Rear ends - explaination

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bthieson

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I'm very interested in swapping out my rear end, but I virtually have no knowledge of how they work or the numbers people are using when referring to the rears. I've been reading and I understand the 8 3/4 rears are supposed to be quite good for all applications. I currently have a 72 Duster with a slant six.

What does the 8 3/4 represent in terms of measurement? When people talk about gear numbers what is it they are referring to. I know that some gears are better for different applications, but in which situation is which better? Like I said, I have absolutely no idea about the rear end at all, and would really appreciate it if someone could run me through a crash course(Not literally).

Thanks,

-Ben
 
8 3/4" is the diameter of the ring gear.

The number of a gear set is the ratio of the amount of revolutions of the pinion(driveline) to the number of revolutions of the ring gear (axles)

Example; 3.55 gears, 3.55 revolution of the pinion to 1 revolution of the axles.

It is all a matter of leverage. The higher the number the more leverage the motor has on the wheels but at the expense of movement. The motor has to move more (rpm) to reach the same distance.

If you have a pry bar that is 24" long and you try to move something using the middle of the bar as a fulcrum point you have 1:1 leverage with 12" being on each side. Now if you add 24" more in the length that you are pushing on you have 3:1 being you are working with 36" and the load is still 12". It will be easier to move but you will have to move your end farther to make the load lift the same distance.

Chuck
 
Here's a quick tutorial.

8 3/4 refers to the size of the ring gear. Bigger is stronger, but takes more power to turn due to inertia and frictional losses. You won't need a 8 3/4 rear with your slant 6.

Gear ratios are stated like the following: 2.73, 3.23, 3.55, 3.73 etc. The ratio is how many turns the driveshaft must make to turn the axels once. Higher (numerical) gears will increase acceleration and pulling power but limit your top end speed.

Lower (numerical) gears will in most cases increase your highway gas mileage and theoretical top speed.

Hope this helps.
 
Awesome, you guys both just made my life so much less stressful. In regards to using the 8 3/4 with my slant six, I plan on putting in a crate 360 next summer, I just want a head start on everything else. So if you want a street and strip car, I would imagine that you would probably want a happy medium for gears then. What would be recommended? Say if you were in the 400-450hp range.

If I was to do a swap out, what kinds of things would be involved in order to get the brakes working with the 5/4.5 bolt pattern. Could I just rip out an entire 8 3/4 setup and throw it right into my Duster?
 
You have to get one from the type of car you have. ie: a-body OR you can get one from a b,c,e body and have it narrowed to fit your a-body Duster. If you get one get the brakes (all of the brakes) with it as it will save some hassels later.
As far as a street/strip car and gear selection, a set of 3:91's would work good with 4:10's being the very biggest gears if you plan to take it on the highway much. Even then the 4:10's are a little much.

Jack
 
That's wicked. Thanks a ton for the information. Is there anything else I should know? In regards to swapping out the rear to the 8 3/4, will clearance with everything else be the same? How about the existing brake setup. Can I just hook it up to my existing Master Cylinder?
 
If you get one out of an a-body it is a straight bolt in. The only other thing you would have to do is change or shorten your driveshaft and make sure of the universal size. Oh yeah, the e-brake cables may be different.

This is assuming you got the shock plates with the rear. I would just get new U-bolts.

Jack
 
Okay, I think I'm just about done with the questions. Just a couple more. If I can't find a used 8 3/4 and wanted to buy the whole rear setup brand new brakes and all, what would you recommend? Do A body 8 3/4 come up for sale on a fairly regular basis, or are they very rare?
 
keep looking until you find a sure grip rear end. you will save $$ if you dont have to change it later on. look in earlier 1/2 ton pickups. mine came out of a 67 1/2 ton. 3.91 with the sure grip. theres also 2 kinds of sure grips and you want the rebuildable one.
 
sweatybetty said:
keep looking until you find a sure grip rear end. you will save $$ if you dont have to change it later on. look in earlier 1/2 ton pickups. mine came out of a 67 1/2 ton. 3.91 with the sure grip. theres also 2 kinds of sure grips and you want the rebuildable one.
Click to expand...
How can you differentiate a sure grip from a non sure grip. What difference is it? Advantages? How can I identify a sure grip?
 
Okay, so I just found a guy locally who has an E-body 8 3/4 rear. What would need to be done in order for it to fit properly? Could I use the drums that are on there?
 
Yes, you could use all of the brake parts. You will have to buy the e-brake cables for an a-body. The e-body rear will have to be shortened to a-body specs and the spring perch's welded on again in the a-body position. I would reccomend new axels that are a-body size but with e-body flange ends to work with the brake parts that came on the rear. I used Yukon axels but there are others. Moser and Strange come to mind. Keep asking questions and read all you can on these rears. It is not rocket science.

Jack
 
When you drop the word weld, I stop right there. I have absolutely no welding experience nor do I know anyone who can weld. There must be an easier way.

Thanks for the info though.
 
Nope, the perch's have to be welded. Whoever cuts the housing down should be able to weld the perches back on for you.

Jack
 
bthieson said:
When you drop the word weld, I stop right there. I have absolutely no welding experience nor do I know anyone who can weld. There must be an easier way.

Thanks for the info though.
Click to expand...

Buy the E-body rear, buy an A-body housing (there's a guy who sells them NEW on Ebay for $300), and swap the center section and brakes over. Sell the empty E-body housing, and use the funds to buy the correct axles. No welding involved.

VM
 
Bthieson, If you still have your 6cyld motor in your car and are not in immediate need of an 8 3/4 unit, my opinion would be to keep looking around for an A-body unit with suregrip and gears in the range that you are searching for(3.55,3.73,3.91,etc). This way when you purchase the unit, all you need to do is bolt it in and connect up your brakes, etc.. The perches will be located in the proper place, the axle lenghts and dimensions will be correct and you'll be good to go. Keep in mind that switching from a 7 1/4 to an 8 3/4 will require different lenght driveshaft, different schockplates and U-bolts. All are available(new,wrecking yards,ebay,A-body forum,etc.) Below is a link on 8 3/4 rears with information you need to know before purchasing. It will give you dimensions and choices for your needs. Check it out. It will help make things clearer for you. Mike

http://www.moparts.com/Tech/Archive/axle/21.html
 
ValiantMike said:
Bthieson, If you still have your 6cyld motor in your car and are not in immediate need of an 8 3/4 unit, my opinion would be to keep looking around for an A-body unit with suregrip and gears in the range that you are searching for(3.55,3.73,3.91,etc). This way when you purchase the unit, all you need to do is bolt it in and connect up your brakes, etc.. The perches will be located in the proper place, the axle lenghts and dimensions will be correct and you'll be good to go. Keep in mind that switching from a 7 1/4 to an 8 3/4 will require different lenght driveshaft, different schockplates and U-bolts. All are available(new,wrecking yards,ebay,A-body forum,etc.) Below is a link on 8 3/4 rears with information you need to know before purchasing. It will give you dimensions and choices for your needs. Check it out. It will help make things clearer for you. Mike

http://www.moparts.com/Tech/Archive/axle/21.html
Click to expand...
Thanks Mike. I guess I am getting a little ahead of myself. I just want to try and have it done before the seasons over. Would you recommend that I swap out my leaf springs while I'm at it? I know this is a whole other can of worms, but are those spring relocation kits really worth it?
(I know this forum is about rear ends, but I'd like to get things done at the same time)
 
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