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Old 11-14-2011, 06:23 PM   #1
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Locking the AMC 20

81 CJ8 with the stock AMC 20 running 33x12.5x15 duratracs

Now keep in mind this is my 60 year old parents Jeep and they are really not going to do anything crazy. They want this Jeep for the area around their cabin up in Ruidoso NM.

So I finally talked him into getting rid of the 2.73 gears and going 4.10.
Next question. Will the AMC 20 rear axle handle a limited slip differential
with this setup?

Difference in price between an open and LS rear carrier is $70.00 more for the LS.

If the axle will handle it then we are going with the LS.

Thanks in advance

Monty

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Old 11-14-2011, 06:33 PM   #2
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I would think so, if its just LS. I am not too familiar with the AMC20, but I know a buddy of mine ran 33s on an AMC20 with a hard locker, which puts more stress on stuff than a limited slip and it never broke on him. Since you would only be going with a LS, it would probably hold up fine.

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Old 11-14-2011, 07:54 PM   #3
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You will be fine. I'm running a lunch box locker. If you haven't you need to upgrade to 1 pc axles...those will break. Going from a open diff to anything does add strain but with 33's and a calm driver it will last many years.

I'd say a detriot trutrac is a good LSD to go with. Both front and back.
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Old 11-14-2011, 08:00 PM   #4
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Been a while since a really followed CJs, but if I remember correctly, it should. The AMC axle is actually stronger than the 44 ( larger gears) either the exception of the 2 piece axle. So if the axles have need changed to 1 piece axles, definitely. Even with 2 piece axles, should hold up unless you start rock buggy competition.

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Old 12-10-2011, 07:56 AM   #5
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X2 to what they said. I've ran the amc since it mine was purchased in '85. The solid axles are a great idea. But you're not competition slamming the poor guy either. The axle will be fine.
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Old 12-10-2011, 09:36 AM   #6
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If keeping the stock shafts, just check for any play in the nut on the outer shaft make sure its good and tight. If worried just upgrade to the 1pc shafts. I think I ran moser or superior in mine.
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Old 12-10-2011, 12:01 PM   #7
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Quote:
Originally Posted by rykemc View Post
81 CJ8 with the stock AMC 20 running 33x12.5x15 duratracs

Now keep in mind this is my 60 year old parents Jeep and they are really not going to do anything crazy. They want this Jeep for the area around their cabin up in Ruidoso NM.

So I finally talked him into getting rid of the 2.73 gears and going 4.10.
Next question. Will the AMC 20 rear axle handle a limited slip differential
with this setup?

Difference in price between an open and LS rear carrier is $70.00 more for the LS.

If the axle will handle it then we are going with the LS.

Thanks in advance

Monty
Quote:
Originally Posted by John in Vegas View Post
Been a while since a really followed CJs, but if I remember correctly, it should. The AMC axle is actually stronger than the 44 ( larger gears) either the exception of the 2 piece axle. So if the axles have need changed to 1 piece axles, definitely. Even with 2 piece axles, should hold up unless you start rock buggy competition.

JohnEGp
Quote:
Originally Posted by Dempsey View Post
You will be fine. I'm running a lunch box locker. If you haven't you need to upgrade to 1 pc axles...those will break. Going from a open diff to anything does add strain but with 33's and a calm driver it will last many years.

I'd say a detriot trutrac is a good LSD to go with. Both front and back.
Quote:
Originally Posted by 4Jeepn View Post
If keeping the stock shafts, just check for any play in the nut on the outer shaft make sure its good and tight. If worried just upgrade to the 1pc shafts. I think I ran moser or superior in mine.
As typed earlier, the AMC model 20 rear end is a good differential. However, since AMC has been out of business for 25 years, aftermarket support is more limited than with Dana axles. This is compounded due to AMCs relatively low popularity for many years.

The AMC 20 has a couple of weak points, and a couple of strengths. Among the strengths, is the large ring and pinion. This differential is plenty strong for tires up to 36 inches, under most circumstances. Weakness include the 2 piece design of the axle. The axle shaft and wheel hub are held together by a crush nut, which requires massive torque to properly seat and hold the axle from spinning within the wheel hub. If I recall correctly, there is also a keyway, but the keyway has no capacity to keep the axle from spinning. For many years, and perhaps still, there were a variety of axle manufacturers building replacement one piece designed axles (semi floating). These axles eliminated the weak spot, but had difficulties with the axle tube seal. The seal was installed over the shaft, and then the bearings were pressed onto the shaft. It was not uncommon to see axle fluid (90wt) leaking into the brakes or on the wheel.

Another solution was to full float the axle shaft using a kit, such as Warn, manufactured. This bolted a spindle to the end of the axle housing, and used wheel hubs similar to a front differential. The full floated design works exceptionally well.

Other than the weak 2 piece axle, the AMC 20s other weakness was the axle tubes, which were press fit into the center section, and were prone to both spinning in the housing, and bending. It was a common thing to weld the tube ends to the housing, to help prevent spinning.

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