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9" rear end vs. 8" rear end question

thegreatjustino

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Red Head Grease Monkey
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I have finally gotten to the point of starting work on a couple of '67 Mustangs I own. My plan is to build one with a 390 and keep the 302/5.0 in the other. One of the cars has an 8" rear end and the other a 9".

Is an 8" rear end sufficient for a fairly stock 90s Mustang 5.0? Or would I be better off tracking down a second Mustang 9" rear end?

Curious what all you builders here think. The current 9" in the Mustang is in good shape and doesn't need any work. The 8" needs a full rebuild. Taking that into consideration, finding another Mustang 9" in good used shape might be around the same amount as rebuilding the 8.
 

Timmy390

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I have finally gotten to the point of starting work on a couple of '67 Mustangs I own. My plan is to build one with a 390 and keep the 302/5.0 in the other. One of the cars has an 8" rear end and the other a 9".

Is an 8" rear end sufficient for a fairly stock 90s Mustang 5.0? Or would I be better off tracking down a second Mustang 9" rear end?

Curious what all you builders here think. The current 9" in the Mustang is in good shape and doesn't need any work. The 8" needs a full rebuild. Taking that into consideration, finding another Mustang 9" in good used shape might be around the same amount as rebuilding the 8.

The Ford 9 inch was replaced by the 8.8 in 1986ish......everything since then has had an 8.8 rear. They're strong..........my 5.8L 14 Shelby with 662HP has an 8.8

Have fun with the 390 build.....if you go with a bigger than stock cam, get rocker shaft end stands.....broken shafts are no fun. Been there done that.....

Tim
 

JWMcCrary

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I broke the center section of 2 8" rears in a 65 Mustang before swapping in a 9 many moons ago. It wasn't stock and had a 4 speed. Stock with an auto trans should be fine. With a higher RPM engine and/or a manual trans I would go ahead and find a 9 inch.


I have finally gotten to the point of starting work on a couple of '67 Mustangs I own. My plan is to build one with a 390 and keep the 302/5.0 in the other. One of the cars has an 8" rear end and the other a 9".

Is an 8" rear end sufficient for a fairly stock 90s Mustang 5.0? Or would I be better off tracking down a second Mustang 9" rear end?

Curious what all you builders here think. The current 9" in the Mustang is in good shape and doesn't need any work. The 8" needs a full rebuild. Taking that into consideration, finding another Mustang 9" in good used shape might be around the same amount as rebuilding the 8.
 

bmc69

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It's been a long time ago now..but I do recall that the old 8" rears were just sufficient for the weak 6-bangers that turned them. Only 4-lug axles too, if I recall correctly.
 

Rustytruck

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Feb 24, 2002
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The 8" rear in 5 lug was standard for a 289 V8. When they went with the 9" rear in 67 was for the 390 engine. If you have a true 67 9 inch housing the rear axles and brakes from the 8 inch housing will fit in the 9 in housing. The rear axles on a 67 mustang 9" have 2 small drilled dimples in the flange end of the axles where the 8" mustang axles had a rectangular recess in the end but all other physical measurements and bearings were the same. Both were small bearing 28 spline. The 8" rear wasn't a bad axle for a 170 hp motor with small tires but back in the day when were were hopping them up the 8" would strip the side gears very easily especially in a clutch driven car once you started playing very hard. You can tell a 67 mustang 9 inch housing by the narrowing of the axle tube out by the leaf spring mount down to the 8 inch tube size.
 

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bmc69

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The 8" rear in 5 lug was standard for a 289 V8. When they went with the 9" rear in 67 was for the 390 engine. If you have a true 67 9 inch housing the rear axles and brakes from the 8 inch housing will fit in the 9 in housing. The rear axles on a 67 mustang 9" have 2 small drilled dimples in the flange end of the axles where the 8" mustang axles had a rectangular recess in the end but all other physical measurements and bearings were the same. Both were small bearing 28 spline. The 8" rear wasn't a bad axle for a 170 hp motor with small tires but back in the day when were were hopping them up the 8" would strip the side gears very easily especially in a clutch driven car once you started playing very hard. You can tell a 67 mustang 9 inch housing by the narrowing of the axle tube out by the leaf spring mount down to the 8 inch tube size.

Hah...right you are. Been 40 years since I had my '66 289. I was confusing the 7" and 8" options with 8" and 9". It was the 7 that was so small and used in 6-cylinder cars. My bad.

I never managed to break the 8 (limited slip) in my '66 despite building the 289 a good bit and having the T-10 4-speed. Wasn't for lack of trying either...
 

RPM289

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I have finally gotten to the point of starting work on a couple of '67 Mustangs I own. My plan is to build one with a 390 and keep the 302/5.0 in the other. One of the cars has an 8" rear end and the other a 9".

Is an 8" rear end sufficient for a fairly stock 90s Mustang 5.0? Or would I be better off tracking down a second Mustang 9" rear end?

