7 1/4 to 8 3/4 rear end swap info needed

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Chryslerkid

Chryslerkid

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I am considering switching over to an 8 3/4 rear end in the near future. First, i want to make an informed decision. i have a '71 dodge dart with a 318/727/7.25 rear end setup. I want to know if i will have to swap out my rear wheels when I do this conversion to 8.75 rear end? the reason i think i may have to is because typically i have seen larger bolt patterns on the 8.75 rear ends unless i am mistaken(which is entirely possible).

Right now i have front discs and rear drum brakes with 19" boyd cottington wheels. I am only putting this out there because even though i will probably go for the 8.75 swap either way, i need to budget out for other costs for example shortening my driveshaft, getting a new set of wheels and tires ect. I am looking for an A-body 8.75 rear end with sure grip for less modifications and proper fit to my needs. Any info and help is always appreciated.
 
To swap a 8-3/4 in place of a 7-1/4, you'll need shock plates. The 7-1/4 is to small and that is for every rear you try and use them on. New U-bolts would be a very good idea.
Then you'll need to shorten the drive shaft accordingly

Most 8-13/4's are small blot.
 
You will find some with small bolt pattern, than I think you need diffrent u-bolts, but your wheels should work with that set up.
 
You didn't say whether the Dart is a Demon/Duster body style or Swinger/Scamp/Valiant style body. If you can find a driveshaft from another body like yours that cane with a 318 (or 340) 727 and the 8 3/4 rear you won't have to cut your shaft.

The driveshaft is a different length on a Swinger/Scamp body style or the Demon/Duster shape, but both are somewhat common driveshafts to find used.

Most stock A body 8 3/4 rears came with the small bolt pattern axles. They are pretty much a bolt in swap with the larger spring plates and u bolts.
 
Did it 20 or more years ago to my Dart, now everything is up to 340 specs except the original 318.
Yep, shock plates and the correct sized U bolts and shorten or replace the drive shaft and you should be home free.
Then you can start playing with easy to swap gears and things. I started out with a 3.91 open rear outa a 71 Dart and later upgraded to a early 742 power lock sure grip with 3.23 gears. I did have to shorten my drive shaft seeing how it has the big 7290 u joint with a adapta joint so it works with my A body rear end. I THINK the shock plates/u bolts from a 8 1/4 will work as well seeing how early A body rears are getting way harder to find and spendy these days.
 
do you remember how much exactly you had to shorten your driveshaft? i have a great drivshaft mechanic that is cheap and will shorten my drievshaft like he did when i swapped from /6 to 318/727. when i was in the process of doing that swap, someone on faro told me the exact length to cut my driveshaft and it worked out great.

on another note...should i buy new shock plates(as in not used)? or are original ones okay to use?
if i buy new shock plates and u bolts, does anyone have a good parts store to recommend?

p.s. my dart is a swinger

thanks for all the great info guys! this is very helpful
 
I think I went 31/4 but it's been many years and you have to measure and all that because you can go two deep or too shallow and have issues bottoming in the trans or the shaft falling out. It was supposed to be for a 727/ 7 1/4 combo though just like yours I think and why not use used shock plates, they are pretty stout so should be fine, mine were in the doner car and are still under mine, though I did later add a rear factory sway bar outa a 76 Valiant cop car, also a HUGE upgrade in handling if you want to go that route. Though those rear bars are made of unobtainium now, and were pretty hard to find then. I also believe if you do the right search you can find the factory length for a Dart 340 auto combo with the 83/4 and that is the length you'd need.

2852228 1967-69 3.25 47.81 727 8-3/4 340 383 Dart
2883693 1970 3.25 47.99 727 8-3/4 340 Dart
 
Chryslerkid said:
do you remember how much exactly you had to shorten your driveshaft? i have a great drivshaft mechanic that is cheap and will shorten my drievshaft like he did when i swapped from /6 to 318/727. when i was in the process of doing that swap, someone on faro told me the exact length to cut my driveshaft and it worked out great.

on another note...should i buy new shock plates(as in not used)? or are original ones okay to use?
if i buy new shock plates and u bolts, does anyone have a good parts store to recommend?

p.s. my dart is a swinger

thanks for all the great info guys! this is very helpful
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Used shock plates are fine, you can find them right here quite often. I would use new U bolts and torque to 45 ft lbs. Re-torque after driving a couple hundred miles.

Driveshaft, I would measure it with the suspension loaded. Check with the person that will be shortening and balancing to see where they want you to measure from.
 
sounds good. I'm trying to tally up all the costs other than the obvious 8.75 rear end. what do you guys think is fair market value for a rebuilt 8.75 A-body rear end with sure grip already in it and rebuilt drum breaks? I'm sure prices vary depending on where you live and the supply and demand market, but i don't want to be ripped off over here in california. it seems that they range from $800-1200 where i am looking.
 
Chryslerkid said:
it seems that they range from $800-1200 where i am looking.
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That's what they range here for a stock, non rebuilt open unit. If you could find one rebuilt, suregrip, large bolt pattern and ready to bolt in, 800 would be a great price IMO.
 
i should've been more specific. generally $800 will buy a non rebuilt rear end, possibly needing some break work. $1200 range will normally buy a rebuilt rear end with or without a sure grip and rebuilt breaks….. what o you think about those prices? i can always rebuilt the breaks myself, but i am not familiar with working on rear ends so id have to take it to a shop or learn as i go.
 
