# 10 or 12 bolt rear end?



## joesweeting (May 8, 2011)

I currently have a 455 with 96 heads and a 223 comp cam with a M20 and a 10 bolt 3.55 rear end. I want to install a 3.23 or 3.08 posi. Should I replace the the 10 bolt with a 12 bolt? I don't want to buy a posi and new gears for 10 bolt and then have it break. I have not dyno'd the engine but my friend Paul helped me rebuild it. He is an ASE master mechanic and estimated about 400-420 hp.

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## BearGFR (Aug 25, 2008)

Depends on how you plan to drive the car. If you're going to "play nice" then the 10-bolt will last a long time. However, if you see drag race style launches in your future, then you'll want something stronger. I put a Moser 9" in mine  It was the one 'aftermarket' piece that truly was a bolt-in and didn't require me to modify everything around it to get it to fit.

Bear


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## joesweeting (May 8, 2011)

I don't plan on drag racing it. I do sometimes burn the tires in the first three gears. The 14" wheels and 3.55 rear end doesn't grab the road well.

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## geeteeohguy (Feb 2, 2008)

Be advised that originally, 455 powered A bodies came with a 12 bolt rear end. There was a good reason for this. With your level of power, I would spend the $$$ on upgrading a 12 bolt rather than a 10 bolt. Do it once, and do it right, and you won't regret it. I have personally run nothing but 10 bolts in my 389-400 powered GTO's over the years. Still am. I have blown a couple up, though. The one in my '65 4 speed car was rebuilt after I blew it up 20+ years ago. I re-geared it to 3.36, and it's been fine since 1990. My slightly modified 389 probably makes about 400 HP.....


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## BearGFR (Aug 25, 2008)

:agree There's a whole lot to be said for 'do it once, do it right'. I really tried to do that with my car and even so, I'm -still- working though little bugaboos on it at least every month it seems. I don't think they're ever truly "done".

If you do go with a 12-bolt, keep a couple things in mind. If you go 'used', there are folks out there who will sell you a "Type O" rear-end out of an Oldsmobile and claim it's a 12 bolt because it has 12 bolts holding on the rear cover. They aren't. Inside they still only have 10 bolts on the ring gear and aren't any stronger. Sometimes they truly just don't know, sometimes they're being dishonest.

On a 12-bolt, I'd recommend making sure you use a c-clip eliminator kit, and make sure it's one that's approved/recommended for street use (not all of them are). If you ever DO go to the track, most sanctioning bodies require them (and besides, they're just safer).

That's one of the reasons I utimately decided to go with a 9" - that, plus the ability to change ratios by swapping pumpkins and they're stronger. (but they do have slightly more parasitic drag)

Bear


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## geeteeohguy (Feb 2, 2008)

A 9" Ford is and always will be superior to a 12 bolt GM. The supported pinion gear is unbeatable. Not cheap, but bulletproof and will last forever.


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## joesweeting (May 8, 2011)

Again, all good information. 12 bolts seem more common, but I will also research installing a ford 9".

Thanks for all the input.

Joe

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## 68greengoat (Sep 15, 2005)

I believe it was Summit who sold a 9" that would bolt right up to an A body. A Moser rear if I'm not mistaken.


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## ALKYGTO (Mar 29, 2010)

*Strange Engineering *makes 12 bolts, 9 inch Ford and Dana 60 (*the ULTIMATE*) rears for A-body GM cars. All top notch stuff and they offer brakes and all accesories for rear axles too. I got all my 12 bolt stuff (aside from the housing) from them and everyone there was very helpful. I highly reccomend them :cheers.

Strange Engineering


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## BearGFR (Aug 25, 2008)

68greengoat said:


> I believe it was Summit who sold a 9" that would bolt right up to an A body. A Moser rear if I'm not mistaken.


I bought mine through Spohn Engineering - the same place I bought nearly all of my suspension parts.
Spohn Engineering

Quality parts for sure. My Moser 9" was a brand new housing with an added drain plug and fill bung. I ordered it with Wilwood rear disc brakes (4-piston Dynalite Pro), nodular iron center section, Wavetrac posi with 3.50 gears, heavy yoke with 1350 series U-joints. I ordered it in bare steel and painted it myself. It bolted right up and was a perfect fit. The only "extra" I didn't order with it was the housing brace.


































































Bear


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## joesweeting (May 8, 2011)

Bear,

Is there any way to determine if a 12 bolt an "O" rear end with out removing the cover? The individual selling it said he could not find any numbers. 

