Full Float Conversion - Hub and Spider Failure

[vuldub]

I have a 1952 M38CDN.  Engine was upgraded to v6 231.  Drive train is original.   Rear is a Dana44.   Last winter I bought a full float kit from Herm and installed it, so I could free wheel the rear hubs while flat towing.  I only drove about 200km on this setup and had a nasty failure.  I am putting it back together, but I cannot figure out why it failed and to simply put new parts in and expect a different result is just dumb.   I have consulted with jeep mechanics locally, on other forums and talked with length with Herm.  Nobody has a solution.  I installed a 19pline full float axle.  Replace the old 10spline side gears with new 19 spline side gears, new spider gears, new pin, reused old thrust washers.  At the hub, I use AVM locking hubs.

Symptoms:

Driving straight on flat gravel roads in 2wd (rear hubs locked) there would be a nasty clunk, engine would race until I reacted and eased off the gas, no power to the wheels (like it had popped into neutral – but it had not), easing off the gas would allow a re-engagement.   Start off once or twice in  40km.  By the end, it happened 30+ times returning to camp over 15km. 

Problems:   

At the Hub:  Upon removal of the locking hubs, I found lots of metallic dust in both hub ends (wheel bearings where spared – dust only inside the locking hubs).  Similar amounts on both the left and the right side.  The AVM hubs had the outer sliding ring cogs worn very badly (source of the metallic dust).  The AVM splined ring was good as was the spline end of the axle.  Wheel bearings were in excellent shape.  The bushing that the stationary hub ring (splined ring) rotates in had become extremely worn and sloppy – to the point that the sliding ring had trouble coupling with the splined ring since the worn bushing along the splined ring allowed it  to wobble a considerable amount.  Some videos about the hub problems: 

Hub Video 1

 

Hub Video 2

 

In the Carrier:  The differential oil was saturated with metal flakes – looks like metallic paint.  The spider gear cogs were worn significantly, such that the spider gear would move up and down ¼”.  The cogs on the spider gears were worn rounded in profile with significant wear on the outer chamfered edges of the cogs.  The side gears were not worn.  The side gear  collars and the axle tupe house where these collars rotate were not worn. Only wear was the spider gear.  Axle splines were perfect.

Question:

Why did the AVM hubs fail?  It appears they were popping in and out and near the end, they were having trouble coupling due to wear on the hub bushing.  Once engaged, there should be no reason for them to pop out.

Why did the spider gear fail?  The floating axle could have “pushed” laterally on the sidegear, forcing the spider to erode, but there no reason for this to happen. 

Action Plan:

I will install a new spider/side gear cluster and new thrust washers.

I have looked at Warn vs AVM hubs and two observations were made.  The AVM hubs use the return spring to engage the hub  whereas the Warn hubs use the return spring to disengage the hubs.  Maybe Warn hubs would stay engaged better.  Warn hubs are well lubricated on the bushing that the spline gear rotates about.  AVM hubs don’t appear lubricated at all there.  I will install Warn hubs this time.

 Thanks in advance. 

[Moses Ludel]

vuldub...I watched your two videos with interest and read your thorough account above.  From what I see, these AVM hubs are designed for an axle shaft that has a very specific and narrower "range" of end float or movement.  These free-wheeling hubs are designed for a front axle, and on a Jeep front axle like your Spicer 25 closed knuckle, the axle shafts have a finite range of end movement.  

In the design of any free-wheeling hub, the position of the splined section is calculated.  The steering knuckle and inner/housing thrust surface (closed knuckle axles) prevents the axle shaft from moving inward past a certain point; there is also a fixed limit on how far outward the shaft can move.  

Note:  OEM truck full floating rear axles have axle shafts with an outer flange that is an integral part of the axle shaft.  They don't float in and out at all.  There are splines at the inner end that engage the side gears of the differential;  at the outer end of each axle shaft, there is an integral drive flange that bolts securely and solidly to the wheel's drive hub.  This wheel hub is supported on a spindle by bearings like your front axle and your rear full floating conversion axle.

