Farley`s Dodge Diplomat Forum

Forum Index => The Library => Topic started by: Reggie on January 01, 2008, 21:31:18

Title: Rear Axles: Chrysler 8 1/4" Rear Axle
Post by: Reggie 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:

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:


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


Aftermarket gear ratios available include but are not limited to:


Photos below:


Sources: