Converting my 489 3.23 open diff. to a sure-grip 3.91 diff.

by **70MagnumChally** » 19 Jan 2006 0:01

Hi, I've been looking around and rear ends just confuse the heck out of me. I don't know exactly how the interior of an open differential compared to a sure-grip differs. I know right now I have an 8 3/4 489 case w/ 3.23 open gears and I would really like to get it to be limited slip w/ 3.91 gears (I believe thats a correct gear size for mopars). I'm pretty new to this as this is my first mopar, so I was wondering what I would need to buy to reuse my casing and make it a sure-grip 3.91 gear unit. Thanks. (Also, I'm on a budget so I wouldn't want to be buying an entire new differential or anything if I don't need to, I'd want to be buying the minimal equipment to get the job done.) Anyone's advice or two cents would be much appreciated, thanks

by **dave-r** » 19 Jan 2006 8:34

As well as the ring and pinion you need the carrier that the ring gear is bolted too. The ring carrier is the bit that is either open or limited slip.

I cannot give you a detailed description here but you will need the workshop manual and a gasket/bearing kit at least. As well as a new crush sleeve and pinion head shims.

It is not a simple job to get right if you have never done this before. In particular you need to be able to measure very very carefully to within a few thou of an inch.

The parts do not just bolt in. They have to be adjusted and shimmed into the exact positions relitive to each other or the teeth on the r and p will not meet exactly right and the diff will be noisy at best and broken at worst.

The pro's use a special tool to measure pinion depth. I use another method of measuring that is just as accurate but not as easy to do.

It is a bit complicated to put into words though.

If you really want to get into this i will help you as much as I can but it is difficult via email and forums.

Start by reading the workshop manual.

by **fbernard** » 19 Jan 2006 9:24

70MagnumChally wrote:Hi, I've been looking around and rear ends just confuse the heck out of me. I don't know exactly how the interior of an open differential compared to a sure-grip differs. I know right now I have an 8 3/4 489 case w/ 3.23 open gears and I would really like to get it to be limited slip w/ 3.91 gears (I believe thats a correct gear size for mopars). I'm pretty new to this as this is my first mopar, so I was wondering what I would need to buy to reuse my casing and make it a sure-grip 3.91 gear unit. Thanks. (Also, I'm on a budget so I wouldn't want to be buying an entire new differential or anything if I don't need to, I'd want to be buying the minimal equipment to get the job done.) Anyone's advice or two cents would be much appreciated, thanks

Since you're in the US, I suggest you ask this question on the moparts.org forum. You'll likely get an answer from a guy named 'DrDiff', who will gladly help and will most likely be able to provide what you need. Not to mention he's cheaper than most retail outfits and only does rear-ends...

by **70MagnumChally** » 20 Jan 2006 9:34

thank you both for your support and ideas. I will talk to DrDiff and see what he will charge, and thanks dave. It'll be a little while until I get to the rearend, I'm still working on my engine right now so I'll let you know when I could use the help

thanks.

Re: Converting my 489 3.23 open diff. to a sure-grip 3.91 di