Your axle shaft(s), as you clearly note, can move or float inward and outward over a wide range without "stops".  This clear drawing (generously furnished by jeepdraw.com) from a vintage Jeep MB service manual helps clarify.  Note the very limited movement inward and outward of the axle shaft:  http://jeepdraw.com/images/jeepdraw/TM10-1513/TM-10-1513-FRONT-AXEL.pdf.  The axle shaft penetration of the drive plate, or the splined section of a free wheeling aftermarket hub, is fixed within a narrow range of end travel.  

So, how can you remedy this?  You have the cross-pin or spacer block within the axle's differential acting as an inner axle shaft stopping point.  An axle shaft cut to the correct length could narrow down the end play or movement to whatever overall range you want.  The outer axle shaft stop would be a solid stopping point at the end, whether a drive plate like the OEM design or a free wheeling hub's axle shaft splined section.

This is not the full picture, though.  The wheel hub would need a 27-splined section (27-spline like your free-wheeling hubs) that is stable and able to firmly stop the axle shaft's outward end float.  This arrangement could not interfere with the design of the wheel hub's engagement and clutching function.  You may not have a wheel hub design that can accommodate these rear axle shafts by providing an outward stopping point for the axle shaft.  Note the construction, design and stamina of the internal parts of the wheel hub.

Note:  I'm not familiar with AVM hubs, I've only used Warn products.  This is not to "bash" AVM, I'm simply not in a position to comment on how these hubs might or might not work with a full-floating rear axle conversion and the need to control axle shaft end float.

Footnote:  If you have a problem with the axle shafts not running "true", you would see marks or signs of rubbing where the axle shafts pass through the spindles.  Even with all of the hub wear, the axle shafts have stayed relatively centered.  This is critical, because the axle shafts must be straight for this arrangement to work at all.  

Additionally, you have no outer end support for the axle shaft like on a closed knuckle front axle or an OEM full-floating rear axle.  The flange-less axle shaft is supported at each end by splines.  This seems to work at the inside with the rugged 44 differential side gears, but you're asking for that same level of support at the outer hub, which is nowhere near as strong a design as the 44 differential's side gears.  Simply put, these hubs may not be able to withstand the torque, especially in 2WD mode when the rear axle has 100% of the powertrain's torque.  

The primary problem seems to be the wide range of end float on these axle shafts...Looking over the front closed knuckle image, it's immediately clear that the outer axle shaft splines (the short or stub axles) are in a fixed range of end movement.  Free-wheeling hubs are designed to provide stable spline engagement with a fixed range of end play at the stub axle.  By design the MB axle depicted has no stub axle end float once the stub axle's outer nut is secured.  Later axle shafts did float slightly end-wise but nothing like the degree of movement your rear axle shafts and these AVM hubs must address.  

As for the AVM hubs:  1) the AVM hubs are likely designed for the axle shaft splines to stay within a narrow and specific range of in-and-out travel, 2) these hubs are not designed to act as a "stop" for the end float of the axle shaft and 3) the axle shaft splines need to apply torque to the AVM hub splines in the same way they would if these hubs were mounted on a Jeep front axle.  Again, these AVM hubs are absorbing twice as much torque in 2WD as they would on the front axle of a "normal" Jeep 4x4.

Moses

[vuldub]

Hello Moses.

Thank you for a prompt and comprehensive reply.  I am a big fan of yours – my CJ Jeep  Rebuilders Manual has plenty of grease and oil stains on the pages.  I call it “the Jeep Bible”. 

The AVM hubs were selected based on the full float kit builder’s recommendations.  He felt they were stronger than the Warn hubs,  which I think I have refuted. I also feel that new Warn hubs are not of the same robustness in the splined ring bushing as Warn hubs from days gone past.

On the floating nature of this setup, I would like to note that I have a centering block that travels on the spider gear center pin.  That block holds the axle from moving inward too far.  As you can see in the video, when the axle is pushed as far it as it can go against the block, it has full spline contact on the free-wheeling hub.  At the hub side, the axle cannot move outward past its own splines on the splined hub ring.  Thus lateral movement in the floating axle does not appear to compromise any spine contact at either the side gear or hub and the float range is rather narrow.

