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Technical discussions specific to 1964-1967, 1968-1970, and 1971-1973 Classic Mustang. Discuss all tech related to in-line six cylinder and V8 powered Vintage Mustangs here.

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Unread 12-28-2008   #1 (permalink)
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Default How strong is the 8 inch rear

I have the 8 inch rear end my 66 fastback. It has limited slip and 325 gears. I am going to swap to lower gears and a 6 speed trans.

I have done a lot of work on these classic mustangs and never seen a problem with the rear ends. Has anyone blown one of these rears up? How strong are they really? I know that axles are the same as the 9 inch 28 spline, but do the 8 inch gears strip if you run 400hp through 4.11's?

Just curious to see what experiences people have had with their 8 inch rears.

Thanks
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Unread 12-29-2008   #2 (permalink)
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Hello. I've never seen anyone blow one up with a small block. You'll twist off the drive shaft or snap a U-joint or something before you blow up the rear end. I have seen some get torn up because of neglect, as in, let the fluid leak all the way out, sit around and get rusty inside and then put 400+ hp in front of it, or, have some idiot set it up with with everything locked down tight, no backlash, no clearance and things sitting in there at strange angles and burn it up or break something, but, if it's been anything close to properly set up and maintained, you won't have any problems out of it.
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Unread 12-29-2008   #3 (permalink)
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Quote:
Originally Posted by Veronica View Post
if it's been anything close to properly set up and maintained, you won't have any problems out of it.
... but ...

I researched it a lot before deciding to stick with the 8" in my soon to be 370 HP Roller Long Block 67 Cougar.

I spoke to quite a few people who said they blew up 8" rearends with far less than 400HP, both "old" and freshly built rearends ... by running drag slicks and launching "at 5000 rpm".

And, if you research it, you'll find that most often, the consensus is that "below 400 HP" is safe for an 8" (depending on how hard you plan on launching with it)

Since my car will never be drag raced, and I know myne was properly set up by a professional shop, I have no problem expecting it to be just fine for my uses ....
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Unread 12-29-2008   #4 (permalink)
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The 8-inch rear end came out in 1962 [1962-1980]. The 8-inch 3rd members from '62-'66 are the weakest versions. Starting in '67-up, the 8-inch 3rd member case was revised [C7OW casting] to include more vertical and cross-webbing on the face of the case to strengthen it.

There are claims of people running these 8-inch rears with 400 hp, and they may be getting away with it, but these rear ends were not engineered to reliably handle that much power and torque --at least not without a lot of beefing up with [expensive] aftermarket parts.

All early V-8 Mustangs came with 8-inch rears except for the HiPo 289s --they came with 9-inch rears. The HiPo 289s were rated at 271 hp [Shelby 289s were rated at 306 hp].

The HiPo 289 carried over into the '67 model Mustangs. Even though the 8-inch 3rd member case had been revised by '67, Ford still did not use the revised 8-inch in the HiPo 289 Mustangs --they used the 9-inch.

Manual transmissions and sticky tires with lots of traction are not the only things that will kill an 8-inch. Most Mustangs suffer from wheel-hop, and this can also kill an 8-inch, even with regular street tires.

There are a few common failure points for an 8-inch; one is in the area of the input driving pinion support on the front of the 3rd member casting. The other is in the area of the pinion [spider] gears inside the differential, or with the differential main caps/bolts.

An example of a driving pinion support failure on an 8-inch [revised case] 3rd member:




An example of the main caps and main cap bolts breaking that lead to this disaster on this 8-inch [revised case] 3rd member:

1.

2.

3.

4.

I suspect the reason why so many people run 8-inch rears, in applications it wasn't really designed to reliably handle, is because a [wrecking yard] 8-inch is much easier to find in the right width for an early Mustang than a 9-inch of like parameters. It's your car and your money, so only you can make the decision of running a rear end that might last, or to run one that will give you no worries.

An 8-inch, beefed up with expensive aftermarket parts, is only as strong as your base-model, stock-configured, run-of-the-mill wrecking yard 9-inch rear.

8-inch rear ends only came with differentials that had 2 pinion gears inside. A 9-inch could have either a 2-pinion differential, or a much stronger 4-pinion differential to distribute the force over a greater surface area.


Some 9-inch rear end info from my site:
The Ford 9-inch Rear End

Differential Types

Third Member Cases

Third Member I.D. Codes
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Unread 12-29-2008   #5 (permalink)
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Quote:
Originally Posted by ultrastang View Post
Manual transmissions and sticky tires with lots of traction are not the only things that will kill an 8-inch. Most Mustangs suffer from wheel-hop, and this can also kill an 8-inch, even with regular street tires.
Facts and Data, Bravo !

