All (’76-89) FMJ-body differentials are interchangeable – with a couple of minor differences.
The ’76-81 FMJ 7¼” came with 2.5” (63.5 mm) tube diameters.
The 7¼” differential tube diameter bumped up to 3.0” (tapered) for ’82-89 M and J bodies.
The ’82-89 7¼” and all FMJ 8¼” use the same shock plates (3.0” (76.2 mm) axle tube).
The 76-81 7¼” differentials use a different shock plates (both parts/both sides - see red circle, below) because the axle tube diameter is smaller.
If you are replacing a 7¼” with an 8¼” into a ’76-81 FMJ, you will also need all of the shock plate parts as well.
Unrelated, but it would also be a great time to just upgrade the ISO-Clamp system to the older style shock plate to begin with.
Red circle is ISO-clamp (has 2 large black rubber pads) and Blue circle is the older style shock plate (no rubber pads).
A quick way to tell if you have an ’82-89 7¼” differential is the 3.0” (76.2 mm) tubes neck down to 2.5” (63.5 mm) just before entering the center section and looks something like this:
This picture only applies to the '82-89 tapered tube 7¼”.
The ’76 to mid-year (early) ’80 FMJ's have 58.5” (1,486 mm) track.
Chrysler considered track to be “mid-tire to mid-tire”. I believe it might be the same as WMS (Wheel Mounting Surface) to WMS – but not measured it to verify.
The late ’80 to ’89 have a 59.5” (1,511 mm) track (each side is 1/2” (12.7 mm) wider).
Both track sizes will interchange – but if you are using aftermarket wheels, you might want to check on clearance between tire-and-leaf spring as well as tire-to-fenderwell before calling the repair “done”.
To recap, the five (5) different differentials that came in (all) FMJ’s are:
- ’76 to mid-year ’80 7¼”, 2.5” (63.5 mm) tube, 58.5” (1,486 mm) track, has a 9-bolt cover.
- Mid-year ’80 to 81 7¼”, 2.5” (63.5 mm) tube, 59.5” (1,511 mm) track., has a 9-bolt cover.
- ’82-89 7¼”, 3.0” (76.2 mm) tube (tapered), 59.5” (1,511 mm) track, has a 10-bolt sorta diamond shaped cover.
- ’76 to mid-year '80 8¼”, 3.0” (76.2 mm) tube, 58.5” (1,486 mm) track, has a 10-bolt oval cover.
- Mid-year ’80 to 89 8¼”, 3.0” (76.2 mm) tube, 59.5” (1,511 mm) track., has a 10-bolt oval over.
The propeller shaft length is approximately 1.6” (40.6 mm) longer for the 7¼” differential OR 1.6” (40.6 mm) shorter for the 8¼” differential – depending on which way you look at it.
An example of a 8¼” (taken by a fellow forum member)
8¼” cover gasket
7¼” Cover Gasket
'76-81
'82-89
BudW
Note: the cover gasket pictures were "borrowed" from the internet.
The ’76-81 FMJ 7¼” came with 2.5” (63.5 mm) tube diameters.
The 7¼” differential tube diameter bumped up to 3.0” (tapered) for ’82-89 M and J bodies.
The ’82-89 7¼” and all FMJ 8¼” use the same shock plates (3.0” (76.2 mm) axle tube).
The 76-81 7¼” differentials use a different shock plates (both parts/both sides - see red circle, below) because the axle tube diameter is smaller.
If you are replacing a 7¼” with an 8¼” into a ’76-81 FMJ, you will also need all of the shock plate parts as well.
Unrelated, but it would also be a great time to just upgrade the ISO-Clamp system to the older style shock plate to begin with.
Red circle is ISO-clamp (has 2 large black rubber pads) and Blue circle is the older style shock plate (no rubber pads).
A quick way to tell if you have an ’82-89 7¼” differential is the 3.0” (76.2 mm) tubes neck down to 2.5” (63.5 mm) just before entering the center section and looks something like this:
This picture only applies to the '82-89 tapered tube 7¼”.
The ’76 to mid-year (early) ’80 FMJ's have 58.5” (1,486 mm) track.
Chrysler considered track to be “mid-tire to mid-tire”. I believe it might be the same as WMS (Wheel Mounting Surface) to WMS – but not measured it to verify.
The late ’80 to ’89 have a 59.5” (1,511 mm) track (each side is 1/2” (12.7 mm) wider).
Both track sizes will interchange – but if you are using aftermarket wheels, you might want to check on clearance between tire-and-leaf spring as well as tire-to-fenderwell before calling the repair “done”.
To recap, the five (5) different differentials that came in (all) FMJ’s are:
- ’76 to mid-year ’80 7¼”, 2.5” (63.5 mm) tube, 58.5” (1,486 mm) track, has a 9-bolt cover.
- Mid-year ’80 to 81 7¼”, 2.5” (63.5 mm) tube, 59.5” (1,511 mm) track., has a 9-bolt cover.
- ’82-89 7¼”, 3.0” (76.2 mm) tube (tapered), 59.5” (1,511 mm) track, has a 10-bolt sorta diamond shaped cover.
- ’76 to mid-year '80 8¼”, 3.0” (76.2 mm) tube, 58.5” (1,486 mm) track, has a 10-bolt oval cover.
- Mid-year ’80 to 89 8¼”, 3.0” (76.2 mm) tube, 59.5” (1,511 mm) track., has a 10-bolt oval over.
The propeller shaft length is approximately 1.6” (40.6 mm) longer for the 7¼” differential OR 1.6” (40.6 mm) shorter for the 8¼” differential – depending on which way you look at it.
An example of a 8¼” (taken by a fellow forum member)
8¼” cover gasket
7¼” Cover Gasket
'76-81
'82-89
BudW
Note: the cover gasket pictures were "borrowed" from the internet.