I neglected to mention that I too had questions regarding the trueness of the axle run.  If the jeep had left the ground and violently returned to earth (before I owned it) then the axle tube may have become bent.  The tubes appear to be pressed into the differential housing and look to be a weak point in the design.  Thus I undertook to assess the trueness.  With everything out of the tube and the hubs/spindles off, I sighted down the tube.  It looked very straight.  Being an analytical scientist by occupation, I didn’t trust my eyeballs completely (as is turns out I think we have a very good perception of “straight” with our binocular vision… I digress).  I took a straight pipe and put it through the axle and observed and measured offsets left and right, up and down.  I looked pretty true to me. Then, I measured from the top of the axle tube outer flange to outer flange and then the bases.  Both measurements were identical.  I clamped a straight block onto each outer flange, to extend the surface up and down, which would accentuate any discrepancies – re-measured top and bottom, back and front – all measured the same. So it appears to be true.  Lastly there was no wear on the spindles where the axle shaft passes through.

Several thoughts come to mind as I read your reply. 

·         -The free-wheeling hub is required to act as a stop for the axle shaft.  Torque in the shaft will translate to lateral movement, thus laterally stressing the hub splined ring.  The weak point in that stress is the bushing that the spline ring rotates about.  That is the bushing that wore out so badly on my hub.   

·         -The free-wheeling hub is required to act as the outer end support for the axle shaft.  The inner end support is the side gear.  There is no bearing in this setup to provide weight support for the axle shaft.  These axle shafts are heavy, so gravity will push them down, and that weight alone may be enough to cause wear in the bushing of the spline ring of the free hub.  Hummm, now how would that impact the spider/side gears?

·         -Many of the full float kits I have seen installed on the internet use internal Warn hubs, not external.  Perhaps internal hubs are integrally stronger weight bearing the axle shaft and stopping outer float.

·         -I have purchase drive flanges which could be used instead of free-wheeling hubs.  They would easily support the axle shaft and provide a strong lateral float stop.  However, it defeats the purpose of having free-wheeling hubs for flat towing.  I could swap the drive flanges in upon reaching my destination.

Initially I was focused on resolving the free hub wear issues and that’s why I made the videos.  However, I also have excess spider gear wear to resolve.   See the "In the Carrier" section of my original post.   I do not want to reassemble with new spider/side gears only to ruin another set.  My current working hypothesis is that the wear in the hub was sufficient to allow the axle shaft to wobble, which translated to excessive wear on the spider gears.  If that’s the case, a stronger Warn hub may fix this issue or just go with drive flanges.   But if there’s another underlying issue, then it needs to be fixed. Thoughts?

 

Regards…Wes

[Moses Ludel]

Wes...I've responded to your comments below in red lettering...Trust this helps...Moses

3 hours ago, vuldub said:

The AVM hubs were selected based on the full float kit builder’s recommendations.  He felt they were stronger than the Warn hubs,  which I think I have refuted. I also feel that new Warn hubs are not of the same robustness in the splined ring bushing as Warn hubs from days gone past.

My only experience is with the robust, earlier Warn hubs.  Your insight here is apt, Wes.

On the floating nature of this setup, I would like to note that I have a centering block that travels on the spider gear center pin.  That block holds the axle from moving inward too far.  As you can see in the video, when the axle is pushed as far it as it can go against the block, it has full spline contact on the free-wheeling hub.  At the hub side, the axle cannot move outward past its own splines on the splined hub ring.  Thus lateral movement in the floating axle does not appear to compromise any spine contact at either the side gear or hub and the float range is rather narrow.

Good observation and attention to detail on limiting the end float of the axle shafts.  Thanks for clarifying that there is adequate spline engagement and limited end float.

