Rear End Swap 7-1/4" to 8-1/4"

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goldduster360

goldduster360

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I got my rear end job partially completed this weekend. I replaced my 7-1/4" 2:76 geared unit with a home built 8-1/4" stuffed with a track-lok limited slip and motive 3:55 ring and pinion. Note the vice grips in the upper left corner of shot holding the brake hose closed. I simply cut it with a razor blade and clamped it with vise grips to facilitate removal. The brake line nut was later heated with a torch then hit with oil which sucked right into the flare fitting/threads preventing the tube from kinking and twisting when turned with the wrench. This technique saved all of the brake lines on the 35 year old doner housing for the 8-1/4 during brake hose and wheel cyl. removal. I also chased the ubolt threads with a tap, torched the nuts and oiled them, made removal with an air wrench possible without breaking them or having to cut them. Having a torch and an air wrench is almost mandatory when working on anything going on 36yrs old.

Here is the little 7-1/4 ready to come out:

baby rear1.JPG
 
Keep us posted, as I'm about to do a transplant myself.

Problems, busted hardware or knuckels etc.

I just wish I was making the upgrade, not enough frequent driver miles yet!:toothy10:

Lcjeff
 
Here is a nice shot of the adapter plate for my 4.5" patterned Rally's. They were installed ten years ago when I went to front disk brakes. Those are summit stainless steel glass pack mufflers in the shot, sounds great with no resonance:

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Here is the two for comparison, both have the A body only 43" spring perch spacing. Note how the little rear is actually wider than the big one. The baby rear measures out at approximately 57.5" from drum face to drum face. I can't remember what the width on the 8-1/4" was but I think it was about 1-1/2" narrower.
That vice grip is a real ham isn't it?:)

reartorear2.JPG
 
I decided to go with the super stock springs, -02 and -03 variants, they are I think the 3,000-3,200 lb car springs. They sat on my garage floor for ten years along with the donor rear waiting to be installed. Note how much more arch the right hand side(-02) has compared to the standard issue 4 leaf spring. The -03 has about the same arch(unloaded) as the stocker as I will show in pic#2. This car sits level with these springs installed. Left and right sides have different combinitions and placement of the leaves. The original springs were not sagging noticeably, they were just too soft to plant the power.

sstock02.JPG


sstock03.JPG
 
Here, the new rear is waiting to be suspended by the SS springs. I simply dragged the rear underneath the car then lifted each end with a jack and supported it on jackstands. I used a jack to hold the nose up. Once the springs were in place I moved the rear end into alignment with everything and then lowered it onto the springs. It's not as scary moving it around when the springs are underneath it because it does not have far to fall. I also allowed the pinion to point down to floor which makes it easy to eyeball if the spring perches are lined up with the front spring hanger mounting locations on the frame of the car. I was within an 1/8" when it came time to clamp everything together. Take a look at the front spring hangar, this is the part people talk about flipping upside down to lower the car. This requires elongating some holes as the stud width is different from top to bottom of hangar so it would only mount in one way at the factory when it was new. Mopar performance sells new hangars with an extra leaf spring bolt hole so you can lower the car without modifying the attaching points on the frame. They are about $65.00/pair. Elongating holes is free though:)

newrearwaiting.JPG
 
I used all new hardware on the brakes, the brake bar(parking brake strut) is a dorman/motormite unit, one of mine was missing, tough part to find these days if you want a new one, I scored this one at advance auto. Napa did not even catalog it. The wheel cylinders were a rockauto score along with the hardware. The wheel cylinders were on clearance for $3.98 each, one has a 5/16" brake bleeder fitting, the other was 7mm, both used the same brake line threads luckily, go figure.

brakes.JPG
 
The AC/Delco shackle bushings, another rockauto deal look heftier than the originals and up to the task, ahh yes that great GM feeling. They come 2/box in spite of the each price shown on rockauto, I found out the hard way so I have a life time supply of them now. They were cheap too, like $2.00/box. I brushed off all the scale and painted all the shackles and perches with dupli color satin black lacquer.

shacklebush.JPG
 
I screwed up sort of on the u-bolts because I bought 7-1/2" ones which were a tad long. I ended up with Superior brand ones at 6" long and sadly the name did not match the quality as they were splayed out much wider than 3-1/8", could not get them to compress with my hands to go through the mounting holes on the shock plates. Had to use my monster sized Harbor Freight vise. The ones which were too long were dorman's and I have to say the quality was there as compared to the Superiors, which had this weird bend in them. The last pic shows the comparison to the dorman unit, the Dorman has a nice round arch compared the Sup.unit, this kink was there before working it over in the vice.

shackle1.JPG


shackle2.JPG


shackle3.JPG


superior.JPG


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You might want to elongate the holes in your shock plates. I had to on mine because the 8 1/4 axle is slightly larger in diameter and even if you compress the ubolt to fit the shock plates it will not sit right on the axle..
HKeller

uboltplate.jpg
 
Here it is nearing completion. If you look closely where the left exhaust pipe makes it's bend before disappearing behind the gas tank you can see two of the front spring perch mounting holes just below it. With the nose of the rear pointing down it is real easy to get the rear squared in with the car before attaching the springs. Just eyeball it for being in line with the spring perch.

rearnear.JPG
 
hkeller said:
You might want to elongate the holes in your shock plates. I had to on mine because the 8 1/4 axle is slightly larger in diameter and even if you compress the ubolt to fit the shock plates it will not sit right on the axle..
HKeller
Click to expand...
I am running the correct 8-1/4" shock plates, the baby ones got tossed to the side, I should have mentioned that. The u-bolts would never slip over the 3" axle on the 8-1/4 if they were compressed to fit a stock 7-1/4" shock plate.
 
