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Author Topic: U-joint to axle centerline on a 6.75" rear end?  (Read 253 times)

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Offline Jauntimus Prime

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U-joint to axle centerline on a 6.75" rear end?
« on: July 11, 2020, 01:39:28 PM »
Hey y'all, been a minute since my last post, but I'm trying to begin the Pinto project in earnest and so am doing some planning.

I've long planned on a T5 conversion, so I know I'll have to shorten a driveshaft. But I think I've decided to go the turbo EFI route... or at least I hope I have, since there's a Merkur engine sitting in my garage. I guess I'm in it now. I've reasoned that if I'm doing this, I'd better plan on eventually upgrading brakes and rear end, which means 5-lug and an 8.8" rear-disc swap might be in the future.

Does anyone know the difference in the pinion length -- or the measurement from pinion yoke u-joint centerline to axle centerline -- between a 6.75" and 8.8" rear? Online I see figures of either 11-5/8" or 11-11/16" for the 8.8, but I can't find any such info on the 6.75.

I hoping that if there's a small enough difference, I can get away with just cutting my driveshaft once :D of course anything about yoke compatibility would be helpful too. Thanks in advance!
Sarsaparilla Jones, '77 C/W
Coming soon: '89 XR4Ti 2.3T, T5, and a big @$$ intercooler

Offline Wittsend

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Re: U-joint to axle centerline on a 6.75" rear end?
« Reply #1 on: July 11, 2020, 03:46:33 PM »
I doubt many have installed a 8.8 when the Mustang II and some Pinto's both with 8" rears pretty much bolt in. FYI, I went from a '73 C-4 Automatic/ 6.75 rear to a T-5/8" rear and the original driveshaft fit perfectly. Years ago I wrote up this article about my Turbo Pinto build (link to part 2 at the end of part 1). https://www.fordpinto.com/general-pinto-talk/so-you-want-to-build-a-turbo-pinto-part-1/msg76893/#msg76893  Others have as well. There are pictured posts of mine as well but you kind of have to hunt for them. I know there are images of the t-5 swap.

Offline Jauntimus Prime

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Re: U-joint to axle centerline on a 6.75" rear end?
« Reply #2 on: July 11, 2020, 06:00:27 PM »
Thanks for the link, Wittsend... lot of good info in there! Crossing my fingers for a *little* less steep of a transition, but I know it never works out that way.

Ah, the 8"... I've definitely barked up that tree. Not ruling that out, but they seem to be much harder to find now than they were a decade ago. I've seen quite a few 8.8s making their way underneath Pintos lately; I find them attractive because they're plentiful and inexpensive, even with limited slip and disc brakes (two features I find very appealing). Sure, I'd have to narrow it and swap over spring pads, but for my trouble I get those features and 5-lug axles to match my Granada rotors up front. (I prob wouldn't even bother thinking about switching from 4-lug, if I didn't strongly suspect I'll want better braking down the line.)

It's worth mentioning that I'm coming from a 4-speed here... I know the T5's a little bit longer (how much exactly I don't remember, I found a great write-up on such a swap that I'll have to dig up... or I could just measure mine once it's out).

Same with the rear end itself... anyone know the best way to get this measurement for my 6.75?
Sarsaparilla Jones, '77 C/W
Coming soon: '89 XR4Ti 2.3T, T5, and a big @$$ intercooler

Offline Wittsend

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Re: U-joint to axle centerline on a 6.75" rear end?
« Reply #3 on: July 11, 2020, 07:26:41 PM »
 Drop a plumb line from the center of the 6.75 hubs on each side to the floor (this will require the wheels off) and draw a line between the two marked points. You can use tight string to form the line or if you have something long and straight use that.  Then you can drop the same plumb from the center of your U-joint to the floor (albeit it somewhat of a guess as to "center" but you can probably get it within 1/32 to 1/16). Measure the yoke point at a right angle to the line and you have your number. The likelihood of someone here measuring a 6.75 (or IF they ever did, remembering it) is pretty nil.

Got you on the 8.8. There are good reasons to use it. Make sure the housing falls properly in line with the driveshaft tunnel. Going from old memory here but I recall something about the Bronco or Explorer 8.8 being close but because of  4x4 the housing is offset??? Maybe someone else can confirm.

Offline Jauntimus Prime

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Re: U-joint to axle centerline on a 6.75" rear end?
« Reply #4 on: July 16, 2020, 07:11:27 PM »
That's great advice, thanks!

And I think you could be right about the truck 8.8... it seems they come in low- and high-pinion versions, where the pinion centerline is either above or below that of the axles, probably to facilitate better driveline angles with the greater ride height of the trucks. There may be left-right offset on different models too, but I can't be sure. I'll be sure to consider that when it's time to narrow the thing.

There was a 2000-ish Saleen Mustang at a nearby wrecker with a 3.27:1 LS 8.8 I could've had for $200 pulled, but I had to pass since I don't have the room to have it sitting around until (probably) next spring when I get around to the swap :/
Sarsaparilla Jones, '77 C/W
Coming soon: '89 XR4Ti 2.3T, T5, and a big @$$ intercooler

Offline Jauntimus Prime

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Re: U-joint to axle centerline on a 6.75" rear end?
« Reply #5 on: July 16, 2020, 07:14:29 PM »
Btw Wittsend I really dig your car! I suppose I'll have questions about it as I move forward with my own wagon. What's your wheel/tire/ride height configuration like?
Sarsaparilla Jones, '77 C/W
Coming soon: '89 XR4Ti 2.3T, T5, and a big @$$ intercooler

Offline Wittsend

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Re: U-joint to axle centerline on a 6.75" rear end?
« Reply #6 on: July 18, 2020, 12:47:00 PM »
Right now I have stock springs. The wheels you see in my profile image are not what is on the car now. Those were 225-60-15" on an 80's 5 spoke wheel. Currently I'm running 205-60-13" on factory Rallye wheels in the rear and 175-70-13" in the front. Trust me the car looks FAR better in pictures than real life.

Offline rob289c

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Re: U-joint to axle centerline on a 6.75" rear end?
« Reply #7 on: July 18, 2020, 07:59:55 PM »
Nice car!  I drove a white '77 wagon from 1983-1988...four years of Navy and 2 years of college.  Very dependable till I got to 144,000...the engine was rattling itself to death.  I replaced it with a blue '76 wagon, then an orange '77 Hatchback.  The white wagon was the best of the three.
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