I neglected to mention that I too had questions regarding the trueness of the axle run.  If the jeep had left the ground and violently returned to earth (before I owned it) then the axle tube may have become bent.  The tubes appear to be pressed into the differential housing and look to be a weak point in the design.  Thus I undertook to assess the trueness.  With everything out of the tube and the hubs/spindles off, I sighted down the tube.  It looked very straight.  Being an analytical scientist by occupation, I didn’t trust my eyeballs completely (as is turns out I think we have a very good perception of “straight” with our binocular vision… I digress).  I took a straight pipe and put it through the axle and observed and measured offsets left and right, up and down.  I looked pretty true to me. Then, I measured from the top of the axle tube outer flange to outer flange and then the bases.  Both measurements were identical.  I clamped a straight block onto each outer flange, to extend the surface up and down, which would accentuate any discrepancies – re-measured top and bottom, back and front – all measured the same. So it appears to be true.  Lastly there was no wear on the spindles where the axle shaft passes through.

Good approach...being scientific helps.  If you can measure with string lengths in "diamond", you can also determine cross-axle straightness at the outer housing flanges.  You likely have a true axle housing.  Axle shafts can be checked more simply by setting the splined ends on V-blocks and rotating the shafts slowly while measuring runout at the middle with a magnetically mounted dial indicator on a flat steel bench top.  

Several thoughts come to mind as I read your reply. 

 

·         -The free-wheeling hub is required to act as a stop for the axle shaft.  Torque in the shaft will translate to lateral movement, thus laterally stressing the hub splined ring.  The weak point in that stress is the bushing that the spline ring rotates about.  That is the bushing that wore out so badly on my hub.   

Makes perfect sense.  Visualize a shaft with torque applied and delivering that torque strictly to the AVM free-wheeling hub spline.  Inertial load says that the splined section wants to move—including laterally/radially, and that's what is tearing up the hub.

·         -The free-wheeling hub is required to act as the outer end support for the axle shaft.  The inner end support is the side gear.  There is no bearing in this setup to provide weight support for the axle shaft.  These axle shafts are heavy, so gravity will push them down, and that weight alone may be enough to cause wear in the bushing of the spline ring of the free hub.  Hummm, now how would that impact the spider/side gears?

As the hub tears up and moves out of center, this of course impacts the direction of load on the different side gear.  But the shaft is fairly long (on one side, anyway, with your offset differential on the M38 and its side-drive Spicer 18 transfer case).  The long shaft would place little oblique load on the side gear.  The short shaft would seemingly suffer more, but the long shaft may move with more radial force.  Academic point if both hubs are failing, a minor point if one hub is taking more abuse than the other.

·         -Many of the full float kits I have seen installed on the internet use internal Warn hubs, not external.  Perhaps internal hubs are integrally stronger weight bearing the axle shaft and stopping outer float.

Not so much weight bearing, though that's a factor, but more the ability of the hub to keep the outer axle shaft on center without the hub disintegrating from the lateral or radial load and thrust.

·         -I have purchase drive flanges which could be used instead of free-wheeling hubs.  They would easily support the axle shaft and provide a strong lateral float stop.  However, it defeats the purpose of having free-wheeling hubs for flat towing.  I could swap the drive flanges in upon reaching my destination.

That's only a partial solution but obviously a good test.  I believe it will work nicely with the plates in place, as this is more like a one-piece axle with integral flange.  (If the outer end of the axle shafts had a threaded section and you added a washer and torqued nut outside the plate, you'd be emulating the front stub axle approach in the PDF diagram for the Jeep-Willys MB.  You would have even better support and no room for lateral shaft movement and flex at the plates.  In your case with the end float and movement of the axle shafts, the test will be how well the splines hold up in the plates and the axle shaft ends.  How will you keep axle lube from creeping out the splines at the plates?

Initially I was focused on resolving the free hub wear issues and that’s why I made the videos.  However, I also have excess spider gear wear to resolve.   See the "In the Carrier" section of my original post.   I do not want to reassemble with new spider/side gears only to ruin another set.  My current working hypothesis is that the wear in the hub was sufficient to allow the axle shaft to wobble, which translated to excessive wear on the spider gears.  If that’s the case, a stronger Warn hub may fix this issue or just go with drive flanges.   But if there’s another underlying issue, then it needs to be fixed. Thoughts?

