7.25 to 8.25 Swap

If using any of the stock wheels - no, either width FMJ differential will interchange. The difference is the rear tires are inwards on the earlier width.
Now if using existing wider aftermarket wheels or wheels of different width than what came on on FMJ's then you might run into interference with either the leaf springs (early diff in later car) or fenderwell lip (later diff in earlier car) due to backspacing difference.

I have heard of using the 8 inch Super Coupe wheels on an later M-body causes problems with fenderwells - but no first hand knowledge of that (just hearsay).
BudW

Edit: IMO the early diffs are way too narrow for the fenderwell. It looks like rear wheels are just too far inwards.

That's good to know, thanks for the info, Bud. I am keeping the stock wheels and I do want the tires flipped inside out, so the whitewalls are out. Eventually I do want to go to a slightly larger tire and hopeful there will be enough clearance for a 215/75R15 like the one in the photo. The slightly larger tires and wider whitewall imo enhance the appearance of the car. I'm also hopeful that the appearance with the wheels being slightly more inward won't look nearly as bad as the Fords from around the same era. I always thought they should have been more outward.

The front wheels are the same position (more or less) for all FMJ's.

This is not a great shot but you can see the rear wheels could come out more.

This using the factory 14" wheels and 195/?r14 tires (not sure of the second number). The car currently has M-body 15" factory steel wheels and 215/70r15 tires (not pictured) - but there is very little difference on the look. The tires, above, are too short for the car, but takeoff performance was great (and speedometer was off, a tad). The 215/70r15's are a tad too tall for the car (and speedometer is off, the other direction) - but both cars share the same tire (and wheel) now.

My wagon has the hump gas tank and different trunk pan - so I can't get a full size spare (215/70r15) to fit. The original 14" Firestone Firehawk polyester belt tire fits just fine (hehe) with its 5 inch wide wheel (which is only 45 years old).

I need to update the trunk pan so I can fit a 15x8" wheel in it - by using a '78 and up trunk pan (and fuel tank).
BudW

I have the narrower 79 axle in my 88. No problems with stock rims and there's no real visual difference. I forget what the biggest tire size is that's been back there

Yes, barely noticeable after looking at his station wagon. Are you the same Mike that has the Fifth Avenue page from the 90's by chance? If so I posted a pic of my neighbor's 83 on that site; I've been a fan of these cars since high school after driving my aunt's 85 lol.

And Bud, I'm not sure yet when I'll start ordering all the parts for my 8.25, but if you order your axle shafts from Dutchman and decide to go for the 29 spline, I'd be interested in knowing how it pans out. Since buying that rear end in November I've been searching for the axle shafts myself and Strange Axles sent me to Dutchman. I've got a few other pressing issues to work on as long as that 7.25 holds out. Thanks for the help and the info, guys!

This is using a 215/70r15 on M-body 15x6.5 inch wheel (which came with the same vehicle using wire hubcaps as used in post # 22).

The '80 and up FMJ differential will be 1/2 inch wider per side as this (or in your case 1/2 inch narrower).


Edit: @dm330 I need to get an axle and haven't been driving the car much because of bent axle (and been working from home last couple of years).
With that said, I do have a pair of 8-3/4 inch housings and drop outs to rebuild as well as an '84 8-1/4 differential - so not sure yet about which to tackle first. The 8-1/4 has 2.9 gears and I got it a long time ago to get rid of the 7-1/4 inch diff with 2.2's currently in my Fifth Ave. Sense I have the big blocks now - I might not use the 8-1/4 diff and just convert both cars to the 8-3/4 diffs, with ISO deletes.
I'm just not going to go in debt and buying parts as I can.
I also really want to go with the Eaton True Trac limited slip - and that is not currently available with 27 spline axles. The wagon currently has limited slip (cone type) and I have an another older cone type limited slip carrier for my other 8-1/4 carrier.

It wouldn't take much for me to just rebuild and paint nice my spare 8-1/4 and toss under the wagon - then figure out what to do with the wagon diff (both are 2.9's). That said, I really want to toss new leaf springs under the car (refer to post # 18 Rear end question) and hadn't figured out what to do there, just yet.