Curious what all you builders here think. The current 9" in the Mustang is in good shape and doesn't need any work. The 8" needs a full rebuild. Taking that into consideration, finding another Mustang 9" in good used shape might be around the same amount as rebuilding the 8.
I would see if I could find a 9in but you can pick up a 8.8 out of later explorers and have rear disc brakes. Have to move spring perches on housing and drive shaft may have to be modified not 100% if it does or not!

Sent from my LGL84VL using Tapatalk
 

JWMcCrary

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I never managed to break the 8 (limited slip) in my '66 despite building the 289 a good bit and having the T-10 4-speed. Wasn't for lack of trying either...

I sure did, broke it twice, the 2nd time there wasn't a single tooth left on the pinion. 1980-81ish, Lincoln locker with a 4 speed and a healthy 289/302. Never broke the 9 inch.
 

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Timmy390

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I've broken a 9.........351W warmed up did it. Split the T-loc in half

Anything can be broken sometimes you just get bad parts.

Tim
 

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Rustytruck

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Hah...right you are. Been 40 years since I had my '66 289. I was confusing the 7" and 8" options with 8" and 9". It was the 7 that was so small and used in 6-cylinder cars. My bad.

I never managed to break the 8 (limited slip) in my '66 despite building the 289 a good bit and having the T-10 4-speed. Wasn't for lack of trying either...

Your not bad it was a long time ago. Yea that 7" with the drain pan cover on the rear. The only thing with them they always leaked gear oil all over themselves I doubt anyone ever broke them with the anemic 6 cylinders used at the time. My sister had a 64-1/2 with the stick and the straight 6. Our Pinto was faster than that thing. But the Mustang looked cooler just keep the hood down and the hubcaps on.
 
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thegreatjustino

thegreatjustino

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My first car was a true '64-1/2 Mustang with the 260 V8. That had an 8" rear end and never had an issue. From the factory, the 260 was rated at 164 horsepower, so not a lot of breaking power.

It's interesting that a couple of you brought up the Explorer 8.8. That was a consideration as well and may be the way I end up going considering the amount of Explorers here in local junkyards.
 

vintage bronco

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I have finally gotten to the point of starting work on a couple of '67 Mustangs I own. My plan is to build one with a 390 and keep the 302/5.0 in the other. One of the cars has an 8" rear end and the other a 9".

Is an 8" rear end sufficient for a fairly stock 90s Mustang 5.0? Or would I be better off tracking down a second Mustang 9" rear end?

Curious what all you builders here think. The current 9" in the Mustang is in good shape and doesn't need any work. The 8" needs a full rebuild. Taking that into consideration, finding another Mustang 9" in good used shape might be around the same amount as rebuilding the 8.

8in Will be fine I have behind 400hp 289 running 4spd trans.
 

El Kabong

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I'd say run the 9 if you can. I broke an 8 behind a built 260 & would think more is better when it comes to diffs.
 

Broncobowsher

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Given the 8" needs a full rebuild, get a 9" Friend had a '67 mustang and went through many 8" and he wasn't abusive, just hard on them. And that was given it was also an automatic which tends to be softer on rear ends. He had more invested in the 8" in the end than what a 9" would have cost, that was just the parts bill.

Stock V8, keep the 8". Fine cruiser axle. Add power and traction with a heavy foot, not the best.
 

broncnaz

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Id agree go 9in if at all possible. The 8in is ok but expensive to build for what you get there's lots more available for the 9in and its just plain stronger. Could be different now but I seem to recall a lack of gear ratios available for the 8in plus if you wanted a traction device you were limited the hard to find factory stuff. Really all boils down to what your looking to do with the car as well if your gonna be on the gas pedal a lot then 9in would be my choice if your just cruising the 8in is fine.
 

fordguy

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anybody got a handy reference on WMS to WMS width on these ford rear ends?
 

Justafordguy

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If the 8" needs to be rebuilt I would just go pickup an 8.8 out of an Explorer from the junkyard. They are cheap, strong, have disk brakes, and usually have limited slip.
 

edmedlin

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There is a reason that most builds (even GMs) use some type of 9" Ford. They are the closest thing to bullet-proof you can find. We use them in racing too. The only issue I have ever had in racing is that if you add rear-steer, it can cause some leaking of the axel seals. Rear steer can cause some binding (rear is kicked out) on straightaways. On 3/8ths mile tracks which dominate asphalt short track racing, you are not on a straight for very long. As long as your ring and pinion lash and pinion angle are right, failures are extremely rare.
 

ntsqd

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I used the 8" in my '67 Ranchero behind a warm street engine that I also flat-towwed the dune buggy with. Only ever had one problem, the crush sleave relaxed. It is the exact same p/n as the 9" part, so going to a 9" isn't going to help any there. Put it back together with a 9" solid spacer kit and never had another problem with that axle. I think 8"s are under-rated and under appreciated. BTW, Pinto V6 8"s were the only 4 lug axled 8"s that I know of. Nice, narrow housings on those, too.

If the car is going to be a handler I think that the lower unsprung weight of the 8" is a bonus and would build the 8". If the car is going to be a drag racer then go 9".
 
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