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Author Topic: Rear Axles: Chrysler 8 1/4" Rear Axle  (Read 23713 times)

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Reggie

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Rear Axles: Chrysler 8 1/4" Rear Axle
« on: January 01, 2008, 21:31:18 »

Chrysler 8 1/4" Rear Axle

Chrysler Corporation's Spicer 8.25" rear was one of two differential types used in M-bodies, with the other being the weaker 7.25" unit.  The 8.25" rear axle first appeared around 1969 in Plymouth Belvederes, Satellites and Dodge Coronets. 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. From 1973 on, the 8.25" rear was used in virtually every rear wheel drive car model produced by Chrysler through 1989 and in trucks and Jeeps through the 1990s. Some truck versions were designated as 8 3/8" or 8.375", but many internal parts interchanged with the 8.25" unit. It should be noted that all 8.25" rear axle assemblies DO NOT necessarily interchange across all models due to different widths.

Built by the Dana Corporation, the housing is cast iron with tubular steel axle tubes pressed and welded in to form a complete carrier and tube assembly. The gear assembly is accessed by removing an inspection cover having 10 bolts. 8.25" axles use either the "7260" or "7290" style U-joints. All 8.25" rears had 3" axle tubes with large bolt pattern (5 X 4.5" bolt circle) axles.  Units with a 2.45:1 gear ratio have a special case that will not accept any other gear ratio. The axles are retained by a c-clip on either side.  All axles were 27 spline until 1997 when they were upgraded to 29 spline.

Although the consensus among many Mopar enthusiasts is that a Sure-Grip (limited slip) equiped 8.25" rear is good for up to 500 crank horsepower, it fares much better in street applications than the strip, and in lighter cars than heavy cars.  Open 8.25" differentials are more prone to fail at higher horsepower levels than Sure-Grip units.  The 8.25" axle has considerable aftermarket support since it was so widely used in the 1970s for performance use.

7.25" To 8.25" M-Body Swap (Originally posted by Malcolm)

If you currently have a 7.25" axle, the 8.25" axle will drop right onto the spring perch pad centers perfectly, requiring no modifications. On early M-body 8.25" rears, 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 mentioned in the old Forum, I run 7 inch rims and 225/70 15's without any problems and there is room for larger tires.

You will need:
  • The donor 8.25" rear. The best bet for locating a sure-grip equipped unit at this point are any police, taxi or fleet cars (see Cop Car Information ). Good donors that will be more difficult to find are 1978 And 1979 LeBaron, Diplomat and Caravelle (Canada) Station Wagons.  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). You can also 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.
  • If available, get the driveshaft from a donor vehicle with a wheelbase and transmission 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.

If your donor axle is from a cop car, it will have 11 inch drums, so just buy new 11 inch ones. If your donor axle has 10 inch drums, you should be able to re-use the brake drums, shoes and all associated brake hardware from your 7 1/4" axle.

It's also a lot safer to do a brake job on everything in the rear while you're at it. 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.

If you find a 727 equipped 4 door M-body with the driveshaft in it, the front yoke will not fit your 904 transmission’s spline. In addition the wagons will have longer drive shafts, which require shortening and re-balancing.

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. Note: NEAPCO (New England Axle Product Company) also has one with the zerk in one of the caps.

Factory axle dimensions (7.25" and 8.25" for all F/J/M cars:

  • Axle flange to flange: 54.34"
  • Perch center to perch center: 44.46"

Factory ratios offered down through the years include but are not limited to:

  • 2.26:1 - This ridiculously high ratio was found in later M-bodies.
  • 2.45:1 - This was the most common ratio for 1978-1979 M-bodies.  It also appeared in many fleet cop cars through 1989.
  • 2.76:1
  • 2.94:1 - This was also common. Usually found in M-body police/taxi/fleet cars.
  • 3.23:1 - This highly desirable ratio was not so common in M-bodies.
  • 3.55:1
  • 3.91:1

Aftermarket gear ratios available include but are not limited to:

  • 2.25:1
  • 2.45:1
  • 2.76:1
  • 2.94:1
  • 3.07:1
  • 3.08:1
  • 3.21:1
  • 3.55:1
  • 3.73:1
  • 3.90:1
  • 3.91:1
  • 4.10:1
  • 4.11:1
  • 4.56:1

Photos below:

  • Front view of an older 8.25" rear in an A-body (Courtesy of Moparts member V194)
  • Rear view of an 8.25" rear found in later M-bodies (Courtesy of Farley's member Woodvark)
  • Exploded view diagram of the 8.25" rear (Courtesy Chrysler RWD Factory Service Manual)

Sources:
« Last Edit: January 30, 2010, 16:39:03 by Reggie »
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