8 3/4 ?'s

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Mopardude

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I just scored another A-body 8 3/4 rear end so now both my A-bodies has one and I can get rid of this B-body 8 3/4 I have. For some crazy reason the guy I bought this new rear end from cut the perches off of it. He gave me some new ones to put on it. He told me they need to be welded on at a 5 degree angle. Does this sound about right? I want to check his numbers before I have someone weld them back on. Also the center hole in these perches are a little over 5/8 of an inch, am I wrong or isn't the hole for A-body perches supposed to be like 1/2 inch?

Also this rear end I got came with a 742 case. So now I have one of each kind of case. I know the 741 is considered the weakest, and thats in the B-body housing anyways, so I am just going to sell that as complete. How does the 489 and 742 stack up against each other? From what I understand is the 742 doesn't have a crush sleeve but the 489 has a bigger pinion.

Also from what I understand from other threads I seen posted lately is that my 74 Dart Sport should have a 7 1/4 that has 3" tubes so the shock plates and u-bolts and brake hardware I should be able to reuse from it. I would like to go caltracs but don't want to spend that kind of money right now. Could I get away with reusing the 7 1/4 springs too?
 
Mopardude said:
I just scored another A-body 8 3/4 rear end so now both my A-bodies has one and I can get rid of this B-body 8 3/4 I have. For some crazy reason the guy I bought this new rear end from cut the perches off of it. He gave me some new ones to put on it. He told me they need to be welded on at a 5 degree angle. Does this sound about right? I want to check his numbers before I have someone weld them back on. Also the center hole in these perches are a little over 5/8 of an inch, am I wrong or isn't the hole for A-body perches supposed to be like 1/2 inch?

Also this rear end I got came with a 742 case. So now I have one of each kind of case. I know the 741 is considered the weakest, and thats in the B-body housing anyways, so I am just going to sell that as complete. How does the 489 and 742 stack up against each other? From what I understand is the 742 doesn't have a crush sleeve but the 489 has a bigger pinion.

Also from what I understand from other threads I seen posted lately is that my 74 Dart Sport should have a 7 1/4 that has 3" tubes so the shock plates and u-bolts and brake hardware I should be able to reuse from it. I would like to go caltracs but don't want to spend that kind of money right now. Could I get away with reusing the 7 1/4 springs too?
Click to expand...

Hi Tim,

I may not be ableto answer all of your questions, but I'll try.

Personally I like the 742 casting myself. IMO they are equally as strong. Of course if you use a 489 casting, be sure to eliminate the crush sleeve with a solid spacer. I am not sure on the later 7 1/4 tubes being 3", maybe someone will chime in on that. If not, you'll need the shock plates etc, for the 8.75.

As for your pinion angle, most guys will set the rear end up in the car with the perches loose and unwelded, so you can adjust your angle. Once you are content with your angle, you can either mark the perches, or tack weld them in place then remove the rear and weld the perches on.

As for the hole in the perches, I know the MP ones have a larger hole. I am sure you could machine up a spacer to go in there to take up the slack if you really wanted to.
 
The 7 1/4" should have smaller axle tubes so the plates and bolts won't fit. If it is an 8.25 it will have 3" tubes and the plates and u bolts will work. Also note that you will have to shorten your driveshaft as the 8.75 is longer fron the ujoint to the axle centerline.

You will have to sleeve your new perches to fit the a-body springs

You could measure your other a-body 8.75 diff and be in the ball park for pinion angle. I would not just go 5 degrees, the best way is to install in car with all the weight on it, measure the angle and tack weld them (or carefully mark) then remove it and weld
 
Duster346 said:
Mopardude said:
I just scored another A-body 8 3/4 rear end so now both my A-bodies has one and I can get rid of this B-body 8 3/4 I have. For some crazy reason the guy I bought this new rear end from cut the perches off of it. He gave me some new ones to put on it. He told me they need to be welded on at a 5 degree angle. Does this sound about right? I want to check his numbers before I have someone weld them back on. Also the center hole in these perches are a little over 5/8 of an inch, am I wrong or isn't the hole for A-body perches supposed to be like 1/2 inch?

Also this rear end I got came with a 742 case. So now I have one of each kind of case. I know the 741 is considered the weakest, and thats in the B-body housing anyways, so I am just going to sell that as complete. How does the 489 and 742 stack up against each other? From what I understand is the 742 doesn't have a crush sleeve but the 489 has a bigger pinion.

Also from what I understand from other threads I seen posted lately is that my 74 Dart Sport should have a 7 1/4 that has 3" tubes so the shock plates and u-bolts and brake hardware I should be able to reuse from it. I would like to go caltracs but don't want to spend that kind of money right now. Could I get away with reusing the 7 1/4 springs too?
Click to expand...

Hi Tim,

I may not be ableto answer all of your questions, but I'll try.

Personally I like the 742 casting myself. IMO they are equally as strong. Of course if you use a 489 casting, be sure to eliminate the crush sleeve with a solid spacer. I am not sure on the later 7 1/4 tubes being 3", maybe someone will chime in on that. If not, you'll need the shock plates etc, for the 8.75.

