Frame width on MTVR MK23 vs M923 Independent tandem axles and Rockwell 5 tons tandem swap-able?

Superthermal

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Are the frame widths between the MTVR and the 900 series close or the same? Has anyone here swapped the independent tandem axles from the MTVR onto the 900 series? I am looking at this for several reasons axle ratio, locker, 7ton vs 5 ton, better CTIS???, rear steering???, space to run chains between 16.00 tires and 16" of suspension. If the frames are close and the sub frame could be attached or welded in place to the 900 series frame I could see this as a very attractive modification. Items to consider here would also be frame height to tire center-line would it be higher or lower. Also since the units are sub assemblies for the tandem axles they could be spaced to custom needs or additional units like three rear axles added to say a M927. Anyone have measurements of the front frame width and rear of each?
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Superthermal

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Why not just buy a MTVR?
I have been looking for one but haven't seen any for sale here in SS and any that are for sale on IRONPLANET are beat to death or are crazy expensive... more than all the mods I can think of on a M923. I am looking for an EMP proof unit too and from what I see the new motor in these MTVRs and trans seem to have a lot going on that may not be EMP proof. This is a priority to me. Not much will work after 9 million volts. I do like the MTVR if handed one for similar pricing as a M923 I would do it out of respect for my son who is a combat engineer in our Marines. It does solve a lot of the problems I see with the 900 series when wheeling.... it has more power, no wheel hop, tuff one speed transfercase, lockers, auto, 16.00s etc... yes it seems like a better deal for what I am trying to achieve. But more $$$. What do you see them selling for? The cargo version 6x6 or the 8x8 version
 

simp5782

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You are really over thinking this. Way too much.

Truck frame rails are generally all 34in.

939 parts are off the shelf civilian available vs MTVR doesn't have anything other than parts at the cat house and u joints/seals. So factor that in to when you gotta replace something on either truck.

I've modified 939s beyond what most people would do. There are always weak links to discover.

Why not look at a 916/920 first generations. Double framed. 400hp+. Tough transfer case. Lockers in both rear axles. Can handle 1600. Will go faster than a MTVR. Parts are off the shelf. Can swap a 13 speed in for more speed and simplicity pretty cheap. Much cheaper than a MTVR too.
 

Russ Knight

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I have been looking for one but haven't seen any for sale here in SS and any that are for sale on IRONPLANET are beat to death or are crazy expensive... more than all the mods I can think of on a M923. I am looking for an EMP proof unit too and from what I see the new motor in these MTVRs and trans seem to have a lot going on that may not be EMP proof. This is a priority to me. Not much will work after 9 million volts. I do like the MTVR if handed one for similar pricing as a M923 I would do it out of respect for my son who is a combat engineer in our Marines. It does solve a lot of the problems I see with the 900 series when wheeling.... it has more power, no wheel hop, tuff one speed transfercase, lockers, auto, 16.00s etc... yes it seems like a better deal for what I am trying to achieve. But more $$$. What do you see them selling for? The cargo version 6x6 or the 8x8 version
I am probably going to sell mine when I get it in good working order. For you to accomplish what you wish to do, and remain under the cost of a MTVR, IMO, to start with, you'd need to buy a non-running MTVR for parts. At current price levels, you will spend in the 6500- 9500 range, maybe more, for something that you won't have a clue about the actual condition of the parts you'll need. Then you'll need a 900 series truck, and unless you are fabrication whiz, you'll spend a small fortune on building the truck and fabricating all the conversion components. Plus, I'm not sure the 8.3 Cummins would generate enough power to make such a truck perform adequately. I know when I put 16.00's on my M925A2, it lugged it down significantly, even with the pump turned up on it. If you decide to buy a MTVR, you'll spend 30-40 thousand on a solid one in good running condition.
 

charlesmann

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If you are wanting to do it for S&Gs, by all means, and post the progress. As others have said, it will be a very expensive endeavor. It would be cheaper to just buy an mtvr. But as simp said, consider part availability for a cobbled together frankenstein rig.
 

