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  #1  
Old 12-21-2002, 11:47 PM
g70challenger g70challenger is offline
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Question 8 3/4 Rear diff and A904 Automatic,OOPS!! replacing a 7.25 not 8.25



I am planning on replacing my 7.25 with an 8.75 on my '70 challenger and need some expert advice. I'm keeping the original A904 but will have it gone through with new clutches, bands, valve body with a TCI trans scat kit, MP deep trans pan, 5:1 kickdown lever, part throttle kick down valve and a four pinion 2:74 steel gearset with a 2200 stall converter. Now the question, I want to put in an 8.75 rear 3:91 sure grip (would consider a 8.25 3.91 sure grip.) can I use my existing drive shaft if a go with a small yoke 8.75? Can I use my backing plates from my old 7.25? is there anything else I should do to make it a sound transplant? I plan to use the car on the street so am I on the right track with my gearing? I want good off the line and midrange performance still using my warmed up 318 that should be good for around 300+ HP Thanks and Merrry Christmas!
George

ps sorry for the mistake thank you in advance for your help!!
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  #2  
Old 12-22-2002, 12:43 AM
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dwc43 dwc43 is offline
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I'd just get the sure grip and gear and put it in the 8 1/4 rear. What most people don't know is that the 8 1/4 was the replacement for the 8 3/4 due to the fact it is easier and cheaper to produce. It is just as strong as the 8 3/4 and you already have it in the car, so that's two plus's already. I've never had a c - clip problem either and that's the question people ask next. If you are worried aqbout it just install the c - clip elimenator kit and your done. I have one in a Duster with a 360 4 spd and it's held up well over the years.
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Old 12-22-2002, 01:49 AM
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ehostler ehostler is offline
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Just because the 8¼ replaced the 8¾, doesn't make it a better rear. It just means that it was cheaper for Chrysler to produce.

If he should ever decide that he doesn't want the 3.91 gears and decides to go with a 4.10 or a 3.55, it's a simple swap out with the 8¾.

If you like the 8¼ rears so much, you can have the one that is in my Coronet. Just get me a 3.91 SG in a 742 or 489 case. I'll trade straight up.


As far as the swap goes, the nose of the 8¾ is .66 inches longer than that of the 8¼. You will need to have the drive shaft shrtend, for it to work right.

I'm not sure if the backing plates will swap
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Old 12-22-2002, 02:12 AM
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dwc43 dwc43 is offline
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I have never said or implied it was better than the 8 3/4. It is equal in every aspect. Now if you oval race and need to change gears often and have several centers available I'd say 8 3/4 all the way just for less hassel and to save time.
I've only broken one 8 1/4 over my life time and that was just dumb luck I think. Littile pin that holds the spider shafts in place broke just after trans shifted to high and slung that shaft out the rear cover. Easily fixed to I might add,and it never broke again.
The only time I broke an 8 3/4 it was the bearing and adjuster that seized some how. I think that might have been a machine shop error since the guy I got it from had just replaced both axle bearings
I dont know about that trade now E. What gear and sure grip does your 8 1/4 have. do you want a whole rear or just the center? Your in Canada right? Can't tell after you start a post you know. Just figuring it would not be worth the cost to ship. Surley you can find one up there can't you?
Sorry,never mind, I see your not in Canada after all. Don't know where I got that Idea from.
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Old 12-22-2002, 03:27 AM
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ehostler ehostler is offline
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Actually, I'm in Virginia. The 8¼ that I have is an open 2.94 setup. I've already got the 8¾ axle housing, axles and backing plates. I'm just needing the 3.91 SG third member. Once I find one at a reasonable cost, I'll probably just donate the old rear to a local salvage yard. I just don't feel that it's worth the cost of shippin
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  #6  
Old 12-22-2002, 03:55 AM
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dwc43 dwc43 is offline
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Hey E, no fair. 2.94:1 for a 3.91:1. You'd have to make it more fair than an even swap now come on.
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Old 12-22-2002, 11:13 AM
g70challenger g70challenger is offline
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:shock: my mistake, actually 7.25 rear end not 8.25

