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M20 / AMC20 Unicorn Axle


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it seems finding info on this jewel is not too simple. as far as i can tell it:

 

-was only on 86 comanches with the metric tonne (ton) package

-has one piece axle shafts

-has a limited slip case

-has thicker axle tubes like on a full size jeep.

-29 spline

-ring gear 8.8+ ? (someone will have to help me here)

 

i wanted to start an information thread on this mythical critter for those who want to know more about it. please add information for those to search on later.

 

here are a few pix of mine. i figured i would snap a few as i left the comanche commuity (for now) ;)

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AMC MODEL 20

Used in 1976-and-later Jeep CJs, the Model 20 rearend is both strong and weak. An 8 3/4-inch-diameter ring gear provides strength; weaknesses are the housing itself and the axle-to-hub retaining method. Converting to one-piece axles or full-floaters gives this axle better stamina.

 

 

http://coloradok5.com/axleguide.shtml

 

is the 20 in the mj's different?

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The AMC20 as used in the CJs had two piece axles. The AMC20 in the MJ and FSJs had one piece axles. The tubes are slightly weaker than those on a D44, however a truss can fix that. On a side note the differential is used in the H1, so that should tell you all you need to know about the strength of the center section.

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as far as my information goes...I've seen two of them. I own one. both were 4.10 gears, but only one was limited slip. one was behind a 2.8l, the other is in my 2.1 turbo diesel.

 

 

metric tonne package only. longbed only (thus 86 only)

 

 

as posted, single piece axle-shafts rather than the splined hubs that AMC20's typically have.

 

 

the only gears I've heard of an MJ AMC20 having were 3.73's, 4.10's, and possibly 4.56 with the auto/diesel combo.

 

 

they are roughly the equivalent of a dana 44 in strength. parts are not that plentiful, and there is not much aftermarket support. brakes are special order, and parts stores think it's a dana 44 in '86 rather than an AMC20.

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as with a LOT of Jeep history, there always seem to be exceptions to the rules. :dunno:

 

My 86 is an MT truck and has 4.10s (2.8/AX-5) and an open diff (far as I know since I haven't opened it, there's no tag and it certainly gives no posi resistance).

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I can tell you what the parts books say:

 

1986/87 Jeep Wagoneer / Cherokee / Comanche parts book dated 1/88

 

 

Page 27.10.6

53001 536 C 1986 4.10 Less locking differential (Trac-Lok)

53001 539 CC 1986 4.10 With Locking Differential (Trac-Lok)

 

Page 27.15.6

83502 504 housing

 

Page 27.25.1 (Non-Locking) & 27.25.5 (Locking)

J8127 072 Gear set (only one set listed)

 

Ratio not listed for J8127072, but a quick search shows they are 4.10's:

http://www.wilsons4x4.com/parts_id.htm

 

Page 27.30.5

53001262 axle shaft

53001524 axle shaft assembly

 

 

 

 

The older parts book gets a little more interesting....

 

1986 Jeep Wagoneer / Cherokee / Comanche parts book dated 3/86

 

actually lists both types of Metric Ton* axles (M20 & D44),

but only the 4.09 ratio D44 (with, or without LSD).

Also, they don't call it a D44, choosing to call it a "10 bolt" instead.

 

I'm guessing they were planning to offer the "10 bolt" if M20 production got low, or maybe as a running change.

Anyone ever find a D44 in an 86'?

 

 

 

 

Other fun facts:

 

The M20's 8.875" ring gear only uses 8 bolts to hold it to the carrier (like a D35, D44's use 10)

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

The 29 spline axle shafts are bigger than an 8.25"s 29 spline shafts

(8.25" shafts fit inside an M20 spider gear, but M20 shafts will not fit inside an 8.25" spider gear).

 

Tho according to some sources, even our '1 piece' MJ axles, and the '1 piece' FSJ shafts are actually 2-pieces, welded together from the factory (instead of bolted together like earlier 2-piece shafts).

 

(it has also been said 84-85 D35-7 9/16" axles are welded 2-piece shafts).

 

 

Also,

I know it has just been posted that the D44 brakes are different from the M20's,

but according to the 86/87 Factory Parts Book, they are the same.

 

34.30.5 Brakes, Rear Drum... 1986 With AMC "Large" Axle

83502 387 (L) PLATE, Backing

83502 386 ® PLATE, Backing

52001 915 -2- DRUM, Brake

 

34.30.7 Brakes, Rear Drum... 1987 With Dana Model 44 Axle

83502 387 (L) PLATE, Backing

83502 386 ® PLATE, Backing

52001 915 -2- DRUM, Brake

 

 

The brake shoes are also the same (83502 385), but all of the brake & e-brake hardware is different,

could've just been a year to year upgrade thing tho :dunno:

 

 

*FWIW, the parts book says "Metric Ton", and not Metric Tonne" :D

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brakes are special order, and parts stores think it's a dana 44 in '86 rather than an AMC20.

Special order?

 

The brakes aren't the same 10" x 2-1/2" that were used with the Dana 44s? AMC was using those brakes all the way back into the 60s, why would they have made the MJ Model 20 different?

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Did you list your truck on Craigslist? i think i E-mailed you about this axle

yes i did, many people contacted me over it and i responded to everyone. only one did not come back... a lucky fella picked it up today.

 

meanwhile, back to the 'ol thread: this mt mj had the big spring pack and some kind of load sensing brake system on it too. interesting...

 

i almost feel bad parting it out because it is such an oddity..

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I'm the one that never e-mailed you back :oops:

 

I was looking for a front axle so i asked you about both,figured if the price was right i would get both of them even though i wasnt really after a cad d30.

 

i would have loved to get the 20 from you but i needed a front axle more at the time,guess i missed out :(

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  • 11 years later...

@Tim. Here's a photo of my AMC 20:

 

This one is sitting in my 1986 Jeep Comanche MJ Long Bed 4x4. I have had some of the members here decrypt the Vin and from what I have been told my Amc 20 is the unicorn that everyone is looking for as for the gear ratio this one has stock 4.10 ratio. @Pete M also has stated multiple times that my mj has the Dealership Add On: winch bumper as well. So unicorns do exists... 

Amc20_MJ.jpg

86_MJ_Front View.jpg

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6 minutes ago, ghetdjc320 said:

 
I’ve come across many amc 20’s in full size Jeeps. Decent axle but I wouldn’t spend much effort hunting one down.

 

I would. It's even stronger than the Dana 44. The downside is that it's also heavier and offers less ground clearance than the Dana 44.

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15 minutes ago, Eagle said:

 

I would. It's even stronger than the Dana 44. The downside is that it's also heavier and offers less ground clearance than the Dana 44.

Maybe stronger R&P than an original MJ 44 but not an HD 44 by any means. I suppose if you have an 86 mj and you want an oem upgrade. But if it’s just a strength perspective, I wouldn’t spend time hunting down an axle that only comes on one production year and will be hard to find and/or expensive. And if your going FSJ amc20 route than grab an Isuzu or passport D44 rear from a 98+ Which is a much stronger option not to mention easier to find and typically cheaper.

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