View Full Version : Rear Ends - 9 Inch, 10 bolt, 12 bolt...What's The difference?

07-27-2007, 01:01:30 AM
Well, I am planning out how I want to build my car, I'm not an excessive planner or anything but I think it's important to have a thought out plan for something like this.

One thing I cannot correctly plan is rear ends, being I just don't understand the difference between them so I'm not sure which one I want. Whats the difference in the different rear ends? Different characteristics and uses? Can you name them, please?

07-27-2007, 01:10:07 AM
Mainly strength. The bigger the gears the stronger the rear end. The "bolt" designation is just a way to indentify the rear end. A 10 bolt is not as strong as a 12 bolt. The bolts are what holds the ring gear to the carrier, but also the cover to the housing so you can see what the size is without removing the cover. The Ford design 9 inch is a whole different animal, and a good one.

07-27-2007, 01:19:58 AM
Cool, because I've seen the 9 inch ones go for cheaper. Thanks for the info.

07-27-2007, 01:28:00 AM
get a 8.5" 10 bolt.... people will tell you they arent strong enuf but they can handle a sh!t load of abuse... my dad had his stock 8.5 in his camaro and blew up two t-10 4 speeds.... the rear end didnt complain once.

if you have an 8.2" 10 bolt junk it.... its useless basicaly. there are two diffrent 8.5" ten bolts... there is 273 and numerically higher 8.5" which has the thinner style carrier, and then there is the 256 which has the thicker carrier. the 256 gear does need a ring gear spacer though if you want to get say 373's. i personally belive the 256 is much strounger then the 273 and numericaly higher because theres is alot more metal.

i have a 256 posi but im putting 373 gears in soon. the only thing im concerned about with the 256 is that when i put 373s and the ring gear spacer on the bolts will be longer and might be able to flex and lossen off when the the gears heat up and cool down. but thats just what i think might happen.

a stock 8.5"ten bolt will handle over 400hp with a standard. with a auto it will might handle around 500hp without blowing up. iv got picks of a 2nd gen doing wheelies with an 8.5" ten bolt.

a 10-bolt is almost as strong as a 12-bolt since its ring-gear diameter is only 0.375 inch smaller than a 12-bolt...

hope everything i wrote makes sence.... long hard day of work and im very tired..... pm me if you have any more questions about ten bolts...

07-27-2007, 01:35:37 PM
The other factor is the way the axles are held in. GM uses a "C" clip to retain the axle. Ford does not. The "C" clip can fail and usually does over 400+hp. When it does, the axle simply slides out of the rear end housing and takes off, taking the wheel and tire with it. There is a ford-style C-clip eliminator setup you can install. A must have if you put in a mini spool, full spool, or even a posi.

07-27-2007, 01:39:27 PM
Also, GM made 3 different rear ends in the 10 bolt style. 8.5 inch up through about 1981, then down to a 7.5 inch through the mid 90's, then back up to 7 5/8 I believe from then out. The early 8.5 is the strongest IIRC, but the new ones came with disks on some applications, and can be upgraded in some cases. Stronger axles are determined by the number of splines on them. You have no doubt head of 28 spline, (Stock GM) up through 33 or even 35 spline axles. The more splines, generally the stronger it is.

07-27-2007, 01:48:24 PM
I have been running a 9" unit with 33 spline axles, .390's and a Detroit locker for over 12 yrs now... same unit. No issues what so ever with the diff....
I abuse the hell outta this setup... the rear has been the only truly bullet-proof piece in the car. I have twisted driveshafts... broken windshields and T-tops... even a tear in the sheetmetal at the top-rear of the drivers side window... and the 9" rear diff just keeps rockin' on.:cool:
I keep killing ST-10's.. LOL! Looking for a TKO next time around:)

Oh yeah... the difference... I haven't been able to kill the 9" unit

night rider
07-27-2007, 04:30:37 PM
Wookie.. The 7.625" (7-5/8") started in 1986

71-81.... 8.5"
82-85.... 7.5" (26 spline axles)
86-mid 91... 7.625" 26 spline
mid 91 to 02... 7.625" 28 spline

big gear head
07-27-2007, 07:02:21 PM
First you need to know what you want to do with the car and what you expect the rear end to handle. We can not recommend a rear end without knowing what your intentions are. The 9 inch should be considered a high end performance rear end and a lot of overkill for most street/strip cars. The 8.5 10 bolt and the 12 bolt Chevy are more than strong enough to handle most street/strip use, and they are lighter and use less power to turn. The differential that you choose will depend on what type of driving you want to do. If this will be a street/strip car then a good cluth type posi like the Eaton would be a good choice. If you will be doing auto cross events and road racing then a Truetrac or Torsen would be a better choice. If it will see more drag strip use than anything enlse then a Detroit Locker would be better. The same thing goes for the gear ratio and all other parts of the rear end. You need to know what you want it to do before you can pick the parts for it.

07-27-2007, 07:06:30 PM
Well I want to have just a tad over 400 HP to the wheels on a 4 speed manual. Mostly street use, but occasional autocrossing or roadracing is a strong possibility.

07-28-2007, 05:36:31 AM
I agree with BGH, but will add the big advantage of a 9" over a 10-12 bolt is the removable center. Its a really convenient design. you can bolt stuff to the face, and weld anywhere on the housing. Strength issues aside, the 9" is a far superior design, imo.

