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Dodge Diplomat Rear Ends (Differentials)

Rear Ends (Differentials) /Gear Ratios:

M-bodies were produced with only two? differential sizes: 7 "" and 8 "". The measurement refers to the size of the ring gear. This gear, which is turned by the pinion gear, changes the direction of the power being transmitted to it. The pinion gear is turned by the driveshaft. The 8 "" units are admittedly not as tough as the 8 "" units that did time behind many muscle car engines, but they are a darn sight tougher than the stock 7 "" rear end, and will stand up well behind all but the most radical high performance engines.


What Type of Rear Does My Car have? (Courtesy of Steve)

You can tell what type of rear end you have by looking at the bolt pattern on the rear access cover and the axle housings (tubes): The 7 "" rear has 9 or 10 bolts with an irregular shape and axle tubes that change in diameter as they approach the differential housing. The 8 "" rear has 10 bolts with an elliptical shape and axle tubes that are consistent in diameter from the drums to the differential housing. The 9 "" rear has 10 bolts?, consistent tube diameter but the bolts are arraigned in a stop sign pattern.

How To Tell What Gear Ratio You Have: (Courtesy of Reggie)

Block your front wheels

On level ground, put the rear of your car up on jackstands (good ones). Make sure that your car is stable.

Put the transmission in neutral.

Chalk mark the pumpkin and the driveshaft.

Chalk mark a wheel where it will be visible.


Count the number of turns of the driveshaft it takes to turn one wheel one time.

The number of times (including fractions) the driveshaft must turn to turn the wheel onetime is roughly your gear ratio.

How To Identify A Sure Grip Differential (Courtesy of Steve Knickerbocker)

Turning one wheel will turn the other wheel in the same direction. The other wheel will turn in the opposite direction on a regular (non-Sure Grip) differential.

Check the fender tag under the hood. If it has A38 or AHB, depending on the year, it's a cop spec M body. Cop M bodies almost always have SG rears.

Pull the oil fill plug. If you can see springs inside the differential carrier, it's a Sure Grip.

Where To Find Sure Grip Rears (Courtesy of Malcolm from Canada)

Since the 1980 year and up on M-bodies (the year they squared them off) the limited slip or Sure-Grip equipped cars are difficult to find. It was still an option but few new owners ordered it.

1978 And 1979 LeBaron, Diplomat And Caravelle Station Wagons are your best bet for locating a sure-grip equipped unit. These cars were owned by families and are usually not punished as violently as former police or fleet cars. In most cases the axles are mint even if the bodies are rotted out (and the fake wood panelling is twisted by the sun). The one I had the wreckers torch out for $120.00 Canadian was clean inside when I removed the rear cover to count the teeth, not even minute filings in the differential oil!

Second best bet are any police, taxi or fleet cars (see the section on how to identify a police car).

Possible Production M-Body 8 1/4 Gear Ratios: (Courtesy of Malcolm from Canada)

2.26:1 This ridiculously high ratio was found in later M-bodies.

2.47:1 This was the most common ratio for 1978-1979.

2.94:1 This was also common. Usually found in police/taxi/fleet cars.

3.23:1 This highly desirable ratio was not so common.


Available Aftermarket 8 1/4 Gear Ratios:

(Courtesy of Toms Rear Ends 634-8431 or (562) 634-8452)

Ratio Available Brands

2.71 Mopar

2.94 Mopar

3.21 Mopar

3.23 U.S. Gear

3.55 Mopar, Richmond Gear, U.S. Gear

3.90 Mopar

3.91 Richmond Gear, U.S. Gear

4.10 *Richmond Gear

4.30 Richmond Gear

4.56 *Richmond Gear

* Note: these sizes are also available from Summit Racing (


Rear End Swaps (Courtesy of Malcolm from Canada)

Changing whole rear ends is a lot easier job than trying to set up a new ring and pinion gear in the stock casing if you're not a well equipped professional. Also, I seem to remember someone telling me that the 8 1/4" rear when equipped with a 2.47:1 ratio gear set would not accommodate other ratios. Maybe I heard it backwards, that the 2.47:1 gear set wouldn't go into other 8 1/4" housings. Anyway you're partially warned.

