don't know but need to know:  (If there is anything else please let me know)

-> 8.2" or 8.5" 10-bolt 

-> 28- or 30-spline

-> Gear ratio

Maybe this will help, if it works

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21 hours ago, MC_Rudy said:

Hey Mike & MC1of80,

Thank you both for the info.  I will look into the Detroit Trutrac. Is there an easy way to figure out if my car has a 28- or 30-spline.  Needless to say, I have a ton of homework to do before jumping into this because I have so much information to figure out.  

Thing I don't know but need to know:  (If there is anything else please let me know)

-> 8.2" or 8.5" 10-bolt 

-> 28- or 30-spline

-> Gear ratio

I believe all 10 bolts were 28 spline stock.  Chances are you have the 8.2" rear if it's stock, but make sure using the photos Steve posted.  Are you planning on buying new gears for the posi?  If you are then it won't really matter what the current gear ratio is.  For an overdrive I would recommend 3.36, 3.55, or 3.73 gears, depending on the intended use for the car.

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Blackhawk, I beg to differ on the rear ratio. He needs to know what ratio to get the corresponding posi unit. 3 series, 4 series etc. Although he may be better off getting a better gear ratio especially if the rear ratio he has now is a 2 something. Finding a posi for a 2 series ratio is usually difficult. 

As Blackhawk stated, the 10 bolts are usually 28 spline and 12 bolts are 30 spline for our cars. 

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That's why I said it won't matter if he's buying new gears, especially when he's posting complete kits that come with new gears.  2.73 or any gear that would fit in a 8.2" 2-series carrier would be painful with a 700R4.

If he's planning on keeping the stock gears, whatever they may be, by all means he should the gear ratio before buying a carrier.

 

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On 5/23/2019 at 10:54 AM, Blackhawk said:

I believe all 10 bolts were 28 spline stock.  Chances are you have the 8.2" rear if it's stock, but make sure using the photos Steve posted.  Are you planning on buying new gears for the posi?  If you are then it won't really matter what the current gear ratio is.  For an overdrive I would recommend 3.36, 3.55, or 3.73 gears, depending on the intended use for the car.

 

On 5/23/2019 at 11:35 AM, MC1of80 said:

Blackhawk, I beg to differ on the rear ratio. He needs to know what ratio to get the corresponding posi unit. 3 series, 4 series etc. Although he may be better off getting a better gear ratio especially if the rear ratio he has now is a 2 something. Finding a posi for a 2 series ratio is usually difficult. 

As Blackhawk stated, the 10 bolts are usually 28 spline and 12 bolts are 30 spline for our cars. 

Thanks cny first gen 71 for the pic of the rear ends as it was super helpful aiding me in identifying the which rear end is on my monte.  

Blackhawk and MC1of80,

I have a 8.2" 10-bolt rear end and I would like to go with the Detroit Trutrac, but I haven't been able to find one that fits my application on summit or jegs.  When I searched the Eaton site the options that I was able to fine was posi limited-slip differential 19603-010, but I have a felling this is not the part you meant.  I do plan to change out the gears when doing the differential and I am thinking about going with a 3.55 or 3.73 gear ratio.  Which if I am understanding all of this correctly I will need a 3-series differential for this application.

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If going thru all the expense and effort to change gears, carriers, etc, wouldn't you prefer a 12 bolt instead? Even if you can find a relatively inexpensive non-posi 12 bolt, at least you'll be putting the rest of those parts in a more durable rear end.

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X2 to what jft69z said.

You are looking at easily 1k to do just about any rear. If you are going to spend that much I would prefer a 12 bolt or at least an 8.5 10bolt. Stronger and more aftermarket parts for it.

 Also send Mike Britcha an IM. I believe he had a few 12 bolts for sale.  Sorry Mike if I misspelled your name. 