Superthermal

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You are really over thinking this. Way too much.

Truck frame rails are generally all 34in.

939 parts are off the shelf civilian available vs MTVR doesn't have anything other than parts at the cat house and u joints/seals. So factor that in to when you gotta replace something on either truck.

I've modified 939s beyond what most people would do. There are always weak links to discover.

Why not look at a 916/920 first generations. Double framed. 400hp+. Tough transfer case. Lockers in both rear axles. Can handle 1600. Will go faster than a MTVR. Parts are off the shelf. Can swap a 13 speed in for more speed and simplicity pretty cheap. Much cheaper than a MTVR too.
SIMP, you are correct in all of these points LOL and YES absolutely I overthink everything LOL! It looks like you too are a tinkerer of all things so I feel at home here, and thanks for the 34" detail. Still mixing the parts in my mind. I like the smaller military units and the 916/920 are just huge for me...not that the M923 is small it is just smaller. The M35 is just a bit too small and has load limits but I am still considering it too with a front and tandam axle swap from a 900 series. I am mixing drive train and gear ratios to see if there is a sweet spot. Thanks for your knowledge on these things.

Yes, MTVR part availability is an issue, very understandable and the MTVR parts that are out there are very expensive too. Duly noted. Thanks again. Still very interested if anyone has tried this modification. Driveshaft, breaks, CTIS integration, locker switching integration, spring rate and wheel center-line etc... and if I did go this way MTVR 30000 transfer case mounting.....

Which brings me to to a question about the 55000 and the 30000 transfer cases. What is the center-line measurement difference in height between the two shafts. They both look giant but is the shaft spread on the 55000 significantly bigger in this respect? The 55000 input driveshaft center-line to the output front shaft center-line compared to the T-138, T1138 and the 30000 case. The 55000 had a significant spread increase over the T1138 in your rig and despite raising the case in the frame the front output shaft looked like it may have needed to travel upward to the front axle where stock would have been almost flat... I couldn't tell very well from the video you made but I was curious how that turned out. The center-line measuerments would be helpful to know if the case will need to move up and down from stock mounting positions to get the same drive shaft angle out to the front. I did think if I was to pick up a M927 with the longer wheel base I may be able to use the 55000 case with longer shafts and place the case in the middle of the skeleton braces of the rear cargo bed and lift it up high enough that it just misses the bottom of the bed. If this is possible so that I didn't need to raise the bed off the frame as much or maybe not at all. This may allow me to even out the shaft lengths too.Again thinking out loud.
 

simp5782

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SIMP, you are correct in all of these points LOL and YES absolutely I overthink everything LOL! It looks like you too are a tinkerer of all things so I feel at home here, and thanks for the 34" detail. Still mixing the parts in my mind. I like the smaller military units and the 916/920 are just huge for me...not that the M923 is small it is just smaller. The M35 is just a bit too small and has load limits but I am still considering it too with a front and tandam axle swap from a 900 series. I am mixing drive train and gear ratios to see if there is a sweet spot. Thanks for your knowledge on these things.

Yes, MTVR part availability is an issue, very understandable and the MTVR parts that are out there are very expensive too. Duly noted. Thanks again. Still very interested if anyone has tried this modification. Driveshaft, breaks, CTIS integration, locker switching integration, spring rate and wheel center-line etc... and if I did go this way MTVR 30000 transfer case mounting.....

