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Rear Wheel Hub Installation on AMC/Jeep CJ with the Model 20 Rear Axle

AMC Jeep Jeep CJ Jeep 4x4 Jeep 4WD Jeep repairs Jeep how-to Jeep CJ-5 Jeep CJ-7 Jeep CJ-8

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#1 Moses Ludel

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Posted 09 March 2014 - 09:03 PM

The axle shaft tapers have "teeth" that cut into the smooth surface of a new rear wheel hub casting.  There is a minimum beginning axle shaft nut torque on Jeep CJs of 250 ft-lbs. For a new hub, this is followed by tightening the axle shaft nut further until you achieve the factory-specified stick-out length of the axle shaft threads (beyond the outer edge of the wheel hub).

 

Note: I've attached the factory procedure in a PDF for those interested in the Model 20 AMC Jeep CJ axle shaft hub installation:

Attached File  AMC Model 20 Axle Hub Installation.pdf   120.03KB   16 downloads

 

See your factory service manual, it will describe this procedure for the Model 15.  For the Jeep CJs, I also go into this installation in-depth within my 1972-86 Jeep CJ Rebuilder's Manual (Bentley Publishers).  Note the way you take this thread stick-out measurement...

 

The amount of torque required on the hub nut can include the use of a 3/4" square drive impact socket and a long handle added to a 3/4" drive breaker bar.  I have used the handle from a hydraulic floor jack (very carefully!) to gain enough leverage with some installations.  Other installs require far less force than this. 

 

It's always about the correct thread stick-out measurement after tightening the nut. Those attempting to achieve the correct thread stick-out length with an impact gun, even a very strong one, may be surprised to find that the impact force is not sufficient to get the hub properly drawn onto the serrated axle shaft taper.

 

All of this said, the AMC factory recommendation is a new hub casting if/when the hub shows any kind of damage.  If you notice, brake work does not require hub removal, and the hub should, ideally, not be removed unless the axle shaft outer bearing requires replacement.  Many do get away with hub reuse if there is no damage to the hub taper or axle shaft "teeth".  If the hub has spun on the axle shaft, there is damage—the hub must be replaced.

 

The hub and axle shaft positioning should be marked before the hub is removed.  If you cannot or do not want to install a new hub, and the old hub appears okay, align the hub at its original position on the axle shaft.  Tighten the axle shaft nut to the initial torque.  I take this a step further and use the thread stick-out method to assure a safe installation. 

 

Once you have installed a hub to an AMC axle shaft in this manner, you will understand why Jeep owners who do not practice this method wind up with the hub spinning on the axle shaft's tapered end or, in the worst case scenario, the wheel hub and wheel/tire assembly can come loose! 

 

There are aftermarket "one-piece" replacement axle shafts available for the Jeep CJ 5x5.5" bolt circle hubs.  These one-piece shafts have a more conventional outer flange for the wheel studs and mounting the brake drum and wheel/tire assembly.  There are several suppliers for these Jeep CJ axle shafts.  However, like you share, your Eagle wheel bolt circle is smaller than a CJ Jeep pattern, and the axle shaft lengths are likely different between the CJ and your Eagle.  (Inner axle shaft splines may differ, too.)

 

As a footnote, the key and keyway on the AMC axle shaft are not intended for withstanding the driving torque at the axle shaft.  This key is mainly to facilitate the installation of the axle shaft nut in the manner I've just described.  (It may provide a very marginal safety factor, but looking at it, you can see that the size of this key cannot withstand axle torque or loads.)  

 

With a new hub, you are actually "cutting" teeth/splines into the smooth hub casting taper as you tighten the axle nut securely.  By securely, the only safe and trusted method is thread stick-out length.  Any attempt to use a torque figure (other than the starting torque of 250 ft-lbs for the CJ Jeep Model 20 axle shaft nuts) is futile.  Actual torque setting can vary from not much past the 250 ft-lbs minimum to the long handle leverage I mention.

 

Related information: Anyone remember the vintage Volkswagen air-cooled era rear axle shaft nuts and the use of a long leverage bar for tightening? AMC is not alone here. And there are the tapered axle shafts on vintage Jeep, Ford, Studebaker, I-H and Chrysler cars and trucks that require a hub puller to remove the wheel hub and brake drum. If you need that kind of puller for an AMC axle, get pointers from my OTC Hub Tool video, click here.

 

When Jeep owners do not follow this hub tightening procedure on the Model 20 axle in a CJ, and especially if they run oversized tires, the hub will spin lose.  This can cause severe parts damage or even the loss of the hub/wheel assembly.

 

Check your shop manual for the AMC AWD Eagle.  I'm curious what the thread stick-out measurement is for the Model 15...

 

Moses



#2 recomer

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Posted 10 March 2014 - 04:14 PM

Moses,

 

Great info! Copying and pasting this into my Eagle folder on my computer here along with the rest of the thread here. I'm posting the page from the FSM for the AMC Eagle and its as you stated, there is  a stick out length specified in the manual. 1 and 3/16th's of an inch. As I said, my intent is to preempt any future axle issues before it happens by wanting to swap out the axle before anything bad happens. Believe it or not, I drive this car quite a bit (its made a Maine to Montana to Virginia trip in the last 3 years and a few other thousand mile trips since I've owned it). Based on what you're saying, I'm gathering as long as I don't abuse it, nor try and run large tires on it, I should have a solid axle for the car.

 

Rich

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#3 Moses Ludel

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Posted 10 March 2014 - 05:33 PM

If you can live with drum rear brakes (like the CJs with the Model 20), this AMC 15 should hold up.  The hub/shaft arrangement is similar to the 20, and you'll now be able to assure safety here.

 

As they say, your AMC Eagle is a true "driver"!  With AWD, too...Fun, Rich...

 

Moses



#4 Kevin's 83-CJ8

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Posted 05 July 2014 - 01:35 PM

This is a great topic. I have had to change my bearings a couple times and have found that the main work involved is the hub. It is hard to get off and hard to get on. I have a friendly local garage that has an arbor press. They have been a huge help. Without that, I would be in trouble. I would like to get the new one-piece axels, but I always want to put the money somewhere else. I broke a ½ Inch breaker bar on the shaft nut. I had to borrow a ¾ inch to get the job done. I learned to keep an eye out for the cotter pin hole once you get it tight enough so you do not pass it and cannot tighten it enough to expose the hole again. If you back off the nut, you have to start all over again.

 



#5 Moses Ludel

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Posted 29 July 2014 - 06:51 AM

According to AMC, tightening is always to the stick-out length of the shaft threads beyond the hub flange.  This can take enormous torque to accomplish.  The starting torque is 250 foot-pounds, and that's to start only.  At that point, you use a ruler or depth gauge/bridge to carefully tighten beyond this point.  You tighten until the thread stick out length is correct...I've used a floor jack handle as an extension on a 3/4-inch drive breaker bar with a 6-point impact socket to reach final torque!

 

Anyone old enough to remember the air-cooled Volkswagen era knows about long breaker bars on axle hardware.  For loosening, I use my air impact gun, very strong.  For installing the nut to proper thread stick-out length tightness, air wrenches/guns seldom develop enough torque.  It's been a hand operation for me!

 

Moses





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