Axle shaft broke 8 3/4

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pjc360

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i know this is a car site but I figured you guys would have a lot of knowledge about the 8 3/4 rear axle and might be able to help me identify why my axle shaft just up and broke in half.
I have a 1991 dodge short bed 4x4 truck that originally had a 9 1/4 in it.
The 9 1/4 was worn out and I decided to throw an 8 3/4 in it that i had in a 74 w100.
It held up great over a year untill today.
I'm driving down a back road going about 20-25 mph when all a sudden the truck starts hopping a little and then I felt what I thought was the transmission slipping.
So I pull over and check transmission fluid level and it's good and the fluid looks good and smells good. I have the truck in 2hi and idling in nuetral and I look down and notice my rear shaft is spinning while the truck is in nuetral.
I thought that was odd, so I lock the hubs in and out the t-case in 4hi and it moves, so I start to pull back on the road and all of a sudden bang! The truck falls on the ground and my drivers side rear tire goes rolling by.
I had to call a tow truck and When I get the truck home I pull both axle shafts and the third member expecting to see some gernaded gears or something.
And when I get the third member off I notice there isn't anything wrong with the gears, so what happened?
The only thing I noticed is the bearing race wasn't on the bearing on the drivers side axle shaft, the one that broke in half.
I don't knowing that happened before it snapped or after.
This is kind of scaring me cause I want to know what happened before I throw it back together cause I don't want that happening on the highway cause that would be very ugly.
Any idea's?
 
ssba said:
Tapered roller bearings or ball bearings?
Click to expand...

I believe they are tapered roller bearings, they have the bearing race and they have the axle shaft adjustment nut on the passenger side axle shaft.
They were the original style axle shaft bearings, I never switched over to the green bearings or whatever you call them.
 
Probably lost the bearing first then the axle shaft.
You may be able to wash the metal out of it and get an axle and new bearings if the gears still look good.
Its gotten hard to find 8 3/4s in my neck of the woods. Their a bit light duty for a 4x4 also in my opinion.
Used 9 1/4s are still plentiful. Its what I would try first for a cheap and fast repair.
 
you did put the same ratio in when you replaced the complete axle ?
 
Ya it's running a 741 case with open 3.55 gears.
I bought a 742 case that has 4.56 gears and I'm going to install them along with a new axle shaft.
I just don't know where I'm going to get a new axle shaft.
I have a spare axle shaft sitting in my storage unit but it's got the passenger side adjuster nut on it.
Does it matter which side the axle shaft goes in? Or can I remove the old bearing and adjustment nut from my spare axle shaft and put a new bearing on it and then put it in on the drivers side?
 
So it doesn't matter that my spare axle shaft was on the passenger side of another 8 3/4, I can just remove the bearing and the adjuster nut and put a new bearing on it and put it in on the drivers side?
 
You can even leave the adjuster on there. just back off the adjuster,stick it in there, bolt it up, making sure there is end play as you go. When she's fully seated, tighten up the end play to spec. The other side wheel should be in the air, during the install.
To answer your question, the axles are identical.At least they were when new. Check the splines.
To figure out why it broke, take the broken parts to a specialist; or post up some pics of the broken ends. Maybe someone on FABO is a specialist; not me.The pics should tell the story.
What width rims? with what backspace? and tire size?
 
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Well I was thinking I would remove the adjuster nut on my spare axle shaft anyways because I'm going to put new bearings on both axle shafts anyways.
Cause the spare axle shaft I have that has the adjuster nut on it the bearing is rusted solid on it and I don't think it would be a good idea to try and use it.
And since I had bearing trouble on the drivers side axle shaft I figure I might as well replace the bearing on the passenger side axle shaft as well since I already have it pulled.
And Before pulling the passenger side axle shaft I felt some movement in the bearing that was never there before.
I think it happened because of the drivers side axle shaft breaking.
 
i think the 70-72 c body 8 3/4 axles are the same or very close to the same length and weight rating as the p/u . i know the the housings are the very close to the same width , had them side by side , if there where no spring mounts you would have a hard time telling them apart .
 
I have an extra axle shaft with the correct wheel bolt pattern.
It just had the adjustment nut on the end. That's why I asked if I can just remove the adjustment nut when I put a new bearing on and then put it in on the drivers side.
And I noticed the passenger side wheel bearing was loose while the drivers side axle shaft was still in. It was broke but it was still in the axle.
That's why I think it may be a good idea to just do both wheel bearings while I have both the axle shafts out.
 
Well guys here are some pics of the axle shaft, I had a few people check them out and they said this wasn't a bearing a failure, the axle shaft was already broke and it just finished breaking.
If you look at the break of the axle shaft, there is a dark spot on the center of the axle shaft and then there is shiny spots on the edge where the fresh break happened while I was driving.

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Odd. Never seen that before. I wouldn't worry about it doing that again.
 
pjc360 said:
Ya it's running a 741 case with open 3.55 gears.
I bought a 742 case that has 4.56 gears and I'm going to install them along with a new axle shaft.
I just don't know where I'm going to get a new axle shaft.
I have a spare axle shaft sitting in my storage unit but it's got the passenger side adjuster nut on it.
Does it matter which side the axle shaft goes in? Or can I remove the old bearing and adjustment nut from my spare axle shaft and put a new bearing on it and then put it in on the drivers side?
Click to expand...
I assume you realize that if you change the rearend ratio you also need to change the gears in the front differential to match the new rear gear ratio
 
It does look like an internal metal flaw that has been present for a long time.
I worked in primary steel production in Gary Indiana, and eliminating gaseous inclusions in metal was a primary concern, because this is one result.
The metal probably had a porous surface at one tiny point, leading to the interior..
If the rear end sat for a long time before you used it, the internal rust in the axle finished it off. It happens. Have the other axle dynafluxed, xrayed, whatever you need to make sure it's not partly fractured and waiting to do the same.
Store axles in a warm dry place with grease on them.
 
The rusty spot at 2 o'clock is part of where the original flaw was, part of the same air bubble in the steel that is seen as the dark
area in the center of the axle.
The shiny part is what's held it together the last year.
 
And realize that the large + offset of those wheels puts a lot more bending torque on the shafts, and it concentrates the peak stress right at the wheel bearing.... which is where it broke.
 
nm9stheham said:
And realize that the large + offset of those wheels puts a lot more bending torque on the shafts, and it concentrates the peak stress right at the wheel bearing.... which is where it broke.
Click to expand...
Excellent point, since I keep things pretty datn stock, I wouldn't have thought much about it. You are quite correct!
 
If you remove the wheel bearings, you will probably find a nick in the shaft caused by a cutting torch.
 
Well my uncle is the one that looked at the pics of the shaft and said that thing was already broke then the shiny stuff is what broke on you.
Yea I realize when I switch gears in the back I'll need to switch the gears in the front as well.
 
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