Note: I have removed a lot of Chrysler differentials in my time (hundreds) and to do it at home, I recommend removing the differential and leaf springs as one assembly. with wheels attached and car on tall jackstands, it can be a somewhat easy one-man job. I have a step by step instructions in this forum somewhere (just do a search using my name and differential removal (or something like that)). If you have access to a lift then go for that. Trying to man-handle the differential over the leaf springs is not for the weak of heart.

Now granted, having limited slip does make it a bit more tricky to move the differential around in the garage (over an open diff), but it still makes it a fairly simple 1-man job. If nothing else, get some cheap vehicle dollies:


My '84 Gran Fury police differential still has the 5-leaf springs and rear sway bar still attached to it. It is also easy to move it about, by myself.

Note: the 8-1/4 uses a shorter propeller shaft over the 7-1/4 inch differential. Not sure if you were aware of that, or not. Other than that, you shouldn't have any surprises on install.

With that said I would recommend a new rear brake hose. Also check both axles for wear at the rear wheel bearings. If either is scored deep enough to catch a fingernail - not all is lost. Using the offset bearings will allow you to keep using the scored axle. Also check the yoke for scoring at the seal. If you can catch a fingernail there (once cleaned), you can get a microsleeve to fix that. If old differential has been sitting for a while, it would be worth to replace all of the bearings. I like/prefer Timken bearings for anything I want to stand behind. If someone wants to just get it built (to sell), then any ole bearing will work. Most any bearing coming from China is junk - which you can feel by putting both in your hands.

If old differential has sat for any length of time, you might have some rust pits in the gears. Rust pits will not weaken anything - but normal driving will produce higher than normal noise. I have been told that having some rust pits on gears actually makes diff run cooler - but no first hand knowledge of that.
BudW

If the car is good I've always done axle swaps by pulling the axle shafts and unbolting the backing plates and leaving them with the car. Then you don't have to touch the brakes and don't have to worry about disconnecting the brake cables or lines and without the backing plates on there the axle easily comes out with the springs in place. Drop it on a piece of cardboard and you can slide it out from under the car.

dm330 said:

Yes, barely noticeable after looking at his station wagon. Are you the same Mike that has the Fifth Avenue page from the 90's by chance? If so I posted a pic of my neighbor's 83 on that site; I've been a fan of these cars since high school after driving my aunt's 85 lol.

Click to expand...

Yes. For years it was the first or one of the first sites that came up in search results. Then Google started showing all the commercialized sites first and it's harder to find now but it's still out there getting occasional updates.

The Yukon and Dana 27 Spline limited slips are also unavailable right now, so I'd be better off upgrading to the 29 spline count once I'm ready to tackle that. I'm going to have the rear springs replaced, too and wanted to have the axle swap and springs replaced at the same time, also upgrading to the 5 springs from 4. A plus, all brakes and lines are in good shape. Car is clean underneath as it is above and inside. It sat for several years before being delivered to me in Ohio and was always a Florida car. Everything rubber was shot and I only put a little over 100 miles on it since getting it, but once all this work is complete, I believe I'll have what I felt the Fifth Avenue should have been in the first place.

Wow Mike that's awesome, do you still have your 84? I will check out your site, it has been a long time since I've looked at it.

Yeah it's still around but it's pretty beat now and mostly just takes up driveway space. Then I got most of what I wanted to get done at the time done on my 88 and moved on to other projects. Right now its sitting in the garage with its rear suspension apart.
The website still looks like a hold over from the millennium days.

Over the years I had both an 85 and an 89 diplomats, police package cars. Both came from the factory with 8-1/4 and 2.94 gears

That said I had to replace the 7-1/4 in my mom's 84 5th avenue original 7-1/4 equipped car. .
And did a 7-1/4 to 8-1/4 on a 90 Dakota as well. In both cases the 8-1/4 bolted right up and no different driveshaft needed for either one