Thanks,

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## BearGFR (Aug 25, 2008)

Yes there is... the general shape of the cover itself is a giveaway.

Here's a decent reference:
diff_id

Bear


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## joesweeting (May 8, 2011)

Thanks, attached is a picture of the 12 bolt I'm considering buying. It is from a 72 Monte Carlo. 

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## ALKYGTO (Mar 29, 2010)

^That's the one you want Joe. Be sure to check and/or change out the axle bearings. That's one weak spot the 12 bolt has, the bearing rides on the actual axle surface and can wear into the axle.

If you plan on racing at all I would recommend Ford axle bearings welded onto the ends and custom axles to eliminate this and the c-clips.

If it's just a streeter I would just clean it up and check the axles at least.


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## joesweeting (May 8, 2011)

Thanks for the info, it is for a street car. It currently has a 2.56 in it. (He thinks, I will verify) I will either switch it with a 3.08 and continue running my M-20 with my 455 or visually inspect the 2.56 and I wait on installation with a TKO 500. The car is a daily driver.

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## ALKYGTO (Mar 29, 2010)

With a 2.56 (or even a 3.08 for that mattter) you'll be ready to run Bonneville with that TKO. Should be good for a top speed of around 300 MPH  :lol:.


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## joesweeting (May 8, 2011)

Unfortunately, I just received a text saying it was sold. The seller said he would check on the price of shipping yesterday and then text me he sold it already today. I should have been more proactive. It was a good deal for a 12 bolt open 2.56 for $200. 

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## Too Many Projects (Nov 15, 2008)

I don't know were you live that Chevy 12 bolts are more common than 10's but a 12 bolt with ANY gear ratio is a steal at $200. That was a $500 axle any day up here. $1000 with a desireable ratio and posi.
Speaking of ratios, you stated your 3.55's didn't grab the pavement well. A taller gear isn't going to make that better. It will lower the rpm on the highway but only make traction worse on hard acceleration.


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## leeklm (Mar 11, 2012)

Too Many Projects said:


> I don't know were you live that Chevy 12 bolts are more common than 10's but a 12 bolt with ANY gear ratio is a steal at $200. That was a $500 axle any day up here. $1000 with a desireable ratio and posi.
> Speaking of ratios, you stated your 3.55's didn't grab the pavement well. A taller gear isn't going to make that better. It will lower the rpm on the highway but only make traction worse on hard acceleration.


:agree I live about 30-40 miles West of you, and was looking for a "stock" 12 bolt last spring, which was about impossible to find. I am to the point where I will just buy a complete rebuilt/aftermarket unit when my 8.2 blows apart.


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## Too Many Projects (Nov 15, 2008)

leeklm said:


> :agree I live about 30-40 miles West of you, and was looking for a "stock" 12 bolt last spring, which was about impossible to find. I am to the point where I will just buy a complete rebuilt/aftermarket unit when my 8.2 blows apart.


I bought my last 12 bolt with 3.31 posi from a guy in No St Paul. He buys and sells them as a hobby. If you want, I can see if he has any.


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## joesweeting (May 8, 2011)

I'm currently stationed in FL. The 12 bolt I wanted to purchase was in CA. That is why the seller was going to check on shipping. I have an app on my phone that notifies me of any 68-72, chevelle or GTO parts listed on craigs list in North America. I have a large list of items I need and if it comes up at a good price I will buy it. I was going with a 3.08 to lower my RPMs and planned on installing a posi to increase traction. 

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## joesweeting (May 8, 2011)

How much torque/hp can a 10 bolt handle?

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## mrvandermey (Jun 15, 2011)

joesweeting said:


> I have an app on my phone that notifies me of any 68-72, chevelle or GTO parts listed on craigs list in North America.


Joe, what app is this? I would be interested in this app.


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## AdamIsAdam (Oct 15, 2012)

mrvandermey said:


> Joe, what app is this? I would be interested in this app.


Me too.

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## joesweeting (May 8, 2011)

Search for Autoguide.com and select GTO forum

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