Swapping rear end question

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jrc4y4

jrc4y4

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Well, thanks to your guys' influence, I am going to swap my rear end rather than doing gears in my 7.25. A guy local has an 8.75 but is open and 2.76 gears and wants a fair amount for it. I found another guy with an 8.25 that he says is out of a 73 Duster. Wants a lot less for it. Unsure yet if it is sure grip or what gears. I plan to go with the 8.25.

That being said, I have a 72 Scamp. Is the 8.25 a direct fit or what all do I need to do? Is the driveshaft fine? Didn't the 73s come with BBP? But the perches and width are all the same? Just want to make sure I am covering my bases. Thanks all.
 

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The 8.25 will be large bolt pattern. You will probably need to shorten your driveshaft. It should bolt right on to your springs but your old u bolts and shock plates won't fit so get those too!
 
I did the same swap several years ago. If your Scamp has a 904 transmission then your driveshaft with the existing 7 1/4 rear end is 53 7/8". I had mine shortened by 1 5/8" when I installed by 8 1/4" and have had no issues.
 
You might consider replacing the leaf springs while you're at it. I just did the 7.25 to 8.25 upgrade on my '74 Swinger and I replaced the springs at the same time. I've already noticed the car looks better and corners better even though I haven't rebuilt the front end yet. Also, if you can get the driveshaft from the same car the 8.25 came out of, there's a chance it could fit your car without cutting. That worked for me (it was about 1.5" shorter than the one that was in my car).
 
The 8 1/4 should have a tag bolted to the rear end with the gear ratio, assuming it hasn't been removed in the past 43 years. My '73 Dart's 8 1/4 had 3.21 gears with no sure grip.

It should be a fairly straight forward swap. The only big thing I can think of is the driveshaft. If you get the driveshaft from the '73 Duster, you may or may not have to change the driveshaft yoke and u joint. The 4 speed and 727 use a large yoke, and the 3 speed and 904 use a small yoke.
 
Where can I get the u bolts and shock plates? What's a fair price to shorten the driveshaft? Is that something best to leave in the car and just trailer it to a drivetrain shop? I got a 904 in it now. Would it be better to get my driveshaft shortened or just hope that the 73 one will work if he has it?
 
jrc4y4 said:
Where can I get the u bolts and shock plates?
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You can get U bolts new online. I know ESPO has them, but I've seen them other places too. You'll need ones that fit the 3" diameter axle housing on the 8 1/4. The shock plates I think you would have to get from a donor car or ask around on this site. Those are also different for the 3" axle housing (the holes are farther apart to fit the larger U bolts).

jrc4y4 said:
What's a fair price to shorten the driveshaft? Is that something best to leave in the car and just trailer it to a drivetrain shop?
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I don't know about pricing, but it's pretty easy to ballpark how much shorter the driveshaft needs to be, and the measurement doesn't need to be that precise. There's some slop built in because of the fore/aft movement of the driveshaft during driving over uneven roads. (Look at the front yoke on your existing driveshaft and you'll see where the shiny part of the yoke has room to slide forward into the transmission. It doesn't matter exactly how much shiny part is visible as long as the yoke doesn't go too far in/out at full up/down displacement of the differential. If your measurement is off by a quarter inch it'll be fine.)

On the old diff and the new one, measure the distance from the axle housing centerline (remember the axle housings are different diameters) to the part of the pinion yoke that contacts the u-joint. The difference in that measurement between the 8 1/4 and the 7 1/4 is how much shorter your driveshaft needs to be. In my case it was about 1.5". You can use that measurement to either find a driveshaft the right length or have yours shortened.

It's super easy to pull the driveshaft. When you remove the 4 little bolts holding the rear u-joint onto the pinion yoke, the driveshaft will just slide rearward out of the transmission and fall out if you don't catch it. If you want to leave it in, you'll need to wire or tie it up to the car body, making sure it can't slide rearward, and you'll need to secure the u-joint bearing caps with tape or a rubber glove or something if you plan to reuse the u-joint.
 
go with the '73 8.25 and you'll be buying new wheels or axles too - - so, what year and car is the 8.75 out of?
 
the 8 1/4 is large bolt pattern. make sure it has decent gears in it. don't want to be stuck with a 2.45 gear. if its a 3.23 sure grip then go for it..

personally i'd spend the extra and go with the 8 3/4. when you are ready for a better set of gears and a sure grip ya just change the center section..

both you'll have to shorten the drive shaft for anyway.
 
abodyjoe said:
the 8 1/4 is large bolt pattern. make sure it has decent gears in it. don't want to be stuck with a 2.45 gear. if its a 3.23 sure grip then go for it..

personally i'd spend the extra and go with the 8 3/4. when you are ready for a better set of gears and a sure grip ya just change the center section..

both you'll have to shorten the drive shaft for anyway.
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Agreed!!! The driveshaft needs work either way, with the small bolt pattern you'll be able to bolt your wheels back on, and it holds its value! You can always big bolt it, and easy gear changes are about an hour to do!!
 
this is kinda where I was headed.. O:) the $$ you save on the 8.25 will be spent on other things to get it done correctly - do the 8.75 - overall an easier swap and you'll be ahead on value when you get to the other end.. some may argue that the 8.25 is "strong enough" - - I believe the 8.75 was a performance upgrade option for a reason... JMHO.
abodyjoe said:
personally i'd spend the extra and go with the 8 3/4. when you are ready for a better set of gears and a sure grip ya just change the center section..

both you'll have to shorten the drive shaft for anyway.
Click to expand...
 
cosgig said:
Agreed!!! The driveshaft needs work either way, with the small bolt pattern you'll be able to bolt your wheels back on, and it holds its value! You can always big bolt it, and easy gear changes are about an hour to do!!
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Good point about the bolt pattern. For me the main reason for swapping to the 8 1/4 in my '74 was to get the big bolt pattern since I wanted to put '73-'76 factory disk brakes on the front. But in '72 you would have had SBP even with disk brakes. If it were my car I'd probably not want to change that.
 
I swapped from 7.25 rear end with 2.93 gears (stock) to a Dana 60 with 3.55 gears and I run 14" rims and a 727 3 speed. One thing you need to factor is "cruising speed"


With my setup, I can go 0-110 pretty dang fast, but at 110 I am doing almost 6000 RPM,

But Cruising speed: I have to cruise at 55-65 mph @ between 2500-2800 rpm, 55-60 feels perfect if I get up to 70 I am doing 3000rpm, and 75 is 3300rpm (which is NOT fun to cruise at because it feels like you need to shift). That being said, I can go from 55mph - 100mph in 4.6 seconds. To compare, my 2010 Chevy Tahoe takes 10.7 seconds, my 2016 300hp Honda pilot takes 9.7 seconds, and my wife's 2014 mini cooper Turbo S (six speed) takes 8.9 seconds. So my set up is great for those "rolling pulls" when I feel like getting on it hard... but not so good for cruising. I also had to have the axles shortened, a new custom 3" aluminum drive line, and new U bots and shock plates (purchased on EBAY..NOT CHEAP!)

Hope this helps...
 
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