So, you believe the spider/pinion gears failed because of the axle shafts running out of center?  That's actually not likely unless you also see wear at the side gears' hubs where they ride in the differential carrier housing—or if there is wear/wobble in the differential case bores for the side gears.  If the side gears run on true center, the only way pinion/spiders could fail would be undue inward thrust of the side gears.  Unlikely but something to consider or at least rule out.  If there is no binding or side tilt between the side gears and pinion/spiders, premature wear would raise suspicions about differential carrier wear or even the heat treating/metallurgy of the spiders and side gears.  Before blaming the differential gears, rule out tilt, bind or inward thrust caused by the flailing and jamming axle shafts.

Hardness and pitch/mesh analysis of the side gears and spider pinions would be a smart safeguard...

Regards…Wes

 

 

[vuldub]

Thank you Moses.  

I used Crown spider/side gears (which failed) but in my rebuild I will use Spicer (at the suggestion of a fellow jeeper).  Maybe the Spicer gears are harder.  

Not sure what you mean by  How will you keep axle lube from creeping out the splines at the plates? Is that a concern?

Cheers.... Wes

[Moses Ludel]

Wes, I don't like to bash products but thought you should at least take the machining and hardness (plus metallurgy) of the side gears and pinions/spiders into account.  Spicer is your assurance of quality and correct tolerances.

Your original semi-floating rear axle shafts with shimmed outer bearings had seals at the axle shaft outer ends with matching, machined seal surfaces on the axle shafts.  There were also inner axle shaft seals just outboard of the differential.  Do your conversion axle shafts have machined surfaces for inner or outer axle shaft seals?  If so, there would be no axle lube seepage out the axle shafts and through the spindles to the splined drive plates.  

What is the axle shaft sealing arrangement?  If there are no seals or machined sealing surfaces on the axle shafts, lubricant can migrate to the ends of the spindles and weep through the drive plate flanges.  Please clarify...

Moses 

[vuldub]

Moses.

The new full float conversion uses an axle seal in the axle tube end.  On the axle a smooth shiny spot has been machined where the seal runs.  The space is now empty where the old grease was pumped in from the grease nipple on the tube end.  Heres a photo

[Moses Ludel]

Wes...Thanks for the added pictures and clarifying...How did the axle shaft outer seals hold up with these axle shafts running out of center?  Was there axle lube outboard of the seals?

The seals should be more effective when you can keep the axle shafts on center.  Judging by your 10-spline original axle shafts, there were no inner axle shaft seals just seals similar to your current layout.  These outer seals, one per side, are the oil seals that prevent axle lube from leaving the housing, and that should work fine if you can control the radial movement or runout of the axle shafts.  

Looks like the key here is to get the axle shafts to ride on center.   That's the job of the free-wheeling hubs or drive plates.  

As for concerns about seepage at the plate splines, with this seal arrangement and the design of the aftermarket axle shafts, water and dirt could get past the outer splines (through the spline gaps) and into your wheel bearings.   A minute amount of wheel bearing lube might seep outward through the splines.  These 19/27-spline axle shafts "float" at the splines, there is no high tension axle shaft hub nut like the OE tapered axle shafts use. Though not much open space/gap, this arrangement is definitely not for stream fording.

There will also be some friction between the outer axle shaft splines and the drive plate splines with the axle shaft moving end-wise.  At the very least, I would apply wheel bearing grease to the splines to reduce wear, this will also help as a moisture barrier.  If I understand correctly, the drive plates will be removed and installed on a regular basis?  If so, you can clean the splines and add fresh wheel bearing grease...A bit messy and not perfect, the sealing issue is still there.

Moses 

[vuldub]

1 hour ago, Moses Ludel said:

How did the axle shaft outer seals hold up with these axle shafts running out of center?  Was there axle lube outboard of the seals?

Though not much open space/gap, this arrangement is definitely not for stream fording.

There will also be some friction between the outer axle shaft splines and the drive plate splines with the axle shaft moving end-wise.  At the very least, I would apply wheel bearing grease to the splines to reduce wear, this will also help as a moisture barrier.  If I understand correctly, the drive plates will be removed and installed on a regular basis?  If so, you can clean the splines and add fresh wheel bearing grease...A bit messy and not perfect, the sealing issue is still there.

