wonso79

What's the difference between an 8" or 9" rear end? I have started my restoration on my 65 mustang and coverting over from a 6 to an 8 but looking for help on the type of rear end to change to. Also 3.55 gears are they good?

2+2GT

Well, the ring gear in one is 8", and…

Seriously, they even share some internal parts.

Forget finding an OE 65-66 9" setup, they were ONLY used on cars equipped with the 289 High Performance engine.

You can buy aftermarket housings, of course, if you really have your heart set on a 9".

The difference is easy- The pinion housing of the 9" overhangs the two lower-right mounting studs, the 8" does not.

andrewmp6

The easiest way i learned was try putting a socket on the lowest bolt if it fits its a 8 inch if it won't go on its a 9 inch.Unless your going to build a high powered engine a 8 inch is fine.

wonso79

O.k. so if I put a 289 the 8" would be fine but what happens if I put a standard 302 in the car?

andrewmp6

Ford used a 8 inch behide all small blocks only the big blocks and few early hipo cars had a 9 inch.The 8 inch can be built up some but if your looking over 400 hp then i would buy a 9 inch housing.If lower then a 8 inch will be fine if you don't run slicks and try to drag race it.

2+2GT

True, if you are using 'normal' street tires, say, 205/70R14, you'll probably never need more than an 8". And of course the aftermarket helps, Currie has an 8" 4-pinion Traction-Lok that's probably stronger than a lot of 9" rears.

There were quite a few 9" Mustangs.

64-67 289HP
67-69 390GT
67-70 GT500 (428)
68 390 2Vpf
68 302 4V in "GT" cars
68-70 428 Cobra Jet
69-70 Boss 302
69-70 Boss 429
69-73 351 2V
69-73 351 4V (includes CJ, BOSS, & HO)
71 429 CJ & SCJ

wonso79

Thanks for the info! I am looking at a 302 with 300-325 horse.

67mustang302

8" is fine for up to 400-500hp range without sticky tires. With sticky tires and hard launches you could break one with 200hp. Running something like a Detroit Tru-Trac helps, since you eliminate side and spider gears the carrier assembly is more stable(reduces axle, bearing and gear issues). The main problem with 8" is the weak pinion support, too much resistance through the rear end and the pinion gear needs someplace to go....usually breaks the pinion mount in the 3rd member and tries to spit the pinion out the front.

I've been running an 8" for years with track use and I abuse my car, close to 400hp, never had issues.

Currie now has an aluminum 3rd member for the 8 that is stronger than the stock castings. I like the 8" actually, less weight and more efficient than a 9". 9" is easier to find parts for though and a LOT stronger....they'll handle more power than you could ever control in a street car. Same with the newer 8.8".

If you're doing a whole rear end swap and starting from scratch though, you might want to consider doing a, 8.8 or a 9....it's not that easy to find 8" housings, but 8.8's and 9's are all over the place.

andrewmp6

I was just saying 64-70 there was probably more 8 inch's used then the 9 inch.The 71-73 they was about even because they dropped the 7.5.I would look for adds on here for a 8 inch or even craigslist has them sometimes.

stephen.osborne1

Has anyone tried those ebay third members for 8"? I've seen some "rebuilt" third members for about 500-600 bucks. I want to put lower gears and trac-loc in my 8" and am wondering if this is a viable solution. I was looking at 9" as well, but after some research, it sounds like I may be better off with the 8"... 100 pounds and less static HP loss...