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So I searched but was unable to find a definitive answer. I have a 1970 cougar xr7 302 and want to upgrade the rear end with a limited slip or traction locking differential. My question is, what size is the rear end in my car? and what is the difference between a 28 spline and a 31, and which do i have? and finally what is the gear ratio as well? Your patience is greatly appreciated. I am a recent convert from import cars!
 

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All 1970 Cougars got 9 inch gears. But it could have been changed. Look at the nuts that hold the center section to the housing. If you can get a deep socket on all of them then it is a 8 inch. If some of the lower ones are shrouded by the case then it is a 9 inch. Your Cougar should have a 351 in it. Only a small number of Eliminators got the Boss 302 in 1970. You might want to add a location to your profile. You might find a good number of other Cougar lovers near by that can help you with things.
Let us know what you find.
 

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The 28 spline are smaller axels then a 31 spline it is the spline teeth they count, From my experience get a limited slip for the street I have a Detroit Locker in Mine will switch some day , If for some reason you have an 8" and are keeping the 302 Leave the rear end in it as well they are fine for street use and a lot lighter as well 70 Cougar Restomod 003.jpg
 

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Thanks guys, it is the 9 inch. Did the 9 come with either 28 or 31, or just the 31? and i found the sticker inside the door with the model information, but its pretty eroded and worn away so is there another way to find out what the original engine was?
 

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the 9 inch rearends came with both 28 and 31 splines. the 31 splines were for mostly the boss 302 and the 428 cj big block cars . More then likely yours is a basic 28 spline 2.75 or a 3.00 geared setup depending if your car has a/c or not.

thanks
pat
 

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Thanks guys, it is the 9 inch. Did the 9 come with either 28 or 31, or just the 31? and i found the sticker inside the door with the model information, but its pretty eroded and worn away so is there another way to find out what the original engine was?
The VIN will have an engine code in it. Yours should be 0F93, the next digit will be a letter. Most likely an H but it could be an M or a Q. H is for a 351 2V which could have been either a Cleveland or a Windsor, no real way of knowing which you had. The M is for a 351 4V which would be a Cleveland. The Q is for a 428CJ.

On the rear there should have been a small metal tag at one of the nuts. This tag will have codes on it that will tell us what it is, including if it is 28 or 31 spline. See if the tag is still there and if it is post the codes on it. Someone here will decode it for you.

To learn more about your car you might want to consider ordering a Marti report. All you need to provide is the VIN and some money. I believe that his website is martiautoworks.com. If I am wrong on the website I'm sure someone will correct me. Also check on the front passenger side of the radiator support for the buck tag, a small metal tag about the size of a normal business card. It will have the VIN and some other codes on it. If that tag is still there post the codes and someone will decode it for you.

Randy Goodling
CCOA #95
 

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I believe 31 spline can be identified by looking at the ends of the axles, if you pull off a rear wheel.

28 spline will have a rectangular indentation in the end / center.
31 spline will have 2 round holes / indentations (see below).

*Edit: here is a nice link that shows them both together, for comparison: http://www.metropartsmarket.com/rearend/measure.html

 
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