Converting 28 to 31 spline

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Converting rear axles from 28 to 31 spline. How difficult of a job is this? What parts would I need to do to this conversion? I have to confirm but I'm pretty sure it's a 9". The car is a 73 Mach 1 Q code. I know the gears were changed from the previous owner which are now 3:89's with an Eaton posi. Since I've gone overboard with everything else in this car I figured the 31's would be good piece of mind with the 408 getting ready to be installed. All the brakes and stuff are off already and the gas tank is getting replaced so I figured with everything out of the way...

I don't "plan" to drag race the car but I was still thinking of getting a set of M/T's streets to drive around in just to have some kind of traction. Should at least be better than a set of BFG's. Thoughts?

 
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So I did just what you are asking.

I called Randy's Ring and Pinion. Told them what I had, told them what I was trying to do and they put together the list of parts for me. The side gears need to be changed and the "Hat" on your posi unit. You need new bearings which fit the housing and the larger 31 spline axle, new axle seals, new axles and you'll probably also have to cut the axles to length (Metal blade on a chop saw, cut a chamfer with your grinder- easy peasy) Fresh fluid for the rear end and a fresh gasket are a good idea at this point.

A few suggestions. . . if you haven't upgraded your driveline to 1350 u joints, now is a good time. If you want rear disc brakes, again, now is a good time to do so since you'll be pressing bearings off and on at this time (or having a shop do it for you) And if your ring gear bolts are not ARP I would change them and properly torque and locktite the new ones,

 

Hemikiller

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The job is no more difficult than what you have already done, with the exception of removing the center section. Hardest part of that is the weight, as long as whoever redid it before didn't use copper washers on the studs like Ford did.

To be honest, if you're just hammering around on the street, you'll most likely never get enough traction to hurt a 28 spline rear end, unless it was ready to fail anyway. Of course, you're only an hour or so from ATCO, so anything is possible. The M/T drag radials are a good tire, a couple friends run them and are very happy.

There's a shop not too far from you that has a good reputation. A bunch of guys on the VMF have bought 8" & 9" center sections from him with great feedback.

http://ford8and9inchrears.com

If you wanted to upgrade to 31 spline, then I'd look to Strange for axles and see what the guys above can do for you for gears. I'd bet they would do a partial trade for your existing 9" towards a 31 spline assembly.

https://www.strangeengineering.net/product/ford-mopar-or-chevy-alloy-axles-ret-plts-brgs-1-2-stud-kit.html/

 
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If you are going to punish it you might want to go to 35 spline. 

I think the MT are made by Cooper tire would have to google. Some are not so good for sticking. You might want to look at a drag street tire if you want great traction.

You should install a drive shaft loop for sure. Can do sever damage if you pop a u joint and it goes crazy under there. Can destroy a car in a split second.

 
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So let me say this...I don't intend to drive like an ass. It will not be taken to redline with every gear every time I go out. I understand that I'd have zero traction from the hole but what if I went with a good set of BFG's G force or something? Wouldn't the slip help not break stuff? I know it's not fun spinning and not going anywhere but I'm talking about a street cruiser and not a drag car. Can I get away with the 28's? Yes, I still think 408 is ok for street cruising!

 

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On BFG's the chance of braking an axle is so low as to be nearly nothing.  I could spin mine into 4th before they would hook at all.
Yeah that doesn't sound like much fun at all. Ok, guess I have to look into this upgrade also.
Pretty much any car with 400~500hp will be able to break traction at will with regular street radials. Running a street legal drag radial is generally a lot safer.

 

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So I know there are a lot of guys here making really good power on the street. What is everyone using for tires?
Buddy of mine has a 70 Roadrunner with a 540" engine/ auto that'll turn low 10's at the strip, he runs Mickey Thompson ET Street R tires all the time. Says it's too dangerous on regular radials. 

https://www.summitracing.com/parts/mtt-3559/overview/https://mickeythompsontires.com/drag-tires/et-street-r

If you want something a little more "street" looking, you can do the Nitto 555R or ET Street S/S

https://www.summitracing.com/parts/nit-180300

https://www.summitracing.com/parts/mtt-3453/overview/

 
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Love the look of the nitto's. Only thing is trying to find something that will fill the wells and still be 28" tall. I haven't decided on a rim size yet. I just know it will have to be bigger than a 15" because of the brakes. I don't want to have a super low profile tire either... With the TKO 600 and 3:89's I'm trying to squeeze out as much 1st gear as possible. The 28" tall tire would help a little. Again, I don't intend to drive like a mad man but I do want to have fun with the car.

 
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I fit the M&H 325-45-17's to my wheel wells, but it required removing the interior and cutting and welding to mini tub the wheel well. The worst contact point is up in the front of the wheel well.

It is a day long job but not particularly hard.

 
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I fit the M&H 325-45-17's to my wheel wells, but it required removing the interior and cutting and welding to mini tub the wheel well.  The worst contact point is up in the front of the wheel well.

It is a day long job but not particularly hard.
I'm assuming width was the issue here. How wide can we go on a 10" wide wheel before rub?

 
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