Moses 

There was no leakage outboard of the seal and I had expected the seals to be in bad shape, but they were not, they were like new - I don't need to replace them.  Odd eh?  Thats good news in a way as the axle wasn't eroding the seal.

If the spindle is sealed (RTV) to the axle flange and  the free-wheeling hub is water tight (gaskets at both the hub and cap), then there shouldn't be a concern for stream fording.  If I use driveplates instead of free-hubs, then there is a gasket at the hub and a grease cap on the end.  How would water penetrate?  

The regular swapping of driveplates for free-hubs is not ideal, as you have pointed out another "con" - gasket reusage.  I might have to make a batch of gaskets.  I have looked harder at the Warn hubs and feel they are much more robust than the AVM ones, so hopefully they will stand up to the job.  The  weak spot is still the spline gear bushing.

[Moses Ludel]

Wes...Ah, the grease caps!...That would suffice if there's an adequate seal at the grease cap, and this would be the case with new caps, used ones if in top shape with good tapers, perhaps enhanced with a thin film of traditional Permatex Super 300 sealant.  Good call...This should work.

You do need a sturdy free-wheeling hub that can handle the torque (100% of powertrain torque when in 2WD) and also stay on center—plus hold up over time!  I like that you're researching contemporary hubs, this is smart and helpful to others, please share your findings.  In addition to Warn, there is Mile Marker:  http://milemarker.com/hubs/.  What vehicle pattern uses these 29-spline hubs with the same hub flange pattern (presumed Jeep CJ or military) as you have?  Is there a heavier duty "truck" application that will fit?

There are hubs that hold up to significant V-8 power in 4WD front axle mode, but again, the rear end has no torque split in 2WD mode: 100% of engine torque, plus the multiplied gear ratio torque, pass through these free-wheeling hubs.  I'm trusting others are getting the significance of rear full-floater conversions when run in 2WD versus a conventional 4x4 system with the free-wheeling hubs at the front only.  Some do the Dana 300 twin-stick, I did this on a Jeep CJ project for OFF-ROAD Magazine in the early '90s, which provides a mode for front end only 2WD.  I used Warn hubs for the twin-stick conversion and that was early-'nineties era free-wheeling hub technology.

I thought about another issue.  You describe the pinion/spiders going away quickly and the need to order new Spicer parts, which we agree is a good move regardless.  Are you aware that the side gears in your axle/differential have thrust shims that provide proper clearance between the pinion/spider teeth and the side gear teeth?  Did you set this clearance with the Crown differential gears?  Did that "kit" come with assorted size thrust shims to perform this procedure?  Will the Spicer kit include side gear shims?  The conical thrust washers behind the pinion/spiders are a fixed thickness, the tooth backlash adjustment is made at the side gear flanges (between the side gear flange and the differential case).  Here is the side gear/pinions/spiders adjustment procedure for your axle, assuming that you have a stock open differential:

Jeep Universal CJ Rear Axle Differential Side Gear Adjustment.pdf

As an interesting footnote, the WWII MB's full-floater rear axle, which carried into the early serial number CJ-2A models, uses an inner axle shaft seal arrangement like the front axles do.  These seals fit at each axle shaft just outboard of the differential case bearings, one at each side of the axle housing.  Your M38 axle is like the later CJ-2A, CJ-3A and CJ-3B; as you note, your axle does not use the inner axle shaft seals, the seals are at the outer sections of the axle shafts, inboard of the end flanges and the OE semi-floater axle shaft bearings.  Your photos depict this very well...