Which brings me to to a question about the 55000 and the 30000 transfer cases. What is the center-line measurement difference in height between the two shafts. They both look giant but is the shaft spread on the 55000 significantly bigger in this respect? The 55000 input driveshaft center-line to the output front shaft center-line compared to the T-138, T1138 and the 30000 case. The 55000 had a significant spread increase over the T1138 in your rig and despite raising the case in the frame the front output shaft looked like it may have needed to travel upward to the front axle where stock would have been almost flat... I couldn't tell very well from the video you made but I was curious how that turned out. The center-line measuerments would be helpful to know if the case will need to move up and down from stock mounting positions to get the same drive shaft angle out to the front. I did think if I was to pick up a M927 with the longer wheel base I may be able to use the 55000 case with longer shafts and place the case in the middle of the skeleton braces of the rear cargo bed and lift it up high enough that it just misses the bottom of the bed. If this is possible so that I didn't need to raise the bed off the frame as much or maybe not at all. This may allow me to even out the shaft lengths too.Again thinking out loud.
You can not run the Oshkosh 30000 in a 939. Unless you just wanted to go everywhere under 40mph

It is an underdrive unit.

 

Superthermal

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You can not run the Oshkosh 30000 in a 939. Unless you just wanted to go everywhere under 40mph

It is an underdrive unit.

SIMP, you are correct again on the 30000 unit and if I used it a different trans w a .7ish OD would be mandatory. The only reason I am considering it at all is if it is smaller shaft to shaft to eliminate loss of ground clearance and moving the bed around. I would rather add an overdrive transmission than loose ground clearance.
I am absolutely in your same shoes thinking the 55000 is the best replacement t-case even with the size constraints... so I guess your the best person to ask on lengths etc. I would gladly sacrifice cutting a hole in the bed and raising the 55000 as high and off center as needed to ensure I lost no ground clearance over the original case. Or at least anything I did loose was only to the larger gear size and case from the center-line of the front output shaft if aligned identical to what the T1138 front output shaft was. If the 55000 had to go in with the bottom end shifted to the drivers side to ensure alignment of the front and rear drive-lines to near stock and the top of the 5500 was shifted to the center or slight passenger side to whatever gave me the least drive-line angle out of the transmission I think that would be fine with me and I think this 55000 could tolerate not being mounted perfectly upright. The lifting or removal of the spare tire carrier and the bed are secondary here for me.
So if I was to keep the original 8.3 and transmission which I am guessing is shorter than the Engine/CAT7155 you have, what are you thinking the length of the divorced shaft to be and would the rear drive-line be around 21-25" or near stock? If shorter than stock if I am going full suspension drop out say only the drivers side rear tandem axle is on a rock and the front tandem axle is being pressed down by the spring pack to its maximum drop is this going to allow the free movement without the drive shaft binding? I will go out tomorrow and measure the total distance between the transmission output shaft and the rear forward axle input yoke on a stock M923A2 and get the total span there to figure the working distance. If you can let me know what the thickness of the 55000 is to the center of the yokes front to back... or can I look that up somewhere? Additionally if you could let me know the center line distances from the input shaft center to the front output shaft center would be handy too if possible.
I very much appreciate your time and input.
Note after making an optimal mounting bracket to place the case in the best possition possible etc... it would be kickin to make it available to all our friends here who would love to punch their rig in reverse for the first time ever and do it without wishing and praying nothing breaks. :)
 

simp5782

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On an A2 truck you won't have to move your transfer case back. In its stock spot it will clear the bed. A2 Trucks motor/transmission overall length is about a foot shorter than that of an A1 truck. The A1 Trucks have a doghouse in the cab to compensate for the bigger engine, the A2s do not. You would want your jackshaft to stay the same length setting the transfer case input yoke there then altering the bigger shafts for the front and rear as needed. 927s have a center shaft on both front and rear drivelines so you only need to extend those.

The cat transmission is the same length as an Allison. I moved the transfer case back so I could have a stronger 4in diameter jackshaft and to get rid of the 62 series mechanics joints.

It's around 26in center input yoke to center output yoke with 1810 half rounds. 17in wide.
 

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simp5782

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55000 is a .98 high 2.66 low compared to the t1138 is 1.72 low. The 939 would only go .5mph in low reverse With the 55000 so it's useless.