Thank You Steve !

.... you've introduced another term which I always hear, but I've never really tried to understand exactly what it is:

Is wheel hop something that happens when you launch hard ?
( I saw another guy describe how he put on drag slicks, warmed 'em up, got a little bit of wheel hop, and "snapped" his 8")

What is the "physics" behind it ? (i.e., what causes it ? )

Does it help to eliminate it by "stiffining up" the shagged rear suspensions that are on lots of our 40 year old cars ?
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Unread 12-29-2008   #6 (permalink)
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Ultrastang, what a great post! I agree with what you say. The 8" is not designed for a performance environment.

I installed a 9" in my 5.0 powered 67 and even though it's heavier weight wise, I can throw anything I want at it with peace of mind. There are some weak early 9" diff. housing castings out there, but I've found that the later ones have close to the same strength as some of the earlier "N" case nodular iron castings. I'll take the weight penalty, give me a 9".

There's one thing that everyone needs to keep in mind, if piecing an axle assembly together. Always check for a fill plug provision to correspond with the axle housing before you buy or install. In 67 Ford changed the fill plug from the axle housing to the differential housing or "Chunk". Installing a pre 67 chunk into a 67 and later axle housing will result in no fill plug. That would be enough to make a preacher cuss.

Wheel hop is a condition that happens on a hard launch. This usually happens with weak springs and/or weak shocks. When the shocks don't have enough dampening resistance to the axle's upward motion, and the weak springs start to wrap up, throwing the drive line angle out of alignment, the wheel hop starts. The sudden tire contact with the pavement on each hop, places strain in the drive line.
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Unread 12-29-2008   #7 (permalink)
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Great stuff Ultrastang!!!!!!
I have gone to a trutrac because I burned up two limited slip 9" chunks with 3.50 and 3.70 gears. On the 3.50 I went hard into 2nd (351 C injected w/ 4R70W 4 spd auto) and felt this horrible vibe out of nowhere that did not go away. grinded up the u-joint and thought that was all but upon turning the pinion by hand with car in air found a "hard spot" took hubs off - same - pulled axles-same its in the chunk. My car would put down 25' marks going to 2nd WITH ONLY ONE WHEEL(left) but smoke both from a roll or stop. was told I was overpowering the limited slip and it was a common failure with 400+ power. to see your site and pics really sheds light on things for me - thank you.
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Unread 12-29-2008   #8 (permalink)
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I stayed up last night reading through your web site ultrastang, great info but a bit late for me. I just spent a small fortune upgrading my 8" to LSD. With 345HP behind it i guess it's only a matter of time.

I think this deserves a sticky but i don't know how to help you get one.

Steve
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Unread 12-29-2008   #9 (permalink)
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Ultrastang sure great to see you on this board you are the guru of 9 inch and 8 inch rearends. I have used your website so many times for info what a great service. I have even referred many people to it. Welcome. Some day I do need to make an order for your disc adapter plates. Just haven't got to it, but I will some day.
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Unread 12-29-2008   #10 (permalink)
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Quote:
Originally Posted by Blackshadow View Post
Ultrastang sure great to see you on this board you are the guru of 9 inch and 8 inch rearends. I have used your website so many times for info what a great service. I have even referred many people to it. Welcome. Some day I do need to make an order for your disc adapter plates. Just haven't got to it, but I will some day.
Glad to help, if my information did help anyone out.

I've actually been a member here for about 7 years [joined Jul 2002], I just don't post a lot here mainly because they tend to get pissy here if you leave a link to your own website. I frequent about three other major Mustang forums, and none of those other sites has a problem if a person leaves their weblink in the signature line there.

I find my "Rookie" status here rather amusing. I'm 42, but was bitten by the Mustang bug when I was 13 [1979]. I don't know the answer to everything, but in roughly 30 years in this hobby, I would say a person has enough experience at this to certainly be more than a common "Rookie". It just goes to show that a person's post statistics really isn't a good measure of what a person may actually know.
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Unread 12-29-2008   #11 (permalink)
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I have found the weak link in the 8 inch the ring and pinion. I have spit out more than my share of them years ago, both from wheel hop and power shifting at high rpm. I finally got everything to hold up with using zoom gears back in the day and a good set of lakewood traction bars with the j bolts. I know thats an old school set up these days but it worked
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Unread 12-29-2008   #12 (permalink)
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Oof! I feel my credit card heating up and vibrating in my wallet.

Maybe I can sell my full 8 inch housing, third member(3.25 with Limited Slip), leafs, shocks, axles and drum brakes. What is this stuff worth? It is all rebuilt with new bearings and clutches, new leafs, new brakes, new KYB's.

