amc 20 question

pootie

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Joined
Oct 25, 2009
Location
charlotte
this is a newbie question, but it is axle specific. so...
Everyone recommends upgrading amc 20's to one piece shafts. Why? What breaks? I understand the two piece design, but i haven't had one apart. Is it just a keyway like what holds a flywheel on a pushmower engine, or are they splined together like the other end of the shaft is splined into the spider gear? When this key shears, does it destroy the shaft, the flange, both, or just the key? I've heard that LOTS of torque on the nut holding everything together will make them much less likely to break. Can anybody help explain these axles to me?
 
pootie said:
this is a newbie question, but it is axle specific. so...
Everyone recommends upgrading amc 20's to one piece shafts. Why? What breaks? I understand the two piece design, but i haven't had one apart. Is it just a keyway like what holds a flywheel on a pushmower engine, or are they splined together like the other end of the shaft is splined into the spider gear? When this key shears, does it destroy the shaft, the flange, both, or just the key? I've heard that LOTS of torque on the nut holding everything together will make them much less likely to break. Can anybody help explain these axles to me?
Click to expand...

The outer end of the shaft is tapered, keyed, and torqued to the flange with a big nut to around 350ft-lbs. Maybe there are some spline-like knurls on the end of the shaft, not sure. The key shears and then the shaft spins. Been there, done that, had to shift into 4Hi and pull myself home with the front axle. I can't say I've ever heard of anyone who's spun a shaft just replace the key and rock on...usually the one piece shaft upgrade occurs. I've also heard people claim they've been running "big tires" for years on a locked AMC20 with stock shafts, just be sure to regularly retorque the big nut.
 
if all that happens is shearing a key letting the shaft spin freely inside the flange, it seems to reason that the key could be replaced, fixing the problem. Granted most people in this sport see this as an opportunity to upgrade, but you could potentially break somehting harder to fix next time. Maybe that could be used as a sort-of fuse. Just thinking out loud now.
 
Your better "fuse" would be the drive shaft, all you need is a 1/2" wrench to swap it.
 
a driveshaft is an easy trail fix, but i was thinking a fuse to save more expensive parts, not necessarily easier to replace parts. a key, nut, and washer kit is $4.99 from 4 wheel drive hardware. if that is all it takes, i would gladly carry a socket everywhere. I already have a hi-lift handle for leverage. But if only cost 5 bucks to fix, i dont think 1 piece shafts for $250 would be very popular. There has to be more in there than a key to transmit torque.
 
found this

"The hub of the two piece shaft is normally splined and held on with a single nut. Often times the hub will spin on the end of the shaft ruining the outer splines of the shaft and allowing the shaft to spin freely in the hub. This is a common problem with the AMC 20 used in '76-'86 CJs. The solution is normally to upgrade to after market one piece shafts. Some people have reported that torquing the shaft nut way beyond specification will help prevent the hub from spinning on the shaft."

But i dont understand having a keyway if the shaft and hub are also splined.
 
Ive had one shear the shaft at the keyway and the whole outer works come off. It was at a stoplight in town. Jeep sat down on the tire/rim and bent up the rear quarter panel. Get the one-piece shafts and go on about your life.
 
Chuckman said:
Ive had one shear the shaft at the keyway and the whole outer works come off. It was at a stoplight in town. Jeep sat down on the tire/rim and bent up the rear quarter panel. Get the one-piece shafts and go on about your life.
Click to expand...

I just bought a 20 with moser shafts installed. Previous owner had a bearing failure and the whole axle shaft (moser) came out at highway speed. his quareter panel survived, but only because he was riding on the brake drum. until it came off, then he was just grinding on the backing plate. well you get the idea. My point is, they all tear up. and they never do it at a convenient location.
 
The splines are very fine, almost like knurling, they're not like the splines that fit into the carrier. Hard to see, but here's a replacement 2 piece kit...
awww.jeepdoc.com_catalog_partspix_AMC2_PIECEKIT.jpg

Once it spins, I bet it tears up either the flange or shaft, if not both.
 
RenegadeT said:
The splines are very fine, almost like knurling, they're not like the splines that fit into the carrier. Once it spins, I bet it tears up either the flange or shaft, if not both.
Click to expand...

probably both.
 
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