The 30000 series case even with a .73 OD, which is standard in a 13spd transmission, on 1600s it would still be at 55mph at 2200rpms. A transmission With a .62 OD which is only in a 4500/4600 series and RTOO units would be 64. With a 3000 series Allison it would be 61.

You could not use 4500/4600 unit behind an 8.3 as they are SAE 1 bellhousings, 8.3s are SAE 2.
 

simp5782

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SIMP, you are correct again on the 30000 unit and if I used it a different trans w a .7ish OD would be mandatory. The only reason I am considering it at all is if it is smaller shaft to shaft to eliminate loss of ground clearance and moving the bed around. I would rather add an overdrive transmission than loose ground clearance.
I am absolutely in your same shoes thinking the 55000 is the best replacement t-case even with the size constraints... so I guess your the best person to ask on lengths etc. I would gladly sacrifice cutting a hole in the bed and raising the 55000 as high and off center as needed to ensure I lost no ground clearance over the original case. Or at least anything I did loose was only to the larger gear size and case from the center-line of the front output shaft if aligned identical to what the T1138 front output shaft was. If the 55000 had to go in with the bottom end shifted to the drivers side to ensure alignment of the front and rear drive-lines to near stock and the top of the 5500 was shifted to the center or slight passenger side to whatever gave me the least drive-line angle out of the transmission I think that would be fine with me and I think this 55000 could tolerate not being mounted perfectly upright. The lifting or removal of the spare tire carrier and the bed are secondary here for me.
So if I was to keep the original 8.3 and transmission which I am guessing is shorter than the Engine/CAT7155 you have, what are you thinking the length of the divorced shaft to be and would the rear drive-line be around 21-25" or near stock? If shorter than stock if I am going full suspension drop out say only the drivers side rear tandem axle is on a rock and the front tandem axle is being pressed down by the spring pack to its maximum drop is this going to allow the free movement without the drive shaft binding? I will go out tomorrow and measure the total distance between the transmission output shaft and the rear forward axle input yoke on a stock M923A2 and get the total span there to figure the working distance. If you can let me know what the thickness of the 55000 is to the center of the yokes front to back... or can I look that up somewhere? Additionally if you could let me know the center line distances from the input shaft center to the front output shaft center would be handy too if possible.
I very much appreciate your time and input.
Note after making an optimal mounting bracket to place the case in the best possition possible etc... it would be kickin to make it available to all our friends here who would love to punch their rig in reverse for the first time ever and do it without wishing and praying nothing breaks. :)
I used the shaft for a 931 transfer case to front tandem. It is 36in if I remember right. That was the shortest you could really go without having slip problems with the axle articulation. Plus I had an easy access to em.
 