Thanks everyone for your input. I am sure glad I found this forum to bounce ideas around.

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Unread 12-30-2008   #13 (permalink)
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Just ignore that "Rookie" BS, it's totally a forum usage deal and has nothing to do with the amount of knowledge that you have. I'm sure that you have too much going on to live on this forum.
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Unread 12-30-2008   #14 (permalink)
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Default 8" or 9" Reliability Statistics ?

.. hmmmm ...
... it'd be really nice to have some more uniform statistics on this

We've got two votes for "9" or nothing"
... maybe I should have posted a poll with the following choices:

1) I run an 8" with 300-400 HP, no problems
(? how many years, modifications to "help" it ? )
2) I run an 8" with 400-500 HP, no problems
3) I have personally grenaded an 8" rearend
(HP ? Trans ? Suspension ? Tires ? Racing ?)
4) I run a 9" with 300-400 HP, no problems
5) I run a 9" with 400-500 HP, no problems
6) I have personally grenaded a 9" rearend
(HP ? Trans ? Suspension ? Tires ? Racing ?)

(anybody know how to do it ... is there a FAQ which tells me how ? )

I'm going to stick with my (hopefully) well built 8'er at 370 or so HP,
hopefully I won't be reporting back with problems anytime soon ...

... looks like my search engine is going to get another workout ...
(gathering more statistics)
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Unread 12-30-2008   #15 (permalink)
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Quote:
Originally Posted by Fastercat View Post
.. hmmmm ...
... it'd be really nice to have some more uniform statistics on this

We've got two votes for "9" or nothing"
... maybe I should have posted a poll with the following choices:

1) I run an 8" with 300-400 HP, no problems
(? how many years, modifications to "help" it ? )
2) I run an 8" with 400-500 HP, no problems
3) I have personally grenaded an 8" rearend
(HP ? Trans ? Suspension ? Tires ? Racing ?)
4) I run a 9" with 300-400 HP, no problems
5) I run a 9" with 400-500 HP, no problems
6) I have personally grenaded a 9" rearend
(HP ? Trans ? Suspension ? Tires ? Racing ?)

(anybody know how to do it ... is there a FAQ which tells me how ? )

I'm going to stick with my (hopefully) well built 8'er at 370 or so HP,
hopefully I won't be reporting back with problems anytime soon ...

... looks like my search engine is going to get another workout ...
(gathering more statistics)
Don't know any real-world statistics of how many 8-inch rears survive punshiment vs. those that don't. Some people get a way with 8-inch rears in high horse power applications, and some people blow 8-inch rears with only mildy warmed over small-blocks, so it's just a coin-toss. In the case of running an 8-inch with a powerful engine, it's pretty much a senario of running on barrowed time.

The weak link for an 8-inch is the 3rd member. This could be the 3rd member case itself, or any of its internals.

The weak link for a 9-inch --assuming we are talking about the 3rd member cases from around 1960-up and not the gray-iron cases from '57-'59, are the 28-spline axles. If you have a 9-inch rear with the single vertical-rib case [or the ultimate --the N-case], and 31-spline axles, it's very difficult to break the 9-inch.

...now that I think about it, there is one other weak link on the 9-inch in reference to the differential...

Going back a bit...The 9-inch rear end first came out in the Ford passenger car and half-ton pickups in 1957. The first version of a Ford differential for the 9-inch that directed power to both rear wheels was called the 'Equalok'. Some time just after the early '60s, the term was changed to 'Limited-Slip'. I don't know this to be a fact, but I suspect the Equalok and Limited-Slip are the same differential, since I can see no difference in their design from one to the other.

The Equalok/Limited-Slip differentials both have the same internal design and they both only have 2-pinion [spider] gears inside. The Ford Limited-Slip was phased out of production at the end of 1968 to be replaced by a much better differential design called the 'Traction-Lok'. However, the 1st-year Traction-Lok had a flaw. The driver's side half of the differential had (4) large holes in its casting. This is a weak area that is prone to breakage if lots of power or torque is applied. The casting will crack between all (4) holes and the center area holding the side bearing will separate from the rest of the differential.

The (4)-hole '69 Traction-Lok differential was a one-year casting only. The Traction-Lok differential was revised by 1970 with a driver's side differential case half that had a solid cover. The revised casting can still break in this area, but is not nearly as prone to breakeage as the '69 version.

1970-1985 Traction-Lok differential on left compared to weaker (4)-hole casting '69 Traction-Lok differential on right:


Another weak '69 (4)-hole differential:


What a failed differential looks like when too much force is applied:
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