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Well jft69z the problem is finding a 12-bolt rear end or a 8.5" 10-bolt rear end that is in the price range of just replacing the parts on my rear end.  I've called many junk yards in my are looking for a 12 bolt rear end and most done have them.  If they do have them they want at least $1500 (non-posi) for the rear end and then I will have to convert that rear-end.  The cheapest 10-bolt posi rear-end I've come across was $1000 (not even sure if it was an 8.5" 10-bolt) but I would have had to get it shipped to me.  With my current rear end (with me spurging on parts) I am looking at about $1100-$1200 installed and that would include a "high-strength" alloy axle kit, richmond gear ring & pinion set, the Detroit TruTrac diffenential, and the miscellaneous thing like fluid, shims, and bearing.  Couldn't speak on which rear end is more durable or better, simple because I lack the knowledge to make a quality case for either.  To be honest I didn't even know a 10-bolt posi rear end was possible until reading post in this forum.  What I can tell you is that I have no plans to race my car, but I do plan to rip up and down the highway every chance I get 😁

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Nothing is cheap anymore, that's for sure. I broke a 10 bolt years ago (in my first '72 Monte). Sheared the pinion gear in half. That corner up the street from my parent's house smelled like gear oil the whole entire hot summer, :grin:

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18 minutes ago, jft69z said:

Nothing is cheap anymore, that's for sure. I broke a 10 bolt years ago (in my first '72 Monte). Sheared the pinion gear in half. That corner up the street from my parent's house smelled like gear oil the whole entire hot summer, :grin:

🤣🤣 I tried to send Mike Brichta an email to see if he still has rear ends for sale, but I got an error message stating he cannot receive emails.  Are the 8.2" 10-bolt rear ends that bad off or is this about beefing up the rear-end?  Lets say I am able to obtain a 12-bolt posi-rear end is it a direct bolt on or will I need to make modifications so that it will fit?

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On 5/22/2019 at 9:52 AM, Mike Brichta said:

One of my 70 Montes came originally with a bop 2- something open rear and i converted it to a 3.55 posi pretty easily. A club member helped me with the conversion with parts and instructions. All I needed to purchase was the posi unit, new ring gear and some shims. The pinion did not need to be touched.  Maybe someone in the club can help you do the same. I didn't know anything about rears and did the conversions by myself over the weekend. 

Good luck!!

Mike

Hey Mike,

By chance do you have any rear-ends for sale for a 72 MC?

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1 hour ago, MC_Rudy said:

  Lets say I am able to obtain a 12-bolt posi-rear end is it a direct bolt on or will I need to make modifications so that it will fit?

Pretty much a bolt in. One of the upper control arms has a notch in it for the 12 bolt as Paul mentioned in the second post. There are pictures around here someplace, but certainly no big deal.

 

Edit: Here is something that explains it pretty good:

https://www.chevelles.com/forums/16-brakes-suspension-steering/733498-informational-post-convert-10-12-bolt-rear-upper-control-arm.html

https://www.lever-family-racing.com/lever-family-racing-home/control-arms-version-2/gm-a-body-64-72/10-bolt-12-bolt-difference/create-12-bolt-arms

https://classicoldsmobile.com/forums/chassis-body-frame-7/informational-post-10-bolt-vs-12-bolt-rear-upper-control-arms-90157/

 

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The "notch" you speak of is on the F41 control arms. Supposedly to clear the bigger 12bolt housing. I have put many 12 bolts into cars originally equipped with 10bolts and the original upper control arms fit. 

Here is a pic of a 12bolt that is in my 71 402 4speed car. The drivers side arm has the notch and reinforcement plate, aka F41 upper arm. 

IMG_20190518_115214.jpg

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I've been looking at 12-bolt rear ends on Facebook and Craigslist and I came across a few, but I am unsure if I can use them.  From the post above I am pretty certain that I will be able to use a rear end from a A-body.  But I am seeing a some from (67-72) C10 trucks,  (80, 82-92, & 88) Camaro, (78) Caprice, and a (69) truck w/ 6 lugs (??)  Can I use any of these?

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As a bolt in application- no. Only a 12 bolt from an a-body will fit. From 64-72. The 64-67 12 bolts are an inch narrower and usually command big dollars, but will bolt into our Montes. 

The others can be made to fit but in my opinion, not cost effective. And the truck 12bolt is a different species all together. 

The Caprice rear is an 8.5 but also not cost effective. Those rears are from 73-91ish. Will bolt into a car of those years. Shocks bolt to front of rear as opposed to ours being bolted to the rear of the rear. All of the other rears you mentioned need fabrication skills or a pro to make it fit properly. Not even mentioning lengths. 

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