how much stronger is the 23 spline than a 21 really?

as the title asks, has anyone actually broke the 21 spline shaft? i have an 89 with a aw4/242 and the aw4 is making some serious whining when put in gear. have a feeling the front pump is going, but its been working good for over two years. so my dilema is either get a replacement aw4/? from the junk yard and upgrade to the 23 spline, or try and fix what i have. id hate to put money and time into something that is far inferior, but ive never actually read or heard anyone breaking the 21 spline. sure the 23 would "seemingly" be stronger, but by how much? this is a trail rig that is getting set up with 4.5 lift and absolutly no bigger than 33 tires ( probably 32s), and 3.55 gears in d30/44 combo for a little while. (4.56 planned later). so has anybody broke a 21 spline shaft or have any experience with this? please dont just comment in with "get the 23 spline, its stronger" without an actuall explenation why. thanks.

More points of contact around the axle spreads the load over a larger surface area.

Think: less splines = weaker = cheaper; more splines = stronger = last longer.

yes i understand that. but with the 21 spline aw4 trans still going strong in so many 200k+ XJs, is it really a weak link? and if i was talking about axle shafts id be more incline to upgrade even at a higher cost since the axle shafts tend to be stressed more. but im talking about a transmission tail shaft. yes it has been upgraded years back to 23 spline, but my question is, how are the 21 spline shafts holding up? we all know the d35 has its weak area and it has been proven time after time with broken shafts so i get it. but i have yet to hear of a broken 21 spline aw4 shaft, especially one that a 23 spline shaft probably would have held up to. think of it this way, if you had a d44 or a 9" and know the dana 60 is even stronger, would you make the investment for the d60, even though its only on a trail rig with 33s? if i was building it as strong as possible, then yes id go with the 23 spline, but is it really needed on something like i mentioned?

2stix said:

yes i understand that. but with the 21 spline aw4 trans still going strong in so many 200k+ XJs, is it really a weak link? and if i was talking about axle shafts id be more incline to upgrade even at a higher cost since the axle shafts tend to be stressed more. but im talking about a transmission tail shaft. yes it has been upgraded years back to 23 spline, but my question is, how are the 21 spline shafts holding up? we all know the d35 has its weak area and it has been proven time after time with broken shafts so i get it. but i have yet to hear of a broken 21 spline aw4 shaft, especially one that a 23 spline shaft probably would have held up to. think of it this way, if you had a d44 or a 9" and know the dana 60 is even stronger, would you make the investment for the d60, even though its only on a trail rig with 33s? if i was building it as strong as possible, then yes id go with the 23 spline, but is it really needed on something like i mentioned?

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How do you use your XJ? DD/Light offroad; Desert speedracer; Rock-crawling? You haven't had a problem to date with the 21 spline, odds are you may never. If you are building a more aggressive vehicle, and like to get home most of the time, then an upgrade might be in order.

Just one of those look at the facts, make up your mind based on what you want to do--or might want to do in the foreseeable future.

I can't speak to the strength differences, but one other thing to consider is the final OD ratio. The earlier AW4s had a lower ratio. It may be important if you're spinning crazy revs on the hgiway.

T = n · τ · w · l · d/2

where

T = torque
n = number of teeth
τ = material shear strength
w = tooth width
l = tooth length
d = pitch diameter

I don't have the measurements of the splines so I couldn't even do a rough calculation. The above formula is a rough calc. and doesn't take into consideration ( reversing loads, lubrication, axial movement or misalignment, load over the length, tolerance of fit between the two parts, etc) the list goes on and on.

joe_peters said:

How do you use your XJ? DD/Light offroad; Desert speedracer; Rock-crawling? You haven't had a problem to date with the 21 spline, odds are you may never. If you are building a more aggressive vehicle, and like to get home most of the time, then an upgrade might be in order.

Just one of those look at the facts, make up your mind based on what you want to do--or might want to do in the foreseeable future.

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HE said that it is a trail vehicle. And honestly, most of my XJs have been Renix-era with AW4's And never had issues with the trans.
Cost-wise, may or may-not be cheaper to find a used trans/possibly t-case to swap. If you pick-up just a trans, I would recommend finding a 21 spline and call it a day.

Johnnie Walker said:

HE said that it is a trail vehicle. And honestly, most of my XJs have been Renix-era with AW4's And never had issues with the trans.
Cost-wise, may or may-not be cheaper to find a used trans/possibly t-case to swap. If you pick-up just a trans, I would recommend finding a 21 spline and call it a day.

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Missed that! :wave1:

vt98xj said:

T = n · τ · w · l · d/2

where

T = torque
n = number of teeth
τ = material shear strength
w = tooth width
l = tooth length
d = pitch diameter

I don't have the measurements of the splines so I couldn't even do a rough calculation. The above formula is a rough calc. and doesn't take into consideration ( reversing loads, lubrication, axial movement or misalignment, load over the length, tolerance of fit between the two parts, etc) the list goes on and on.

