Amc 20?

Amc 20?
T

Tugger

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Location
North West Missouri
Vehicle(s)
1985 CJ7 with a shot 2.5 four banger and a 4 speed. All original and bone stock.
First off, how strong is the AMC20 rear end and what kind of horse power can it handle? I'm putting a 350 Pontiac in my CJ7 that will be making 300 hp and the torque should be a little over 400 and I'm planning on a set of 31's. This is going to be a everyday driver that will see some playing in the mud once in a while. Other than that it shouldn't see much abuse.
Thanks in advance for any help.
 
The first thing you need to be aware of is that in stock form, the 2piece axle is a weak design...especially behind any kind of horsepower or torque.

That said, the corp 20 is a very strong axle once the 2-piece design is addressed. A 1-piece kit is cheap and a huge upgrade over stock. The next two concerns are the thin axle tube walls, they are prone to bend but generally running larger then 36" tires and heavy rock crawling is the culprit. The last problem is under torque, the tubes can spin on the housing. A simple fix with a welder solves that problem.

The 20 has a larger ring and pinion then a Dana 44 and axle diameter difference is negligable. Once the problems I stated are addressed, it is, on paper, a stronger axle.

Im running 430 hp and about 470 ftlbs of torque through a healthy 401 and have had no issues whatsoever with my 20. Im also running 36's, lockers, and a compound low tranny. Torque is abundant and Im not afraid to use it. My biggest issue is keeping front axles together.
 
^^What he said^^
I have the Superior 1 piece axles and a detroit locker behind my 350 with no problems after 6 years of abuse.
 
Thanks for taking the time to help. I was kind of wondering how good the 2 piece axles were.
Now one more question. I can't find a tag on the rear end for the ratio. Was a standard ratio used or did it depend on the engine that was in the Cj? If it helps it's a 1985 with a 2.5.
 
The two piece stock axles tend to strip the splines where the hub is attached. It doesn't seem to take much to spin one.

Most 4 poppers came with 4:10 gears. But with jeep you never know.
 
Old Dog said:
The two piece stock axles tend to strip the splines where the hub is attached. It doesn't seem to take much to spin one.

Most 4 poppers came with 4:10 gears. But with jeep you never know.
Click to expand...

yep! i spun one on a stock CJ7 in city traffic.

the latest addition to my Jeep collection is bone stock and came with a spun axle. that's part of the reason the owner sold it.... didn't know what was wrong and wasn't going to pay a shop to look at it (score for me :D)

that's two spun 20's on bone stock AMC 258 i6 / 4.2l 's with tires no larger that 235's.

after a Moser one piece upgrade my first 20 was a beast and took everything i could throw at it. big tires, rocks, mud, hill climbs and general youthful neglect.

my latest project will be getting a one piece upgrade. if i were in your shoes i would upgrade to a one piece and forget about it.
 
I have an chevy 350 with 350HP and over 400 Ft of torque running a model 20 with Moser axles for over 15 years. I beat it good and the axle is going strong.
 
Tugger said:
Thanks for taking the time to help. I was kind of wondering how good the 2 piece axles were.
Now one more question. I can't find a tag on the rear end for the ratio. Was a standard ratio used or did it depend on the engine that was in the Cj? If it helps it's a 1985 with a 2.5.
Click to expand...
Like Old Dog said, it's "probably" 4.10's....my 84 originally had a 4 banger, 5spd, and 4.10's
 
I've had all the bad experiences above and spring wrap too. I don't know much about mud, but that torque on leafs could cause u to break lots of u joints, dshaft or pinion. All but my pinion broke, but teeth were chewed when I pulled to 456 it and add ladder bar/truss.
 
I had a 81 CJ7 donor jeep that had a Iron Duke, SR 4sp., & 3.54 gears. tag for gears was on front diff.
 
If the tags not there, you can just jack up rear and put in nuetral. Spin tire a full rotation and have someone count drive shaft turns. For a more accurate measure do two spins and divide by two.

My buddy thought he has 3.73s for a couple years, was 3.07 LOL!
 
Tugger said:
Thanks for taking the time to help. I was kind of wondering how good the 2 piece axles were.
Now one more question. I can't find a tag on the rear end for the ratio. Was a standard ratio used or did it depend on the engine that was in the Cj? If it helps it's a 1985 with a 2.5.
Click to expand...

Like previously said it should be 4.10

heres a sight that go's over the gear ratios for the axles.. truly the best way figure it out is to get the numbers off the ring gear but you can use this for a quick guide

Jeep Axle: AMC 20

It says the stamp should be on the left side ofthe diff casting before the tube starts but on my CJ its on the right side. there is also a x&y variant that is not mentioned

According to the FSM it's 3.07. The letter code for a 3.07 standard diff is "X". With Trak Lok it is "Y"

you can also look at the tag on your front diff if its still there.
 
Jimbos76cj said:
Like previously said it should be 4.10

heres a sight that go's over the gear ratios for the axles.. truly the best way figure it out is to get the numbers off the ring gear but you can use this for a quick guide

Jeep Axle: AMC 20

It says the stamp should be on the left side ofthe diff casting before the tube starts but on my CJ its on the right side. there is also a x&y variant that is not mentioned

According to the FSM it's 3.07. The letter code for a 3.07 standard diff is "X". With Trak Lok it is "Y"

you can also look at the tag on your front diff if its still there.
Click to expand...
Thanks for posting that site! Lots of good info on there.:)
 
Several of you great jeep owners have posted about an upgrade for the 20.... I have searched a few places for the one piece upgrade... could any of you help point me in the right direction? any help would appreciated...
Thanks guys
 
Jcwhitney.com
Morris4x4.com
4wd.com and lots of others.
Jcwhitney will give u 20% off coupons occasionally when u sign up and u can save some cash if u not in a hurry. I got mine there for about $220 shipped. I think u get a discount for being part of this forum but not sure how. There are different brands so some may be stronger but I don't know who's best.
 
Damnitutk said:
Several of you great jeep owners have posted about an upgrade for the 20.... I have searched a few places for the one piece upgrade... could any of you help point me in the right direction? any help would appreciated...
Thanks guys
Click to expand...

The best ones are Moser or Superior if you are running 33's or bigger I go with one of them. Summit racing usually has good prices,
 
Old Dog said:
The best ones are Moser or Superior if you are running 33's or bigger I go with one of them. Summit racing usually has good prices,
Click to expand...

Yep. I got my Superior axles through Summit years ago. To my knowledge, they still offer them.
 
Old Dog said:
The two piece stock axles tend to strip the splines where the hub is attached. It doesn't seem to take much to spin one.
Click to expand...

Why not weld the hub onto the axel?
 
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