First off thanks for your info, but I made a mistake. The rear end is actually an 7.25. Still wondering what I need to do to make an 8.75 or even an 8.25 rear end swap. (drive shaft, yokes, backing plates etc.) Everything else from my previous post is correct, sorry. Still want to do the sure grip and 3:91 gearing

Again, thanks and happy holidays!
George
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  #8  
Old 12-22-2002, 11:47 AM
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340MAN 340MAN is offline
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Do you know that you could put a 60's all the way to 1970 "b" body 8 3/4 rear under your e- body. This would yeild a little over 1" extra room for rubber on each side.
Simple install - Pull the frt spring mounting plate and drill holes 1" to the inside of original holes. There is enough room and causes no safty issues.
Use the offset rear hangers from M.P. on the rear.
The pinion angle still should be a factory specs, but I always double check. You would not have to moved spring perches and bolt right up.
Wa La - Factory narrowed bolt in semi narrowed rear end. All of your existing cables ,lines etc.. will fit ok.
Thought I would throw this at you before you spend the time changing out the rear end. Why not get more for you efforts.

Good luck & Merry Christmas

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Old 12-22-2002, 01:54 PM
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The Dartman The Dartman is offline
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The 7 1/4 backing plates will not work on an 8 3/4 housing due to a four bolt flange versus a five bolt flange, and the differences between brake drums and hardware. Keeping the 7 1/4 brake hardware because is was recently replaced is not worth the headaches it will cause IF you can modify them to fit.

I'm not too familiar with B and E bodies, but if 340MAN is correct, a B body 8 3/4 is probably easier to find and lets you run larger tires, I'd say go for it. You should be able to get everything you need drum to drum to make that swap work.

I've replaced two 8 1/4's in both a Dodge Dart Sport and A Plymouth Duster, and I have never run into any problems with the .66 difference that Ehostler mentions. Usually most driveshafts have enough clearence front to back to go a 1/2 inch or so either way without causing problems. IN fact in both of these swaps the driveshaft was actually a tad shorter than I would've liked...

Dartman
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Old 12-22-2002, 03:14 PM
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ehostler ehostler is offline
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By sacraficing that .66 inches, you are limiting slip yoke travel. If the slip yoke ever needs to go full travel, you can damage the output shaft on the tranny. I have come across two cars that this was done to. The slip yoke was stuck on the output shafts of both.

The nose of the 8¾ is 2.26 inches longer than the 7¼. That is difference that will be noticed. You will have to get the driveshaft cu
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  #11  
Old 12-24-2002, 10:56 AM
moparmac moparmac is offline
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Not true Dartman, on my particular car, the brakes swapped on the new 8.75 flange. I didn't have to disconnect the e-brake. I took the old backing plates off the 7.25 and then slid them directly on the 8.75 with no fitment problems at all!!
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  #12  
Old 12-24-2002, 01:11 PM
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The Dartman The Dartman is offline
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Did your 7 1/4 have four bolt flanges? Not doubting your knowledge about the 7 1/4, but are your sure this was a 7 1/4 (stop sign diff plate)? I know for a fact that the A body 8 1/4 rearend backing plates are a direct, exact swap. The 7 1/4 I bought was specifically bought for the backing plates, brake hardware, and drums to go on another 8 3/4. I got a big suprize when I took the backing plates off and found 4 bolt flanges that did not line up to the 5 bolt flanges on the 8 3/4, as well as the rearend hub relief in the backing plate being too small to even fit onto the 8 3/4 axle housing.

Is it possible that the 7 1/4 rearend changed (bolt flanges, hubs) from 72 to 73 when the bolt pattern changed from 4 to 4.5? The rearend I had was a '71 A body 8 3/4, and a '72 A body 7 1/4, and the backing plates wouldn't work without extensive modifications.

This could be the only way that both Moparmac and I can be right.

Dartman
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