Gear swaps between cases

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Penstarpurist

Penstarpurist

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Hopefully this is a quick question with an easy answer. I talked to a couple drivetrain shops including six states distributors and no one could give me a for sure answer. I have a 489 case center section with a 3.23 sure grip but want to change the gears to something a little steeper. So I also have a 741 open center section with 3.55’s. Can I pull the 3.55’s ring gear out of the 741 and put it in the 489 center section to replace the 3.23’s? Also to add to the dilemma if it is, the 489 is a big pinion and the 741 is a small pinion. Thanks in advance for the information.
 
I'm sure the experts will chime it, but from my personal experience, I'm going to say no, you cannot. When I was ring and pinion shopping, most of the sets specifically call out which case they are for, and I believe there were different part numbers for 3.55's for a 489 case and 741 case, etc.
 
Absolutely not. Bearing diameters are way different and won’t interchange. The carriers can interchange with a bearing swap. But not the gears.
 
Penstarpurist said:
Hopefully this is a quick question with an easy answer. I talked to a couple drivetrain shops including six states distributors and no one could give me a for sure answer. I have a 489 case center section with a 3.23 sure grip but want to change the gears to something a little steeper. So I also have a 741 open center section with 3.55’s. Can I pull the 3.55’s ring gear out of the 741 and put it in the 489 center section to replace the 3.23’s? Also to add to the dilemma if it is, the 489 is a big pinion and the 741 is a small pinion. Thanks in advance for the information.
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Ok , why not swap the SG out of the 489 third member and install it in the 741 3.55 third member?
 
SGBARRACUDA said:
Ok , why not swap the SG out of the 489 third member and install it in the 741 3.55 third member?
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When we were building the drivetrain for the car the driveline was way off lengthwise as we went from a 904 transmission to a built 727 and the rearend was a 7.25 so when my son had the driveline made for the car he initially was going to use the big pinion 489 with the 3.23 sure grip. So he had the driveline built with a big pinion yoke. I’m sure we can just use a conversion u joint to make it work with the small pinion. I think he was just hoping to use the 489 case. Thanks for the information.
 
Pinion size has nothing to do with which yoke size you have. Measure both yokes. They may be the same u- joint.
 
SGBARRACUDA said:
Pinion size has nothing to do with which yoke size you have. Measure both yokes. They may be the same u- joint.
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That would be a really good thing, I wasn’t sure if they would possibly be the same as when six states built the driveline for us they asked if we were running a big pinion or small pinion so I just figured it would make a difference. I’ll take some measurements and let you know what it looks like. If it’s all doable then the answer will be swapping the sure grip into the 741 to get the 3.55’s. Of course I did see that dr diff has sure grip units in stock. Maybe the better answer is to get one of those and have a couple sets of sure grips.
 
What they were really asking is if you had a 7290 big u-joint yoke or a 7260 small u-joint yoke. They are pretty obvious when you look at the two.
 
Penstarpurist said:
That would be a really good thing, I wasn’t sure if they would possibly be the same as when six states built the driveline for us they asked if we were running a big pinion or small pinion so I just figured it would make a difference. I’ll take some measurements and let you know what it looks like. If it’s all doable then the answer will be swapping the sure grip into the 741 to get the 3.55’s. Of course I did see that dr diff has sure grip units in stock. Maybe the better answer is to get one of those and have a couple sets of sure grips.
Click to expand...

To add to the info that Roy posted above, here's a pretty good guide to the 8 .75 differentials that Ma Mopar has used over the years.

Guide to the Chrysler 8-3/4" rear axle - www.Mopar.nl
 
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