Like I said typically an 8 3/4 A body rear complete with open diff would run you around the 800 mark not rebuilt, at least around here anyway. If you look at some of the prices for just the center sections sold on here, they can vary from 350 to 550 for a suregrip center. So doing the math I would say a complete rear, with suregrip, rebuilt, is worth the 1200.

That's been my experience and just my opinion. Some will say they can find them cheaper, if you can, better yet. I would also try and find one with good drums, as they can be hard to find. Large bolt pattern is a huge plus.

IF you find one rebuilt, ask for some documentation as to parts etc just for peace of mind. Also when you check it out, spin one drum in a forward rotation and make sure the other side turns in the same direction, that will tell you if it's a suregrip or not, at least from an honest seller.
 
thanks! ill be sure to do all of the above. well off to find a new 8 3/4 rear end. wish me luck….im sure ill have more questions as i am installing and learning at the same time.
 
Chryslerkid said:
thanks! ill be sure to do all of the above. well off to find a new 8 3/4 rear end. wish me luck….im sure ill have more questions as i am installing and learning at the same time.
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I see you're in Cali., ask here, there might be someone around you from on the board here that could fix you up with a rear.

Here's one in Stockton, don't know how far that is from you. 800 bucks. He doesn't say whether it's a suregrip or not. You could offer a bit less if interested, you never know. He might even throw in the shock plates.

http://stockton.craigslist.org/pts/4206884967.html
 
Ok, to clear up a few things here!! All stock non- modified A body 8 3/4" rear ends were small bolt pattern! If you happen to find one in unmodified condition it will have a 4" bolt pattern and you should be able to use your current wheel/tire setup. You can also use your 7 1/4" shock plates if you enlongate the holes. The axle tube from your 7 1/4" is 2 1/2" and the tube from all other Mopar rear ends is 3" so there is not much grinding to do, but you will need the correct u-bolts. If you are going to buy shock plates, try to make sure that they are from an A body as there is a small difference in the drop length for the shock bolt from other B and E bodies, which use the same shock plate!

Agree with the measuring of your driveshaft length with the suspension loaded and the car on the ground! Many people measure the length differently, so maybe a call to your guy is in order to see what he prefers the measurement to be! At any rate, the yoke should be pushed into the tail shaft all the way, then backed out about 2", no less and no more. This will give you some travel in and out and still have plenty of contact in the middle range!

I thinking is $1200-1500 for a totally rebuilt ready to drop in unit with full documentation and receipts! Hope this helps, and good luck with the swap!! Geof
 
cosgig said:
Ok, to clear up a few things here!! All stock non- modified A body 8 3/4" rear ends were small bolt pattern!
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Right, but since he was talking rebuilt, I was suggesting finding one with LBP as many times people will upgrade the axles, or at the very least drill them for 4.5".
 
Yep Bruce, gotta have a watchful eye for that! That's why I included the actual measurement of the bolt pattern, nobody mentions that!! Merry Christmas too by the way, hope you are living it up right!! Heck,'I hope everybody here in FABO land is enjoying the best Christmas ever!! Geof
 
cosgig said:
Yep Bruce, gotta have a watchful eye for that! That's why I included the actual measurement of the bolt pattern, nobody mentions that!! Merry Christmas too by the way, hope you are living it up right!! Heck,'I hope everybody here in FABO land is enjoying the best Christmas ever!! Geof
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Merry Christmas back at ya. Quiet here now, kids went visiting after stuffin' their faces. lol.
 
i will refer back to this thread after looking for a little bit longer online for a rear end for sale. i have one last question… what are some clear signs of a bad rear end in need of a rebuild? in other words, how can i tell if someone is trying to sell me a lemon???
 
Chryslerkid said:
i will refer back to this thread after looking for a little bit longer online for a rear end for sale. i have one last question… what are some clear signs of a bad rear end in need of a rebuild? in other words, how can i tell if someone is trying to sell me a lemon???
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Receipts on a rebuild are a great start but not a guarantee. You may try to farm out the pieces as many of us have done, but you may end up coming out of pocket for as much as a complete unit or more. I pieced together all the parts used & only bought the axles & U-bolts new but had a reputable shop inspect the Powerlok & set up the gears. It bought me some piece of mind.
 
Any chance anyone here could point me to someone to deal with for a full assy 8-3/4 in south western Ontario Canada (near Brantford ON)


Thanks
 
65_Dart_GT said:
Any chance anyone here could point me to someone to deal with for a full assy 8-3/4 in south western Ontario Canada (near Brantford ON)


Thanks
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I have one across the border in Fetroit if that’s in your wheelhouse. Message me and we can talk about it.
 
65_Dart_GT said:
Any chance anyone here could point me to someone to deal with for a full assy 8-3/4 in south western Ontario Canada (near Brantford ON)


Thanks
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i have one for sale located near Windsor , Ontario ,, rebuilt sure grip with 3.91 gears ,housing cleaned and painted,,stock 5 on 4 axles with right hand threaded studs on both -- like new backing plates ---- will send you a PM
 
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