As for your pinion angle, most guys will set the rear end up in the car with the perches loose and unwelded, so you can adjust your angle. Once you are content with your angle, you can either mark the perches, or tack weld them in place then remove the rear and weld the perches on.

As for the hole in the perches, I know the MP ones have a larger hole. I am sure you could machine up a spacer to go in there to take up the slack if you really wanted to.
Click to expand...

Yea I can steal the shock plates off of the B-Body rear i have if I have too. If I take this rear end to a shop that does this sort of thing they should know the right angle to mount the perches and they could probably fab up that hole better than I ever could, right?
 
I just called a local shop that specializes in Mopar's they will rebuild and setup my rear ready to bolt in. I just need to find out that pinion angle. Where could I find a spec sheet or info on this?
 
I can make you a set of pins that fit...i made my own. the only thing is that you'd have to wait til next week, because I'm off til this weekend, and I do all my fabbing at work in my free time......and if it's a busy weekend, it may be later next week....BUT they will probably cost less, AND you get a whole new pin instead of just a sleeve for your old ones....

here's a pic:

Leaf_pins.jpg
 
green67cuda said:
I can make you a set of pins that fit...i made my own. the only thing is that you'd have to wait til next week, because I'm off til this weekend, and I do all my fabbing at work in my free time......and if it's a busy weekend, it may be later next week....BUT they will probably cost less, AND you get a whole new pin instead of just a sleeve for your old ones....

here's a pic:

Leaf_pins.jpg
Click to expand...


Thanks I just may take you up on that offer!



Could someone detail out how to go about mocking up this rear under my car to setup the perches? If it isn't all that bad maybe I will just do it myself. Any have my brother or a buddy come weld it up.
 
best way is to put the perches on the installed leafs, set the axle housing on the perches (centered), put the U-bolts on, but don't torque them down...just snug, install the center section and the axles, no need for the brakes, and install the tires.

set the car on the ground, and you should (maybe need a jack under the pinion yoke) be able to rotate the housing to get the right angle.
then tack the perches in place on the housing (don't weld them solid or you might damage the springs), and remove everything again so you can weld the perches solid

i know it's a lot of work, but it's probably the best way to do it....
 
green67cuda said:
best way is to put the perches on the installed leafs, set the axle housing on the perches (centered), put the U-bolts on, but don't torque them down...just snug, install the center section and the axles, no need for the brakes, and install the tires.

set the car on the ground, and you should (maybe need a jack under the pinion yoke) be able to rotate the housing to get the right angle.
then tack the perches in place on the housing (don't weld them solid or you might damage the springs), and remove everything again so you can weld the perches solid

i know it's a lot of work, but it's probably the best way to do it....
Click to expand...

Ok sounds all good but how do you know if you have the right angle? What do you need to line the pinion up too?
 
you'll need to listen to someone else on this, BUT
here's my take on it....

i'm naturally a ford guy, and what i've ALWAYS heard, read, said, and done is make it "nose down" in relation to the engine/tranny centerline

most engines sit around 3 degrees, angled back. so if you put your angle finder on the tranny yoke, you measure 2 degrees, for instance (the tailshaft of the tranny is lower than the input by 2 degrees)
thn the pinion angle, depending on application, should be (for a street car) 3-ish degrees nose down, relative to the prior measurement...SO...since the engine/tranny is angled back 2 degrees on the angle finder, you'd go 3 degrees forward from the 2 degrees, which would be 1 degree, on the opposite side of zero on the angle finder

if the engine/tranny were at 5 degrees, you'd roll the pinion forward 3 degrees from 5, which would be 2, on the same side of zero.....

if the engine/tranny were angled FORWARD, say, 1 degree, you'd still roll the pinion forward to get the 3 degrees in relatiuon, and it would be at 4 degrees....


NOW....what the Mopar Chassis Manual says is that it needs to be relative to the driveshaft.....but i see a flaw in that because if the driveshaft was angled 5 degrees down from the engine/tranny, and you went 3 degrees from there, you'd be at 8 degrees from the engine/tranny...

and the whole point of pinion angle is to have the pinion at the same angle as the engine/tranny under normal load (in reality, IDEAL is 1 degree difference at normal load to promote u-joint roller turning), to prevent driveline vibration and conflict

that's why the pinion angel for a leaf-sprung drag car is more than for a leaf sprung street car...in drag racing, there is more load on it to rotate the pinion up, so you need more inital angle.....



i THINK i covered everything i was trying to say
another BUT:

:worthles:

pictures would be GRAND to explain myself.......
 
i agree with both, as they are both saying the same thing in different words....

they both say that ideally, the engine/tranny centerline should be parallel to the pinion.....exactly true, and that is at normal load. The info Mark Williams gave stated that in that example, he was referring to something like a 4-link setup....AND
There is a general misunderstanding about "dropping the pinion down" several degrees. This is a practice that should
be applied only to leaf spring cars without any traction control devices where springs can “wrap” and change pinion angle.
Click to expand...

as far as traction devices, the better the traction sontrol, the less pinion angle required.