Superthermal

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Cool! Thanks SIMP!
OK I made it back with some measurements that I did take rapidly on the way with the family to the movies so dont sell the house they are exact but they should be close enough for our estimation here :)
Measuerment made from the stock M923A2:
Rear driveshaft length 31" from center of U-joints
Front trans to T-case shaft "divorced" shaft length was 19 3/4".
The thickness front to back from U-joint in to U-joint out of the T1138 T-case was 28 1/2"
(Your measurement of the 55000 is 26", this means that we get 2 1/2" extra shaft either front or rear if everything else stays stock.)
This gives an approx length overall between the transmission rear shaft and the Rear axle Input shaft of 79 1/4"
The center line of the T1138T-case input shaft is 11 1/2" from passenger side inside edge of frame
The center line of the T1138T-case input shaft is 17 1/2" from drivers side inside edge of frame
This makes the stock T1138 T-case input shaft 3" shifted to the passenger side.
The distance between the T1138 T-case center of the input shaft to the center of the outputs shafts is 13 1/2"
The center line outputs shafts F&R in relative terms from the frame:
Center of shaft outputs to axles is 6 3/4" from the driver side inside edge of frame or 7 3/4 offset to drivers side
The height is not clear in my scribble note... not sure if the center of the shaft or bottom of the T1138 case was 4" below the frame or if this was the shaft center line from the bottom of the frame... I will need to go back to measure that. Also I think from what I wrote down was that the bottom of the case was 5 1/2 " higher than the bottom of the fuel tank frame mount. Sorry I missed that one.
So the 55000 is 17" wide and there is approx 29" between the frame rails gives us 6" of shifting max. To land the input shaft in the same center line of the previous case (not that this maters much since the input is going to be higher up in the frame with the 55000 and we anticipate mounting the case at an angle in any way to minimize the angle of the input shaft) we will shift the top input shaft center 3" to the passenger side and the bottom output shaft of the case 7 3/4" or possibly more to the drivers side.
Now all we need is two things... me to confirm the height of the output shaft of the T1138 in relation to the frame and the distance on the 55000 between the center input shaft and output shaft to know how high the case will be than the old case to land the output shafts identically in the same place. This will also let me know how much angle will be on the shaft between the trans and t-case. Ill confirm that measurement tomorrow. I am also going to confirm from the center line of the output shaft how much "stock headroom" is available and this will let me know how high the bed would need to move out of the way.
Thanks guys.
 

simp5782

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Cool! Thanks SIMP!
OK I made it back with some measurements that I did take rapidly on the way with the family to the movies so dont sell the house they are exact but they should be close enough for our estimation here :)
Measuerment made from the stock M923A2:
Rear driveshaft length 31" from center of U-joints
Front trans to T-case shaft "divorced" shaft length was 19 3/4".
The thickness front to back from U-joint in to U-joint out of the T1138 T-case was 28 1/2"
(Your measurement of the 55000 is 26", this means that we get 2 1/2" extra shaft either front or rear if everything else stays stock.)
This gives an approx length overall between the transmission rear shaft and the Rear axle Input shaft of 79 1/4"
The center line of the T1138T-case input shaft is 11 1/2" from passenger side inside edge of frame
The center line of the T1138T-case input shaft is 17 1/2" from drivers side inside edge of frame
This makes the stock T1138 T-case input shaft 3" shifted to the passenger side.
The distance between the T1138 T-case center of the input shaft to the center of the outputs shafts is 13 1/2"
The center line outputs shafts F&R in relative terms from the frame:
Center of shaft outputs to axles is 6 3/4" from the driver side inside edge of frame or 7 3/4 offset to drivers side
The height is not clear in my scribble note... not sure if the center of the shaft or bottom of the T1138 case was 4" below the frame or if this was the shaft center line from the bottom of the frame... I will need to go back to measure that. Also I think from what I wrote down was that the bottom of the case was 5 1/2 " higher than the bottom of the fuel tank frame mount. Sorry I missed that one.
So the 55000 is 17" wide and there is approx 29" between the frame rails gives us 6" of shifting max. To land the input shaft in the same center line of the previous case (not that this maters much since the input is going to be higher up in the frame with the 55000 and we anticipate mounting the case at an angle in any way to minimize the angle of the input shaft) we will shift the top input shaft center 3" to the passenger side and the bottom output shaft of the case 7 3/4" or possibly more to the drivers side.
Now all we need is two things... me to confirm the height of the output shaft of the T1138 in relation to the frame and the distance on the 55000 between the center input shaft and output shaft to know how high the case will be than the old case to land the output shafts identically in the same place. This will also let me know how much angle will be on the shaft between the trans and t-case. Ill confirm that measurement tomorrow. I am also going to confirm from the center line of the output shaft how much "stock headroom" is available and this will let me know how high the bed would need to move out of the way.
Thanks guys.
Attached

Stock case angle in green with shaft installed.

55000 case with no shaft installed.

Video is the shaft from the CAT to the 55000. Remember the cat is higher on the frame than the stock Allison since it is an SAE1 unit.

 

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