07-28-2007, 08:35:34 AM
Or you could always go find a dana 60 ;)

07-28-2007, 10:05:14 AM
Never hurt a 10 or 12 bolt.
8.5" 10 bolt is nearly as strong as the 12 bolt. They both have large pinion dia. the 8.2 is smaller.
The C-clips are OK but if you need better axles, get c-clip eliminators, or 9" housing ends.

I have broken a 9" doing holeshots in the driveway.
Pinion deflection, perhaps,

All have benefits, none are perfect.
All, properly prepped,will handle more than what a sane person will throw at them.

07-28-2007, 10:41:14 AM
As has been stated above, there are many factors in choosing a rear end and it's componits. No matter which rear end you choose, there are things that remain the same. I've seen every kind of rear end broken when certain rules were broken.

Rule 1: wheel hop/traction control. If the rear end is allowed to wheel hop, it will break a drive line part. Think about this. When the wheel is on the road (under load) it has all the force in one direction). When it hops in the air (off the road) it unloads almost all of that force. Then it contacts the road putting the drive line under load again. Multilply this by the number of times that it happens and you'll see a HUGE amount of force being applied in opposite directions which will result in the weakest link in the drive line exploding. Bottom line, controlling wheel hop is VERY important.

Rule 2: driveability. Total power to the road will dictate what rear end you'll choose. You stated that your engine will be producing 400 hp to the wheels. What engine are you building as that's a lot of horse power to acheive with most engines and it might not be a "happy" streetable combination--nasty cam with little or no vacuum at idle = no vacuum assisted brakes = a bear of a car to drive in traffic. It's nice to dream about haveing a high horse power engine but then actually driving it is another. It's also nice to go fast but eventually you will want/need to stop. The vehicle will stop but the QUALITY and QUANTITY of the stop is VERY important. Be reasonalbe with your build especially if the majority of your driving is going to be on the street. BEEN THERE DONE THAT! An engine with a Isky Z85 cam ( ) cannot be driven on the street in a reasonable everyday.

Rule 3: cost. If you have an unlimited budget, then by all means buy the biggest baddest rear end you can. If however, you're like most of us, you can only spend what you have in a buget. BUT spend it once. Don't settle for a setup that won't last but also don't get the BIBS (bigger is better syndrome) which is the most common mistake (we've all made). The BIBS can apply to everything in life but especially to cars. EX: if a 650 CFM Holley is good then a 1050 CFM Holley HAS to be better. NOT! You need to consider each and every drive line componit to INVEST your money in the best parts that match each and every part front to back. BTW, you are planning on putting a safety bell housing and SFI approved flywheel in aren't you especially if you put that 400 wheel HP engine in. Add another $300 to $1000 to your building budget ( ). You'll also have to have heavy duty u-joints, drive shaft loop, better parts in the rear (gears, posi device, axles, axle retaining = c-clip eliminators OR Ford style axle retention), etc. It all adds up. Bottom line, the more horse power you make the more money you WILL BE investing in drive line parts.

Strength of the differential: as stated the 8.5" GM 10 bolt corperate rear end will accept a lot of HP. Next in line is the GM 12 bolt CAR rear end (the truck 12 bolt is NEVER to be used for high performance use). Next in line is the Dana 60. And the ultimate for all out HP is the Ford 9". Each has it's limits on cost and the amount of power it can handle. One drawback to the for 9" is that because of way that the pinion gear intersects the ring gear (which makes it stronger) it has more paracitic drag which means that it takes more horse power to turn it (somewhere around 3% to 5% I think). Not a big consideration but something to factor in especially if you build a lower and more streetalbe engine.

07-28-2007, 10:57:11 AM
On a serious note, I try to keep mine where the tires will break loose before any serious damage is done to anything. I remove the weak link so to say and just let the tires spin.

If your building a race car though, this won't work for you.;)

07-28-2007, 12:35:05 PM
Well I realize that the HP is a lot to shoot for, but I'm not dead set on it. I wanted to use a carbed 383 stroker motor.....what kind of streetable-friendly HP can I make out of that and what is a suitable rear end for mostly street driving? I will be driving spirited, though. so take that into account. If I'm not going to make a lot of HP, I don't think the 9" is right for me, especially considering what you said about parasitic drag.

I'm not building a race car, nor do I race. I just want a fast car that will last too. I'm tired of having slow cars because of having no money.

big gear head
07-28-2007, 10:31:43 PM
For what you want to do I would build the 8.5 10 bolt that is in the car now. I would use a Truetrac and a good set of 30 spline c clip axles. I would also brace the spring perches and weld the axle tubes to the center casting. A 3.42 gear would be good if you don't have overdrive.

Here is how the axle tubes need to be welded.

07-29-2007, 12:51:15 AM
Oh, so stock is 8.5" 10 bolt? I just need good gears with LSD then and put some disc brakes on each end! Thanks for the link, I'll look into that when I'm ready to tackle that area.

night rider
07-29-2007, 09:43:47 AM
yes use the 8.5" you have.. Get a set of good gears in the ratio you need, along with a good limited slip unit like the Eaton or truetrack, a set of aftermarket 30 spline axles, etc.

07-30-2007, 09:55:04 PM
I like the 9" housing fabed to bolt right in under your camaro. you can pick a axle package in 31 or 35 or 40 spline. I have a f*rd 9" center thats a 31 spline 4 pinion trac loc. so my center and some axles and a pre fabed housing I could do a rear priced right. but then I keep thinking a 35 spline & a detroit locker would live 4 ever, and rear disc are easy too. yeah it might be over kill but in a good way. like this