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There is nothing inherrently wrong with the 20, especially the later years such as what you have in yours. I agree, if its not broke, don't fix it. When it does go, dana 35's and 44's are bolt-in (assuming you find an MJ 44), otherwise you can look at dana 44's, ford 8.8's, etc and a weld-in/custom drivehaft/brakes type of job.

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Yes you have an AMC20. The AMC20 from the MJ is a very desirable axle.


You can ID your gear ratio by looking for the stamped code on the housing.




D 2.73 Open

DD 2.73 Trac-Lok

B 3.31 Open

BB 3.31 Trac-Lok

A 3.54 Open

AA 3.54 Trac-Lok

H 3.73 Open

HH 3.73 Trac-Lok

C 4.10 Open

CC 4.10 Trac-Lok



Most likely your MJ has a "C" stamped on it (4.10 ratio); as the majority of the AMC20's found in the MJ have that ratio.

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Don't believe all the above. The AMC20 axle is junk and should be scrapped. Nothing but a boat anchor.







So where do you live? I'll come get it. :yes: :cheers:



:rotf: :rotf: :rotf: :rotf: :rotf: :rotf: :rotf: :rotf: :rotf: :rotf: :rotf: :rotf: :rotf: :rotf: :rotf: :rotf: :rotf: :rotf: :rotf: :rotf: :rotf: :rotf: :rotf: :rotf: :rotf: :rotf: :rotf: :rotf: :rotf: :rotf: :rotf:

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"should" be 4.10 since that's all that was officially offered with the 86 metric ton package.




officially all MJ M20's were 4.10 geared, available with, or without trac-lock.



Is it a good axle?


The shafts are 29 spline, and one piece (unlike the AMC automotive & CJ axles).

They are slightly bigger than the 29spline shafts in a Mopar 8.25" axle.


The ring gear is just smaller than a Ford 9", and the same size as a 12 bolt Chevy (8.875").

The only bad part is they spec'd it out to use just 8 bolts to hold the ring gear to the carrier (most axles this size have either 10, or 12 bolts).


The pinion gear is bigger than a D44's, and the same size as a D60's pinion.


It uses a crush spacer to set the pinion bearing preload.

Easier, and more cost effective when they originally made them, but the crush spacer can be replaced with shims if needed for heavy duty applications.


This is the same ring & pinion set used in Military Hummers & Civi H1's.

Even tho Hummers don't get our cool 'pith helmet' diff cover, it's the same center section under the cover.


Typical M20 tubes are on the thin side, and the center castings are usually as well, but I've never seen an MJ M20 cut up to know if that's true for our version too.


The wheel bearings are tapered, preload type, that require setting up with shims behind one wheel bearing flange, and a special crosspin to be used in the differential.


AMC guys have set these up in 10 second race cars, but most people switch to something else for reliability when they start putting serious power to the ground.



I think it's a good axle. :thumbsup:

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to drain you'll need to pop off the cover (leave one of the top bolts in kinda loose to help prevent dropping the cover and spilling oil everywhere). takes plain ol' 75-90 gear oil (if it has a posi inside, then you'll want to add in the proper additive). for the life of me I don't remember where the fill plug is. :hmm: and the truck is over at my grandma's. poke your head around and I'm sure you'll find it. :thumbsup:

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