Gsoleski

So I purchased a complete ford 9" center section, 31 spline posi, 4.11 gears, new bearings, bolt in ready basically. Turns out this 31 spline unit is actually 28. I know I know my fault for not counting like I should have. What i'm wondering is if I can just take out the 28 spline unit and put in a 31? If so what would be needed? Are the gear sets the same? I'm more confused on the bearings sizes than anything. The case number is C7AW-G. I can get more numbers or pictures to anyone that can help out.

big gear head

You can change it out. Check the backlash before you do anything and do a pattern check. Take pictures of the pattern and write down the backlash number to use later. Get a 31 spline differential and either put new bearings on it or remove the bearings from the other differential and press them on the new one. Remove the bearing caps and lay them aside so that you know which side they came off of. Remove the differential and then remove the ring gear. Install the ring gear on the new differential with red Locktite on the bolts. Oil the bearings and put it back in the case. Be sure that when you put the spanners back in you do not cross thread them. They must go in just right or they will be cross threaded. Put the caps on carefully so that the threads line up with the spanners. Torque the cap bolts to 60 foot pounds. Use a spanner wrench to tighten the spanners and set the backlash to the number that you recorded earlier. Get the bearings tight and then do a pattern check. It should look the same as it did before. Put the lock tabs back on with blue Locktite on the bolts. Now you are ready to go.

cosmick

In the beginning, 28-spline meant 2.89" carrier bearings, while 31-spline got 3.06", but these days there are bearings, and probably carriers, ( or maybe I got that backward ) that let you put 31-splines in 2.89" housings.

bigal55

iTrader: (1)

Now all you need is a set of 31 spline axels that match the housing length!
more $$$$$$$$

big gear head

All of the 28 and 31 spline differentials take the same size bearings (ID). There are cases that use the 2.89 and 3.06 (OD) bearings (most C7AW cases use 2.89), but the differential bearing journals are the same size.

AlkyIROC

As for the posi, one size fits all. Buy a 31 spline posi. You can install 2.50 or 7.00 gears on it. There's no series like i the GM diffs.

Although we use posi as a generic term, the name posi is actually a GM product.

C7AW-G is common. There's probably a million of them out there. I ran 31 spline axles in a spool in one for many years. I managed to pull the center out to fix a leak and found the pinion support cracked. It was only a matter of time before it spit the pinion out the side of the case. I can't remember but I think I was in the low to mid 9's at the time. I've progressed past the OEM stuff. Now run a bolt through Moser aluminum center. Aluminum spool, aluminum Daytona pinion bearing housing. I can lift the whole center section into the diff with one hand.

I painted a line down the length of the axles. Every time I pull the axles, I check the line and the splines. Even though I'm putting a lot of power through 31 spline axles, nothing has twisted yet. I could easily upgrade to 33 spline but to go to 35+ splines, I would need to buy a lot of new parts because I would need a case, spool, etc for the bigger bearings. 31 is all I need so I use them.

cosmick

Oh, thanks! I happily stand corrected! You da man!