If you want anything steeper than 3.23:1 (without changing gears) you'll need to find a 26 year old 8 3/4" rear, in good shape and then make it fit an M-body

(See "7 "" or 8 "" To 8 "" Swap" below). The other choice is to get someone to drop in an aftermarket ring and pinion with your desired ratio into your 8 1/4". See "Available Aftermarket 8 1/4 Gear Ratios" above.

3.55:1 was on the option list into the late seventies, but they're as rare as hen's teeth in 8 1/4" housings. You will have to hunt F-Bodies (Aspen/Volare), older M-Bodies (Lebaron/Diplomat/Gran Fury) and J-Bodies (Cordoba/Imperial/Mirada) as they'll all drop right if you find the ratio you want.


7 1/4" To 8 1/4" Swap (Courtesy of Malcolm)

If you currently have a 7 "" axle (9 bolts), the 8 "" (10 bolts) axle will drop right onto the spring perch pad centers perfectly, requiring no modifications. The measurement from the outside of the left drum to the outside of the right is one inch narrower than the later M-bodies, but there should be no tire clearance problems. As I have mentioned in the Forum, I run 7 inch rims and 225/70 15's without any problems and there's room for larger tires.

You will need:

The donor 8 "" rear - if available, get the driveshaft from a donor vehicle similar to yours, i.e. 2-door, 4-door or wagon. If the driveshaft is not available, you will have to take your present driveshaft to a driveshaft shop, (along with the measurement from the transmission to the new rear end) and have it shortened or lengthened accordingly, and re-balanced.

You should be able to re-use your brake drums, shoes and all associated brake hardware from your 7 1/4" axle as they both use 10 inch drums. It's a lot safer to do a brake job on everything in the rear while you're at it though.

In an afterthought, once I'd completed the swap, I wondered why I didn't pirate the 11 inch drum's, backing plates and associated brake hardware off a larger car with an 8 1/4" axle in it. Give it a try, I read somewhere it's a bolt in operation.

It would be smart to replace both the brake lines on the axle and the rear brake hose (from body to axle) while you're there. They're cheap and also easy to get at while the axle's out.

Great time to replace the rear shocks too if money permits. New axle U bolts and a buddy to help you lift the axles out and in and you're all set.

It's possible you could find a 4 door M-body with the driveshaft in it, but these cars as well as the wagons likely had 727 transmissions in them meaning the front yoke will not fit your transmission"s spline. In addition the wagons have longer drive shafts, which require shortening and re-balancing.

I'm a bad test case on the driveshaft question because I'd already swapped to a 727 before I stuck the 8 1/4" axle in. Because of this I couldn't compare the stock driveshaft for length for you. It doesn't matter anyway, as I mentioned earlier, if you have the lighter A-999 tranny you'll need a different shaft anyway.

So, either find that mint condition 8 1/4" sure-grip rear axle in a 4 door 1980-1989 M-body (good luck) or bring your original driveshaft and the wagon's down to the truck centre. There they'll put the correct end from one on the other shaft, shorten and balance it all for 70 bucks Canadian (that's what I paid two years ago).

TRW makes a greasable heavy duty U-joint (part # 20124) that takes a beating and is well advised if you're going to spell your name on the highway in twin streaks of rubber (spelling "Malcolm" used to take a while until I installed the 360). Back to the U-joint.

The nipple is on the end of one of the caps not in the valley of the inner cross where you can't get a grease gun on.

7 "" or 8 "" To 8 "" Swap

The venerable 8 "" rear is a stout unit, which was used on all but the ultra high performance Mopar passenger cars until 1972. It is easily identifiable because it has no rear cover. The entire unit can be removed from the axle housing, making it the easiest rear end to change gears in. It is the only rear end that was offered by Chrysler with this design, The 9 "" rear has hexagonal shaped 12 bolt cover, and the 9 "" (Dana 60) rear has a 10 bolt cover with an irregular shape You can swap in a unit from an A-body (1960-1972 Valiant, 1964-1969 Barracuda, 1971-1972 Scamp, 1961-1962 Lancer, 1963-1972 Dart, 1971-1972 Demon). They may be too narrow for all I know. I'm unsure, but who cares as they were pillaged out of wrecking yards before the seventies were over.


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