I guess it's better to say that in a perfect world, the pinion and tranny centerlines would be parallel, with a slight height difference, which would provide a 1-2 degree angle in relation to the driveshaft (like said earlier, this promotes roller motion inside the u-joint)

BUT, it's not a perfect world, and the rear axle height and angle is always changing, so we say "at ride height and under normal load (for the application)"

the 3-5 degrees suggested for pinion angle is an estimation for a leaf sprung street car's normal loaded pinion rotation

ok...went and read again......
they ARE both saying the same thing, but the car-craft article is a bit more detailed in specifying that it depends all on the chassis setup and usage of the vehicle....the MW one is like a paraphrasing of the Car Craft article. not that it is, just like a paraphrase.....

the car craft article also has the opinions of a few people...the opinions are different because of their specialties....

Bickel's recommendations are based on racing (it says he's a pro-stock chassis guy)
What Currie says in the article about 1-3 degrees at the tranny and 1-3 degrees at the pinion is referring to at normal load, but it doesn't say it

again, both are right, and ideally, the engine/tranny and pinion are close to the same height and at the same angle, which puts the driveshaft in a straight line or at a small angle AT NORMAL HEIGHT/LOAD


wow...i keep talking....here's more....
take a truck, for example. let's say, like on my truck, the pinion and tranny aren't REAL far off height-wise.....i'd follow the above guidelines...

however, if i lifted the truck 6", i'm opening another can of worms.....
i had a 78 bronco lifted 6" (in the suspension, anyway).
If i followed the above guidelines, i may have the equal and opposite angles at each end of the driveshaft, but the height difference is so great that the u-joint angle would be larger than 1-3 degrees, which would eat the u-joint in short time....to solve this, we put in a double cardan shaft and rotate the pinion up so that (under load), the pinion and driveshaft are in a straight line, and the double cardan joint at the front handles the angle
 
AH...just thought of another way to say it.....

picture the engine/tranny centerline and the pinion centerline. it is proper to have them parallel under normal load/ride height. ideally, you want enough height difference between the two in this loaded state to make the driveshaft at a 1-2 degree angle in relation to the engine, tranny, and pinion angles (so if the engine/tranny and the pinion happen to indicate zero degrees, then we want enough height difference to make the driveshaft indicate 1-2 degrees, either direction, doesn't matter)
this would be exactly right....
NOW....take the load off the driveline, and what happens? the pinion turns downward. how much? 3-5 degrees is estimated for a street car 5-7 for a drag car...with leafs
1/2 degree estimated for a 4-link, and 1-2 degrees for a ladder bar

and there are your angles to build into it....
so the reason we add in the extra angle when we are setting it up is that we can't set it up under normal load, because that would mean the car was either moving, or on a chassis dyno, and the driveline would be spinning

there are so many ways to put it!!!!!!!!!!!!!!
 
maybe at work this weekend, after i get done with Mopardude's pins, i'll write a tech article on this, edit it, and make pictures for it, to REALLY clear this up.....and include the reasons why there are so many different opinions on the topic....

i think it'd be a LONG article with LOTS of drawings.....

and maybe Adam will want it on the tech archives....
maybe i could sell my article to some mags....LOL
 
Ok I talked to my brother today he fabricates Firetrucks for a living and I guess he has moved axels on trucks before. He said where they work they put all pinion angles at 4 degrees but he doesn't know where that number cam from thats just the way he was taught how to do it. We put our heads together and we are wondering if we measured the angle of the 7 1/4 before we took it out it to use it as a guidline or since the 8 3/4 pumkin is longer would it change the angle? Also I have a 71 duster with an 8 3/4 under it could we measure that to get an idea also? Also I forgot to bring this up with my brother but are we going to need a drive shaft to figure this out? I am going to need a shorter one and I can't really order it till I have the rear in place to measure for it. Also whats the best way to get the car up in the air to work on and yet have the car sitting like it should? Can we put jack stands under the rear end than under the lower control arms on the front?

Green67Cuda will those pieces you fabed up also work with caltracs?
 
when i did mine, I left the front on the floor and put jackstands under the rear framerails....to get your checks and measurements, put the car back on the ground and bounce the suspension to settle it....
it'll be a tight squeeeze, but I was able to get my measurements with the car completely on the ground
(for the pinion measurement, i slid up under from the back of the car, and for the engine/tranny measurement, you can reach in from the side and use the starter motor, the engine oil pan rail, the water pump face, harmonic balancer face, to to get your measurement....

as far as the pins working for caltracs.....i don't know.....if caltracs use a 5/16 threaded section on the pin, they will work....I'll see if I can get hold of them and find out what the center pins are, and i can make them to fit their monoleafs if you prefer.....i'm thinking they're going to be the same, but i'll just have to use a shorter bolt

let me know if you are going to use monos, and which brand if you are, or stock type leafs.....i'll need to know how many leafs if you aren't using monos....

if you are using caltracs with your stock springs, they will work anyway, and I will just need to know the # of leafs you are using
 
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