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Discussion Starter #1
Hello everyone!

I'm looking at rebuilding the stock rearend in my 70 cougar. The car currently has 3.0 Open gears with 28-spline axles. I've recently put a new motor in the car thats pushing close to 500 hp/ft-lbs at the flywheel, likely somewhere around 400 at the differential. I have a new set of 3.7 gears to toss in, and was thinking I might want to upgrade to 31 spline axles and a posi of some type while I'm in there. The rub of it is I've never done any rear end work before and I find it a bit daunting. There are different bearing sizes, different carrier bearings, etc and I don't even know where to start to find this information so I have the right parts before I even get off the ground. It is an Eliminator, so I dont really want to modify the housing. Some general questions:

1) At 400 ft-lbs, will the stock 28-splines take the power reliably or should I upgrade to 31 spline axles?
2) Will 31-spline axles fit where 28-spline axles do currently or do they require a larger bearing?
3) what am I not smart enough to consider that I should be when doing a rear end rebuild?

Thanks!
 

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Contributing Sr Motorhead
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5,470 Posts
I'm running 28-spline axles behind my 480hp 393W - in the 8.8"rear of my '89 Crown Vic! With an automatic you should be good, but if you had a stick, I'd strongly recommend 31s or even 33s if you have a stick and run slicks at the track!
 

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31 vs 28: I would stick with 28 until they broke (see sig)...
But to do it you will need:
The axles (factory or Moser)
The differential carrier for 31 splines
That carrier will need to be the kind that accepts the bearings to fit where the 28 spline diff bearings 2.8x" were or you'll have to get a carrier for the 3.025" bearing. It's cheaper of course to match the diff to the case.
Doing the rear end just requires mechanical knowledge and the ability to follow instructions. Nothing is difficult technically about doing the conversion.

I would just stick a 28 spl posi with the 3.70s in there and go have fun. If you're going to go racing and know you're going to leave 5000 RPM you *need* to upgrade.

Bill
 

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Contributing Sr Motorhead
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Long long ago, before my buddy Terry owned it, his '69 Boss 429 was being actively raced at the Midwest dragstrips. Not stock by any means. Welded in roll bar, welded on ladder bars,... The original owner was snapping the 31-spline axles. They'd let go withought any warning. He found that 28's would twist long before breaking. So he painted a line down the length of the 28-spline axle shaft, and when they'd take a twist of more than half a turn, he'd replace them.

Anyway, I'd follow dfwcatsclub's advice and use what you've got unless you're racing or habitually dumping the clutch at more than 4000rpm.
 

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Ford 8 and 9 inch rear axles are the easiest, you can assemble the the pinion on the bench, assemble the carrier and install, set backlash, use permatex prussian blue and check tooth contact pattern, with a good chart it will tell you to add or remove shims from the pinion. I've done a few of these with no special tools they ran great and quite. I'm sure the 28 spline will be fine, my sons running about 400 horse with a 28 spline 8" with no problems yet.
 
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