Here it is ready to mount the wheels. I had to on the rh side use a pry bar to force the spring shackle off the trunk floor extension before compressing the spring to ride height. The high arch of the -02 spring makes it very short before putting the weight of the car on it which pulls the shackles towards the front of the car and up into the floor. Once sitting on the ground I tightened the shackles down compressing the new bushings. When raised back off the ground the friction in the bushings should keep the spring from pulling the shackle back into the trunk floor. The new 10" brake drums were bought from Advance auto through parts america for $18.99 each including freight as part of a summer promotion. The wheel cylinders practically bleed themselves within five minutes after cracking open the bleeders on each wheel and removing the master cyl. cover. Bye bye wheel adapters.

brakesdone.JPG
 
Her she is sitting on the ground, a tad higher than I hoped for, I will probably flip the front spring hangers upside down to lower it some. Since my 7-1/4 was about 1.5" wider than the 8-1/4" and it also had an extra 1" of width per side from the wheel adapters my wheels now sit about 1-3/4" closer inboard than they did before.

Car sits level from side to side though, measures at 24-1/2" to the top of each bumper jack slot. I new those slots were good for something. Man this thing is stiff, hopping up and down on the rear bumper created maybe an inch of travel. I hope this thing isn't going to be a kidney buster.

Only thing left to do is modify the driveshaft. It measures center to center at 51-7/8". Since the 8-1/4's axle center line to yoke is 1.6" deeper, subtracting that amount means the new center to center length would need to be 50.28", or just 50-1/4". I will remove the yolk and push it into the trans and take those measurements too as a sanity check. Will post the results later.

grounded.JPG
 
looks good to me. i would drive it around some with a full tank of gas. give it some time to settle down before you go flipping springs....
 
I removed the yoke from the driveshaft this evening, stuck it in the trans and lined up the wear marks with the trans seal so it is sitting in exactly like it was before the teardown. Yoke sticks out 3-1/2" from the end of tailshaft housing(not the seal) to the u-joint hole centers. Measurement between both yokes came out to 50" from center to center, about 1/4" shorter than I originally guestimated.

Difference could be perhaps because the car sits 3-4" higher than it once did. Since there is about 5" of trans shaft contacting the inside of the yoke at 50" I feel comfortable with this measurement. If it comes within 1/2" plus or minus I am not sure there would be any consequences. I can push the yoke in about 2" into the trans before it bottoms from where I intend it to mate up with the trans.

Anyway, this was the easiest part of the job. Now tomorrow I will drop the driveshaft off at the machine shop and let them do their thing.

*****Edited in:
The driveshaft actually measured in at 51-7/16 not 51-7/8, that's why my measuement under the car was off by about 1/4". Never trust your memory, always write it down. So going off of my original corrected math, 49.84 would be the center to center guestimate. Underneath the car measurement came in at 50". Sanity check completed. Always do both measurements.

So to recap for anyone who does not want to measure or just wants to find a donor shaft the correct length before starting the swap:
For all 108"(Duster, Dart Sport, Demon, Some Valiants before 73?) wheelbase A Bodies with a 904 automatic(or any other trans the same length and shaft depth), an 8-1/4" rear end, 50" from center to center is the number. I will measure the yoke again and post that because I have heard not all are the same length. I am pretty sure mine was 7" from u-joint center to tip.*****
 
No pun intended, got the shaft back today, measured exactly 50" center to center. Here are some before and after shots. Last picture shows how I started measuring eye to eye. It's also much easier to measure with ujoints installed or to partially press in a bearing cup, that way you have a lip to catch the tape on. Just measure from left of each hole to find the center to center length.

measurebefore.JPG


MEASUREAFTER.JPG


measurebeforefront.JPG
 
Here are a couple shots showing a factory weld versus an aftermarket weld. It's not as pretty as the factory one but it does not have any burn holes in it and looks like it made good penetration. There is quite a bit more metal on the new weld than the old one. There was also quite a bit of weight added after repair to balance it. This shaft did not have any factory weights on it. Perhaps all the extra metal in the weld necessitated some balance weight or maybe the factory did not balance the shaft with the front yoke installed.

yokebefore.JPG


yoke after.JPG
 
Just to verify the front yoke was 7" eye to tip. Depth came in at 5-3/4".

yokelength.JPG


yokedepth.JPG
 
Well, since the driveshaft guy actually cut this part to the length he was given I have no choice but to be happy with it. So does it fit?

shaftinstalled.jpg
 
Only thing left to do is add a couple quarts of gear lube and she will be good to go. I am going to spin the axle over a bit while she is still on jack stands to make sure all the axle bearings, clutch packs and pinion/carrier bearings get a good splash before running any power through it. I will probably do this tomorrow night then saturday she is going on her maiden voyage!!!!!

fillerup.jpg
 
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