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Jeez, it's just a Jeep, not a rocket. It runs, it breaks, it gets fixed, and so on.

Sorry but I may be UNDERthinking this, especially with all the variables, big rocks, little rocks, snow, sand, gravel, pavement and DRIVER.

I have never seen a transmission output shaft break, for what it's worth. Of course I've only really been working on them for about a year and a half now. Never seen a broken one in the JY either, but plenty of broken transfer cases (mostly housings and chains) posted on here.

The only reason I would go from 21 spline to 23 spline is that around here, it is very rare that you will see a 90 or older XJ in the JY, most of them rusted out and were crushed years ago. Finding spare parts for a 23 spline based drivetrain is FAR easier.

kastein said:

The only reason I would go from 21 spline to 23 spline is that around here, it is very rare that you will see a 90 or older XJ in the JY, most of them rusted out and were crushed years ago. Finding spare parts for a 23 spline based drivetrain is FAR easier.

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A most excellent point.

kastein said:

I have never seen a transmission output shaft break, for what it's worth. Of course I've only really been working on them for about a year and a half now. Never seen a broken one in the JY either, but plenty of broken transfer cases (mostly housings and chains) posted on here.

The only reason I would go from 21 spline to 23 spline is that around here, it is very rare that you will see a 90 or older XJ in the JY, most of them rusted out and were crushed years ago. Finding spare parts for a 23 spline based drivetrain is FAR easier.

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true, but it's also not that hard to swap the input shaft on a 23 spline t-case to a 21 spline input.
I've done it on a picnic table in a campground with none of the proper tools.

I would probably just find the lowest mileage AW4/231 combo I could and swap it all out as a unit.
It's already proven that the 231 is stronger than a 242 (the picnic table incident was a 242 that snapped a mainshaft), and the aftermarket support for the 231 is much stronger with SYEs and the like, something that you'll likely need eventually in a trail only rig.
The AW4 is not worth rebuilding, there are way too many good OEM units in yards to justify the price tag of a rebuild.
I would take this oppurtunity to upgrade the t-case, because you're much more likely to destroy a 242 than an AW4 output.

87manche said:

true, but it's also not that hard to swap the input shaft on a 23 spline t-case to a 21 spline input.
I've done it on a picnic table in a campground with none of the proper tools.

I would probably just find the lowest mileage AW4/231 combo I could and swap it all out as a unit.
It's already proven that the 231 is stronger than a 242 (the picnic table incident was a 242 that snapped a mainshaft), and the aftermarket support for the 231 is much stronger with SYEs and the like, something that you'll likely need eventually in a trail only rig.
The AW4 is not worth rebuilding, there are way too many good OEM units in yards to justify the price tag of a rebuild.
I would take this oppurtunity to upgrade the t-case, because you're much more likely to destroy a 242 than an AW4 output.

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now thats what i was looking for. someone who has some actuall experience. thank you. and some great points aswell came up. availability is definatly something to be concerned with. i have definatly read many times that the benefit of the 231 over the 242 makes a good idea to upgrade. and that is something i will definatly consider. can the cable driven speedo gear be swapped into a 23 spline 231? ive heard the best years for a somewhat simple swap into my 89 would be 92-95. i honestly dont know if i would miss the 242 full time benefit since i havnt drivin the thing except for the last few days around town. just finally got it going. it seems like a cool thing to have when driving in big bear with snow and ice on the ground, but that is such a small percentage of the time this vehicle will be used. mostly desert treking and exploring with my son. of course i would love to go on some local runs with chapter members, but if my jeep isnt capable of doing a certian trail, i have no problem finding another route. at my age ive done my share of trying to climb, menouver, and concour any and all obsticles. just building a decent trail rig that will hopefully be as reliable as possible. no matter what, stuff breaks. and if the 242 is a major weak point in the drive train, i will absolutly consider swapping to a 231. thanks again for all your comments, and please keep them coming.

The 242 is not as bad as some make it out to be, there is just more support for the 231, the 242 doesnt have the room to upgrade the chain due to the differential built inside. I would say if it works good now, then depending on what deals come up that you choose to go for, then consider swapping it out.

You could still use a 231 on those ice and snow covered roads, you just can't drive on dry pavement with a 231 without binding it up.

It really depends on what you're doing with it. If it's just for easy trailriding, then the 242 will likely hold up fine, but it does have a much smaller mainshaft diameter and is a more complicated case internally.

this is what happens when you beat on a 242.
http://www.youtube.com/watch?v=4OFfdB5EqvA

You can also get just the input gear from many of the gear vendors. Then you can adapt your 242 to a 23 spline AW4. Honestly though, for what you'd spend on the input gear, you could probably just buy the t-case that's mated to the AW4 in the junkyard.