Moses

[vuldub]

On 2/13/2017 at 8:54 AM, Moses Ludel said:

I thought about another issue.  You describe the pinion/spiders going away quickly and the need to order new Spicer parts, which we agree is a good move regardless.  Are you aware that the side gears in your axle/differential have thrust shims that provide proper clearance between the pinion/spider teeth and the side gear teeth?  Did you set this clearance with the Crown differential gears?  Did that "kit" come with assorted size thrust shims to perform this procedure?  Will the Spicer kit include side gear shims?  The conical thrust washers behind the pinion/spiders are a fixed thickness, the tooth backlash adjustment is made at the side gear flanges (between the side gear flange and the differential case).  Here is the side gear/pinions/spiders adjustment procedure for your axle, assuming that you have a stock open differential:

Moses.  I did have the side gear thrust washers in place.  It was my understanding that there was only one per side as that's all that came with the cluster set.   In looking now, I haven't found a shim set for this purpose.  In the pdf there is a statement "measure the clearance between the side gears and the case as illustrated" but there is no illustration.  I found this one in my books.

[vuldub]

Moses.  I uploaded two video for you to watch:

https://youtu.be/X3-SDb37XOs

https://youtu.be/Id4kjOWqckY

Note the sidegear clearance in my old 10 spline with new thrust washers is 0.55mm and the new 19 spline  was 0.65mm.  The tolerance in the book you provided is a maximum of 0.15mm.  Is it possible they never began shimming the sidegears until 1967?  See this parts listing:

https://www.amazon.com/Omix-Ada-16512-20-Differential-Pinion-Bearing/dp/compatibility-chart/B000FQ1MTC

In all my books I do not have anything describing this sidegear shimming.  My M38 power train manual TM-9-1804B only shows one thrust washer in the parts diagram and repair description.   My "Spicer Axle Maintenance Manual" shows this:

[Moses Ludel]

Wes...If the differential carrier is not worn perceptibly at the side gear thrust surfaces, the OE thickness spacer shims are likely fine.  It is possible that your earlier application has no remedy for the pinion/side gear backlash, however, you should keep the information I furnished in mind.  The point:  Side gears and pinion/spiders must have a reasonable clearance and yet not have excessive backlash.  

Use your judgment and assess why the relatively new spiders failed or wore out.  Rule out excessive gear backlash.  If there is too much backlash, and if the later Jeep CJ service guide is our reference, see whether fit-up shims or at least a thicker side gear spacer is available.  Measure each side separately, you have the later reference for desired backlash.  If you need to reduce backlash, see if later side gear shims will fit the earlier side gears and differential case.

You don't want tight gears, nor do you want excessive backlash.  One approach that can help here is to use titanium dioxide (yellow gear pattern paste used for ring-and-pinion backlash checks).  Load the differential gears somewhat to get an accurate contact pattern, and note the pattern(s)...Excess backlash can usually be heard.  Too tight will overheat parts.

The service information I furnished does not suggest sticking two OE spacers together as an option.  It is understood that the OE spacers are only one spacer per side just like you installed them.  The sizing shims described, similar to axle pinion shaft and axle carrier bearing shims, apparently come in a variety of widths.  (I've always replaced worn parts, so this "kit" of shims is foreign to me.)  

There was not enough information to make sense of how shims are used...If only a few thousandths of an inch are needed as a correction, that would require a thin shim and not another spacer.  I gave some thought to this, and there is a major concern:  Logically, a somewhat thicker shim or spacer needs to face the moving side gear and also the carrier case thrust surface.  A thinner shim in either position would tear up in service...Even a stack of shims, in any orientation, is unlikely to work, as the the pinions/spiders and side gears produce considerable thrust load.

As you note, the OE spacers will only fit one per side.  In the case of wear, I could see a single, thicker spacer than the original being used in place of the original spacer—still only one spacer per side.  In any case, common sense says to avoid placing a thin shim(s) next to a moving side gear's shoulder or the differential case face.  A stack of shims is not a thrust spacer.

As described in the later Kaiser/Jeep CJ manual, this must have been a band aid remedy, a likely short-term solution.  The right solution for thrust surface, side gear and pinions/spiders wear would be either a single, thicker spacer at each side or a new case, pinions and side gears.  Realistically, the measurement of acceptable backlash is useful as a means for determining the degree of parts wear and when to replace parts.  Does that make better sense?

Moses

[vuldub]

Thanks Moses.  It's clearer now.