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Offline popbumper

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Rear end and axle swap questions
« on: February 17, 2015, 02:12:32 PM »
OK guys, here's my scenario...

My wagon has a 6.75" rear end.
I have an 8" rear end from a '78 sedan.
To make the restore easier, I figured I'd go ahead and restore the 8" unit while off the car, then slip it under with new springs when it's changeout time (rather than jack the car up for a while).

QUESTIONS:
1) I KNOW the gear ratios are different. WHAT kind of place could change out the gears for me in the rear end to make them closer to what the 6.75" has (I DON'T want to change ratios, it's a stock 2.3).
2) Does this involve changing JUST gears, or axles, or both?
3) I guess this would be the time to go to five lug axles, if I chose to. Thoughts?
4) Or should I just find a disc brake rear end (seems like the most difficult choice)

Thanks guys, I just DON'T know this mechanical stuff that well. But throw me bodywork, and I'm ready to roll :).

Chris
Restoring a 1976 MPG wagon - purchased 6/08

Offline dave1987

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Re: Rear end and axle swap questions
« Reply #1 on: February 17, 2015, 04:03:39 PM »
1) Not sure what your original gear ratio is in the 675, but I had 2.73 in mine. I swapped out my 675 for an 8 with 3.11 gears and the stock 4 speed was good for nothing but city driving. If you do any highway driving, you will want a higher geared axle like the 2.73 or swap out the transmission for a T9 from a merkur.

2) Gear change would be the pinion gear and the ring gear.  Pinion gear mates with the ring gear in the axle, your flange for the driveshaft bolts to the pinion gear. The ring gear is bolted to the carrier, and your axles slide into the carrier unit.

3) If you are wanting to do fancier rims, or something other than the stock 13", yes, now would be a good time for 5 lug. Even if you are planning to do a turbo swap or V8, 5 lug would be good to do now.

4) If you are doing any type of performance upgrades in the future, or higher HP motor swaps in general, disc brakes are very nice to have, but I wouldn't say mandatory. I will be doing 5 lug conversion all around and disc brakes at the rear when I do my V8 swap.
1978 Ford Pinto Sedan - Family owned since new

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Offline popbumper

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Re: Rear end and axle swap questions
« Reply #2 on: February 17, 2015, 04:07:51 PM »
Not sure what my 6.75" ratio is, it's an MPG model, I just don't know how to tell easily. Neither do I know my 8" rear ratio, maybe it's on a tag on the housing or something such. Again, not too keen on this stuff.

Chris
Restoring a 1976 MPG wagon - purchased 6/08

Offline dick1172762

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Re: Rear end and axle swap questions
« Reply #3 on: February 17, 2015, 04:16:16 PM »
Count how many times the yoke turns over for one turn of the brake drum. 3 turns to one would be 3:00 to one
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Offline popbumper

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Re: Rear end and axle swap questions
« Reply #4 on: February 17, 2015, 04:18:42 PM »
Standard shift - can this be accomplished with the transmission in neutral?
Restoring a 1976 MPG wagon - purchased 6/08

Offline dave1987

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Re: Rear end and axle swap questions
« Reply #5 on: February 17, 2015, 10:37:32 PM »
Yeah, it can. I usually do a chalk line (vertical) on the inside tire, and then another line on the yoke (horizonal) and spin the tire by hand until the yoke line comes back to where it started at, while counting the revolutions of the wheel/tire. You can get pretty close to the actual gear ratio that way too. Then just look up rear axle ratio options for that axle and you can tell which you have. The mark on the tire usually doesn't end up exactly where it started at because of the fraction about of turns on most ratios, unless you have a 3.00:1 like dick stated, then the tire would rotate exactly three turns to one turn of the yoke.
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Remembering Jeff Fitcher with every drive in my 78 Sedan.

I am a Pinto Surgeon. Fixing problems and giving Pintos a chance to live again is more than a hobby, it's a passion!

Offline Jauntimus Prime

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Re: Rear end and axle swap questions
« Reply #6 on: March 01, 2015, 01:42:57 PM »
Does your 6.75 have a tag on it? If it does, there'll be two lines of letters and numbers. The second line begins with three digits with a space where the decimal would go (2 73; 3 18 etc.). That's your ratio (granted it's the factory diff).

Barring that, there's a single digit axle code on your door VIN sticker. If you google Ford Pinto axle codes, you should be able to find it.
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Offline dick1172762

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Re: Rear end and axle swap questions
« Reply #7 on: March 01, 2015, 03:05:17 PM »
Yeah, it can. I usually do a chalk line (vertical) on the inside tire, and then another line on the yoke (horizonal) and spin the tire by hand until the yoke line comes back to where it started at, while counting the revolutions of the wheel/tire. You can get pretty close to the actual gear ratio that way too. Then just look up rear axle ratio options for that axle and you can tell which you have. The mark on the tire usually doesn't end up exactly where it started at because of the fraction about of turns on most ratios, unless you have a 3.00:1 like dick stated, then the tire would rotate exactly three turns to one turn of the yoke.
   You've got it backwards! 3.00 gear would be driveshaft 3 turns, tire 1 turn.
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Offline Wittsend

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Re: Rear end and axle swap questions
« Reply #8 on: March 01, 2015, 09:33:17 PM »
3.00 should be pretty easy. Telling a 3.40 from a 3.55 can be a bit more difficult. Unless it is a posi style rear you need to keep one of the wheels from turning. I'd use a definitive pointer and markings. Take all the slop out of the rotation before you begin. I'd also have someone slowly (but continuously) turn the tire and drag my hand on the driveshaft - again to eliminate slop. The difference between the 3.40 and the 3.55 is only about a 16th of a rotation.