8.75 Rear Gear Change Question

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SgtLee511

SgtLee511

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I have a 1972 Plymouth 340 Duster, this is has a untouched factory, 8.75 limited slip. Original 3.23 gearing. I am wanting to upgrade to 3.55 or 3.91, most likely 3.55's. I have no performance enhancement knowledge for this project. How much trouble is it to make this rear end a positraction. What must be done? Or what are my options.
 
SgtLee511 said:
I have a 1972 Plymouth 340 Duster, this is has a untouched factory, 8.75 limited slip. Original 3.23 gearing. I am wanting to upgrade to 3.55 or 3.91, most likely 3.55's. I have no performance enhancement knowledge for this project. How much trouble is it to make this rear end a positraction. What must be done? Or what are my options.
Click to expand...

I'm confused, you say you have a limited slip but want to make it a positraction.

If you have a limited slip you already have it.
 
SgtLee511 said:
I said Im enept at this rear end stuff
Click to expand...

LOL, gotta start somewhere.

If you aren't completely sure of what you have, jack both rear wheels off the ground and spin one tire and watch the other side. If it turns opposite it's an open rear, if it turns the same way it's a suregrip.
 
If you own a Mopar, you have 1) a open rear end or 2) a Sure Grip rear end, end of this part of your question.
Next to change from a 3.23 gear ratio to 3.55 or any other is pretty straight forward.
Now to accomplish the gear change properly so that it holds up to abuse and does not whine, that is a whole different mater.
With no knowledge of doing this, you would be better off to buy a third member from someone like Randy's Rears,Dr. Diff or one of the other reputable vendors.
 
Moser offers a 3rd member starting at $1090 plus upgrades, ready to drop in.
 
Sgt Lee, just so you know limited slip is a term indicating that the differential is a locking type that just slips a little when you turn corners. When you turn a corner the inside wheels travels less distance than the outside wheel so you have to have some sort of device that allows the wheels to turn at different speeds (a limited slip differential). The other type of differential is the open differential that has no locking device to lock the wheels together when your going straight. They generally pull mostly on one wheel. A lot of guys call them a "one wheel peel" because when you do a burn out it only spins one wheel

Positraction is the term Chevrolet called their limited slip unit. Sure grip is the term Mopar called their limited slip unit. It's really just a matter of terms but when said wrong can rile some guys, LOL...

Changing gears isn't for the faint of heart. They have to be set up within .001~.002" of an inch or the wear pattern won't be right and they may howl like a banshee and/or wear out real fast. It takes special tools and know how to install new gears. If you have an open differential (sounds like you do) you'll also need a sure grip (limited slip) unit. You'll want it installed at the same time as the gears. Since you have no experience in setting up gears it'd be in your best interest to buy one already done from a professional. I have seen them sell on the for sale forum for $400-800 depending on the gear ratio, type of sure grip (cone type or clutch type), and the age of parts.

Just so you know, any Mopar 8-3/4 case (741, 742, or 489) will bolt into your rearend housing which is great but there were a few different u-joint sizes used in the different body styles. It's a 99% chance yours is the small 7260 U-joint because that's what was used in nearly every A-body. Make sure you verify that when buying a rearend. Sucks when you buy something and have to modify things to make it work.
 
If you have a 489 case, I have a set of nice 3.91s I'd make a deal on. Their for sale on here. My buddy recently paid someone to swap them out. We took the third member out ourselves (no biggie) and took it to a specialist. I think he charged $300 to swap the gears, put in an installation kit and set everything up.
 
I think my rear end guy charged me $40 the last time he did a gear swap for me! It was a shimmable 742 case, which I run in all my cars! As mentioned above, any case can be used, but I believe you must get a gear set specific to your case number, as the pinion shaft diameter is different for each case! Geof
 
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