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Hey, Foresure, they are not the same, I just looked at the two,the 6 is a lot smaller then the v-8,with the v-8 having a bigger u-joint,bigger shaft. These are extra shafts I pulled from parts cars.
 

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Discussion Starter #22
So I'm going to have to change out the shaft as well? Even if I'm keeping my 6 cyl engine and auto trans?
 

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Although I eventually got around to sticking a V8 , when I put my eight inch rear in I still had the 200 six engine and its C4 in my '67. The driveshaft I got from a 302 C4 Mustang fit perfectly. I threw my original driveshaft away, it wouldn't work.
 

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Discussion Starter #24
You mean that when you put the 8" rear end in, while you still had the 200 engine and C4 trans, the drive shaft did NOT work? Or that when you put the V8 in there, the drive shaft did not work? I'm sorry guys, I'm getting a lot of mixed info here, it seems.
 

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Wait a minute now. I'm not changing my engine or my transmission. I'm keeping my stock 200 I6 in the car, as well as the C4 tranny, I'm not changing those out.

This is why I said your current driveshaft will work (and it will work just absolutely perfectly fine). I'm all for safety, but sometimes people seem to assume 6 cyl cars are made out of tinfoil or something!

Putting an 8" rear end into the car isn't going to increase the torque stress placed on your driveshaft. It doesn't matter if the driveshaft can stand the stress of a small block- because you don't have a small block V8. Even if you did, its not like the 289 produces THAT much more low end torque than the 200 (and that's the crucial stat- torque).

Out of curiosity I went on RockAuto to look up the universal joint at the back of the driveshaft and the site lists the exact same rear u-joint for both the 200 and 289 with automatic transmission. That means you may luck out and simply be able to disconnect from the 7.25" and reconnect to the 8" with absolutely no modification necessary. In fact, I suspect this will be the case.
 

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Discussion Starter #26
Wait a minute now. I'm not changing my engine or my transmission. I'm keeping my stock 200 I6 in the car, as well as the C4 tranny, I'm not changing those out.

This is why I said your current driveshaft will work (and it will work just absolutely perfectly fine). I'm all for safety, but sometimes people seem to assume 6 cyl cars are made out of tinfoil or something!

Putting an 8" rear end into the car isn't going to increase the torque stress placed on your driveshaft. It doesn't matter if the driveshaft can stand the stress of a small block- because you don't have a small block V8. Even if you did, its not like the 289 produces THAT much more low end torque than the 200 (and that's the crucial stat- torque).

Out of curiosity I went on RockAuto to look up the universal joint at the back of the driveshaft and the site lists the exact same rear u-joint for both the 200 and 289 with automatic transmission. That means you may luck out and simply be able to disconnect from the 7.25" and reconnect to the 8" with absolutely no modification necessary. In fact, I suspect this will be the case.
GREAT! That is a huge load off my mind. I wasn't really understanding why an 8" rear end would make my drive shaft not work or be unsafe if I wasn't changing the engine or transmission. Hopefully I'll be able to tackle this and the front brakes within the next week or two. When I do, I'll be sure to take pictures and post the progression of the project here. I'm darn excited!!
 

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A that point I was swapping JUST the rear end. I changed driveshafts basically because the rear ujoint was a mismatch. The six's ujoint did not fit the 8's pinion. That and I didn't like the looks of the two-piece rubber insulated driveshaft.
By all means try your driveshaft for fitment first. You can check the rear joint as soon as you pull the driveshaft and have the new rear at hand.
 

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Mine didn't have the rubber insulator- as GypsyR wisely points out, once you disconnect your driveshaft from the 7.25", you can check for fitment with the new rear end (just be ready to plug the back of your transmission to avoid loosing fluid- my manual didn't leak any fluid when I pulled the pinion, but I don't know if an automatic is likely to leak).

Hopefully, you'll be able to bolt the existing u-joint right up to the new rear end using the same u-bolts and nuts. If not, you will probably be able to go one of two ways. Either get a "universal u-joint" like the one at this link: http://www.cjponyparts.com/universal-u-joint-kit/p/M4635A/, or take the driveshaft to a shop and tell them you need the back end of the driveshaft fit with a u-joint that will work with an 8" Ford rear end.
 

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Somehow My brain just skiped over the part where he was keeping the six.

In any case, as long as youve got the 3.22" wide u-joint, you should be fine fitting into the 8" differential.

It will be pretty obvious it the u-joint is too small to fit right. There have just been several first had accounts of 6cyl '67 cars that came with the smaller u-joints that will not fit in an 8".

So, it sounds like some 6cyl auto cars came with the larger u-joint.

It doesn't matter if the driveshaft can stand the stress of a small block- because you don't have a small block V8. Even if you did, its not like the 289 produces THAT much more low end torque than the 200 (and that's the crucial stat- torque).
He's just running with the six, .......BUT the 6cyl drivetrain is certainly NOT up to handling the HP/Torque/weight (whatever) of even the mildest smallblock. In my first car (a '67 I6 coupe) if replaced a blown 200-I6 with a tired (stock) junkyard 302-2v out of an early 70s car. I ran that combo on the stock 6cyl chasis for at least a year. In that span I broke at least 2 (maybe more) u-joints. Eventually the 7.5" differential broke the spyder gears which forced be to track down an 8" rear for a replacement. How many times have you heard of a stock-ish smallblock breaking u-joints on an 8" rear end?? ...not very often, for me.
 

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Does anybody out there have an Ford Mustang 7.25 differential? Blew out the spider gears. Looking before spending the money on an 8" swap.
Thanks
Lenny
 

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I assume all 7.25" rear ends use the same axle spline count. I'll pull the differential out of the housing for $100. You don't have your location in your profile so if I have to ship it to you it will be $100 + shipping.
 

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I put a 8in out of a 70 Mustang in my 67 I6 coupe.

Everything fit like a glove, u-joints included.

OP, I hope that you are at least changing the rear end fluid and pulling the drums for a look at the axle seals before the install.

Its a lot easier to fix problems on a pair of saw horses vs on you back under the car.
 
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