upgrading to a 8.25 rear end from 7.25. what to know?

SonOfaTomP

SonOfaTomP

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let me preface by saying, i don't know that much about rear differentials.

the rear Diff on my 88 Civ Gran Fury has begun to whine quite loudly. from all i've heard this rear diff doesn't seem well liked.

now i'm not really trying to make a hot rod or anything, i just use this car as usual daily transportation. weighing my options. upgrading to a 8.25 seems like a popular choice. possibly cheaper than fixing the 7.25

what gearing do these cars need? and from what cars is the rear axle compatible? not far from me is a 8.25 from a 97 Cherokee with 3.55 gears. only 300 bucks. and there was a police 8.25 up in PA i'm unsure if is still available from a Diplomat.
 
I think the Cherokee axle might be too wide and you'd have to get the axle tubes cut down and get shorter axle shafts.

I'm not a Jeep guy. So I don't know for certain.

If you can get an 8 1/4 off of any Aspen, Volare, Diplomat, etc., it'll bolt right in.

Who cares about the gearing. If you find an 8 1/4 that fits, buy it, you won't regret it. The 7 1/4's in these cars are all stupid low gears anyways. You're not going to find a cheap axle with a crazy ratio. I wouldn't even worry about it. Just find an axle that fits and put it in.

The 7 1/4s suck. I ended up scrapping my 7 1/4, it was in my way after the swap and I couldn't find anyone who wanted it for free. The howl was eliminated and now I can hear all the other problems the car has.

There are B body axles out there you can make fit, but it's so much easier to slap in an interchangeable assembly and just have it fit.

Don't forget you'll need a shorter drive shaft. I got mine out of a Volare. Axle is out of an old Diplomat police car. (and yeah i lost a sway bar bushing while i was installing that and slapped in a cheap rubber one, im a complete hack, i had left over green paint and painted the axle too) 20250808_202200.jpg20250810_184400.jpg
 
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i've heard of people putting Jeep and Ford Explorer Diffs on these cars. must have taken some cutting. thanks for the info. i might have a lead on a Dip one. pain in the ass to take off in a junkyard?
 
I think what people swap in is 8.8 Fords out of a Mustang. If I recall properly, they don't need to be cutdown, but the perches do need to be set. You can get rear disc brakes and limited slip fairly cheap as a result.
 
SonOfaTomP said:
i aim to be there. not sure how many other users here will be
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you could probably find one cheap there. do it.

Idk if you can fit an 8 1/4 in the trunk of an M body. maybe you can get it to fit enough to tie down the trunk lid and add a red flag to the axle hanging out the back.

The 8 1/4 is heavier than the 7 1/4. Just to give you an idea, I can pick up a 7 1/4 by myself. I have to roll/drag an 8 1/4. So if you get most of it into the trunk and tied down, it shouldn't flop around.
 
SonOfaTomP said:
pain in the ass to take off in a junkyard?
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The junkyard one will probably have the drive shaft you need. It'll be a bit of work. Not a lot of fun in a junkyard, especially if they don't let you bring power tools/cutting tools
 
The hardest part of getting them out is dealing with rusty UBolt. I always cut them. In a junk yard you might get lucky and it'll break otherwise you'll need a cordless impact or get a work out with a ratchet and breaker bar.
 
95 and older XJs (Cherokee) had a 60.5” track width and perches at like 44” center to center.
Your M should have a track width of 59.5” and perches as 44.46” center to center. Would work easy if you wanted some big ol wide tires out back, lol
I would go with an 8.25 out of an M along with the drive shaft and sway bars or a Ford 8.8. Parts for 8.25 are getting expensive new.
 
LSM360 said:
I would try to get the M body one and grab the anti-sway bar and as much of the hardware for it too.
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If the car has one.

3.55 gear ratio will put huge smile on your face when you take off (even with a stock /6 or 318-2). It will be a "mostly in-town car gear ratio", though.

For in-town/highway driving, a 3.2 ratio is spot on. 2.9 a close second. AMC/Jeep used a 3.0 ratio - but never had my hands on one, yet.

The gears are easier to find, than the differential assembly.

The 8-1/4" differential has been around for a long time, so replacement parts will be much easier than 7-1/4". The Jeeps were the last to use the 8-1/4".
Most Jeeps use taller tires, so 3.2. 3.5 and even 3.9's can be found.
3.9's are a really, really fun gear to drive in town. Not so much on the highway, though.

My '86 Fifth Ave has 2.2 gears.
My '77 wagon has 2.9's.
Both cars have the same engine and transmission.
From a dead stop, my wagon will circle my Fifth Ave about 3 times before I get across an 4-lane intersection.
2.2 gears suck !

I have not played with a Ford 8.8", so can not comment there.
BudW
 
Note: replacing your 7-1/4" with almost any other size rear differential, will require a shorter propeller shaft. In most cases, you can get your original propeller shaft shortened.
BudW
 
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