74 Duster - I want new gears! 3.91

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Richard McLaughlin

Richard McLaughlin

1974 Duster 340
Joined
Oct 10, 2016
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Location
New Brunswick
Hey guys,

I think I will "upgrade" my gears this summer.

I have a 74 Duster with a 340. I was thinking 3.91 (was recommended from rumblefish).

What do I need to buy to do the gear swap? The whole center component? Or just the gears?

Thanks!
Richard
 
image.jpeg
Would I just get a kit like this and put it in the current housing?
 
Looks like I have a ford 9 bolt rear end.
However my gears seem bad!
Turned tire once and the drive shaft didn't even rotate 2 full turns! Took turning tire about 1 and 1/4 times for the drive shaft to turn twice.
 
Richard McLaughlin said:
Looks like I have a ford 9 bolt rear end.
However my gears seem bad!
Turned tire once and the drive shaft didn't even rotate 2 full turns! Took turning tire about 1 and 1/4 times for the drive shaft to turn twice.
Click to expand...

Take a picture and post it. You probably have a 8 3/4 diff.
 
If yours is already a sure grip you just need the ring and pinion gear set. If this is the case you have a 489 case when asked.

If yours is not a sure grip and you want a sure grip you need to find a whole center section. New is up to $1400. (Doctor diff), used probably in the 600-900 range depending on condition.
 
4mulas said:
If yours is already a sure grip you just need the ring and pinion gear set. If this is the case you have a 489 case when asked.

If yours is not a sure grip and you want a sure grip you need to find a whole center section. New is up to $1400. (Doctor diff), used probably in the 600-900 range depending on condition.
Click to expand...
Ok. What is a sure grip? And what is the advantage ?
 
Sure Grip is the Mopar name for a LSD (Limited Slip Differential). Also known as a "Posi" by the great unwashed.

Simply stated, limited slip diffs apply drive to both rear wheels. This is advantageous in that you get better grip and make pretty parallel stripes on the tarmac under heavy acceleration :). The Sure Grip diff was normally an extra cost option back in the day. The 8 3/4 was available as a Sure Grip or a "single spinner".

To determine what you have, jack up the rear end, put the car in neutral and spin a rear wheel by hand. If both rear wheels turn in the same direction, you have a Sure Grip. If the two wheels turn in opposite directions, you don't.
 
4mulas said:
If yours is already a sure grip you just need the ring and pinion gear set. If this is the case you have a 489 case when asked.

If yours is not a sure grip and you want a sure grip you need to find a whole center section. New is up to $1400. (Doctor diff), used probably in the 600-900 range depending on condition.
Click to expand...

Ummmmmmmm

No, he can just buy the sure grip differential and ring and pinion. Housing is pretty much the same, he just has to make sure he gets the right carrier bearings.

Mopar 8 3/4" (8.75) Clutch Type Sure-Grip ("Powr-lok")

Mopar 8 3/4" (8.75) bearing rebuild kit

Mopar 8 3/4" (8.75) Ring and Pinion for 741, 742, or 489 case

It's also worth noting that '74 Duster's didn't come with 8 3/4 rear axles, so, you may want to do a little more research to find out what exactly it is that you have there. Based on your other thread, you have an open diff right now so if you want to drive both wheels at the same time you'd need to add the sure grip. And, before you buy new rims as per a few of your other threads as well, you'll want to measure that rear axle. Because it may not be an A-body width 8 3/4.

As far as gear choice, 3.91's will be a lot of fun off the line, and absolutely miserable on the freeway for more than 20 minutes. Not sure what your planned use for the car is, but if it includes driving at 65mph or more for an hour or more at a time you may want to reconsider 3.91's unless you're installing an overdrive as well.
 
72bluNblu said:
Ummmmmmmm

No, he can just buy the sure grip differential and ring and pinion. Housing is pretty much the same, he just has to make sure he gets the right carrier bearings.

Mopar 8 3/4" (8.75) Clutch Type Sure-Grip ("Powr-lok")

Mopar 8 3/4" (8.75) bearing rebuild kit

Mopar 8 3/4" (8.75) Ring and Pinion for 741, 742, or 489 case

It's also worth noting that '74 Duster's didn't come with 8 3/4 rear axles, so, you may want to do a little more research to find out what exactly it is that you have there. Based on your other thread, you have an open diff right now so if you want to drive both wheels at the same time you'd need to add the sure grip. And, before you buy new rims as per a few of your other threads as well, you'll want to measure that rear axle. Because it may not be an A-body width 8 3/4.

As far as gear choice, 3.91's will be a lot of fun off the line, and absolutely miserable on the freeway for more than 20 minutes. Not sure what your planned use for the car is, but if it includes driving at 65mph or more for an hour or more at a time you may want to reconsider 3.91's unless you're installing an overdrive as well.
Click to expand...

You're right, but considering the level of questions that have been coming I thought it would be much easier for him to just replace the whole third member with something like this: Mopar 8 3/4" (8.75) 489 Case Third Member or a used unit hence the price range variance which I gave.

Blunblu is right. In that you should determine if it is an A body or maybe a retrofitted B body diff or their in there to avoid a "why my wheels I wanted to use don't fit" post.
 
No worries, I just didn't want anyone to think that the cases were somehow different.

Finding another 3rd member in good used condition is usually the cheapest/easiest way to go, I have yet to rebuild a 3rd member for any of my cars and a few of them run 3rd members I picked up at pickNpull. That will probably change when I finally get around to installing the T56 in my Duster as I want to go to 3.91's and a True-trac limited slip. But other than that one everything I run is factory as far as I know.
 
4mulas said:
You're right, but considering the level of questions that have been coming I thought it would be much easier for him to just replace the whole third member with something like this: Mopar 8 3/4" (8.75) 489 Case Third Member or a used unit hence the price range variance which I gave.

Blunblu is right. In that you should determine if it is an A body or maybe a retrofitted B body diff or their in there to avoid a "why my wheels I wanted to use don't fit" post.
Click to expand...
Ok ok. How do I determine this? The bolt pattern is 5x4.5 if that helps?
 
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