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transfer case regear thoughts for M939 series

lindsey97

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I am interested in changing the gearing in the Rockwell T1138 transfer case to something besides overdrive.

Truck is a M923A0 with 16.00r20 tires on HEMTT rims. Looking for a way to get the final drive ratio back to stock, in order to gain performance.

So would having custom gears built be an option, or will parts from the earlier M809 5 ton series fit in the later case? Do the M809 series of trucks have overdrive in the transfer case?

I have an extra T1138 M939 transfer case to experiment on, and may take it to Inland truck parts in Oklahoma city to see what they think will fit inside.
 

gimpyrobb

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I have not compared them side by side to offer any better info.

On another note, when we swapped one into Opcom's m818 we had no clue they were overdrive till he got home, so I'd imagine they are similarly sized. I'd say the probability is high.
 

red

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No. The T138 case is physically larger than the T1138.

You can install an air switch to disable/enable the sprag on the T138 case. Would also be much cheaper than trying to get custom gears made. I have one that I pulled out of a m51a2 dump truck that I'll sell for a good price.
 

simp5782

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Going to bring this one back as I am looking at going to a stronger case but wanting to keep some road speed but still be able to utilize low range if I need to with the 16 speed.

The T1138 would blow apart in low range. I shouldn't need it but still. They are getting low on supply and no sense on blowing one up when someone can use it. The T138 I don't think would survive either. Plus the 1:1 high and low range is 1.98:1

I scored a HEMTT transfer case from Warren. Oshkosh 55000 High range .98:1 and 2.66:1 low range. Should not have the blow apart issues in low range especially when I go to a big cam with a lil more umph. Transmission should be fine. I see that is uses the standard spicer joint flanges so its easy on U joints to be mated. Even finding a companion flange should be simple.

I am looking to utilize it due to it being a cockeyed mounting on it to bring the front driveline angle down a little more so that my driveline angle around the CAT isn't so extreme. It is 140lbs heavier than the T1138 so just fabbing up a different mount system is easy. may have to lengthen a shaft or shorten one. Should not be an issue. It does not appear to be much thicker than the T1138.

with the .83 over drive on the CAT with the .98 high range and 16.00s I would be turning 70 @ 2250rpms. with the big cam it should have no issue holding up the top end.

Looked at the specs on the MTVR 7 ton case and unless you upgraded to the 7 speed Allison 4070P its a killer on speed. It is a single speed ratio at 1.271:1 it would limit my .83 to 52mph.
 
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74M35A2

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You could change out the trans to one with more gears. You could also add more power to your engine to push the lower ratio, or update the engine via a swap?
 

red

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Big Cams don't like regularly sustained over 2100rpm either (if wanting a long life out of it). Realistically no engine of that size likes being over 2000-2100rpm for long. The smaller engines like the 8.3L will last longer at those rpm's.

Supposedly the tcase from the m916-m920 has other gearing options available. Think it was Soni posting about it years ago.
 

simp5782

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You could change out the trans to one with more gears. You could also add more power to your engine to push the lower ratio, or update the engine via a swap?
I doubt the T1138 will take the power of a big cam with some umph thru the CAT without it going caput. No sense in breaking it when a heavier case like the HEMTT would benefit me. The T1138 front drive line angle is pretty difficult with the how the transmission sits. with a HEMTT case since they sit vertical. I could easy canter it to the drivers side to line up the front tandem axle plus the front driveline angle is more towards the drivers frame rail.

The PLS unit we believe has an overdrive unit built into it. Different transmission is out of the question. Too much work into this one to deal with all that over again. and a clutch is not gonna happen. You will see how old it gets in traffic or city driving. You may run into the same issue as the power going thru your transmission is going to be a lot more than before and something has to give. In your case your torque rods will be first. Ive blown T1138s in low range with a stock 250, G177s went a quarter turn, dug in and boom. nothing major. Seeing breaks as the T138 case doesn't have any case splits vs the T1138. its about internal pressure. You take yours to the offroad park and you pop that clutch trying to rock it out of a hole sometime and I bet something is going to give.

My plan is to go to a big cam so being that the T1138 was not put in the bigger 915 series trucks means it probably wont stand up to it. The 915 series truck transfer cases are massive. If not bigger than the HEMTT case. I know one guy here swapped the HEMTT case into a 916 since the supply of 916 cases is slim if not none. Plus it is only a single speed case.

Researching so far the PLS case is a lot different with its PTO unit, but the round drum on the rear of it has me confused. CSMDavis thinks its an overdrive unit. I found it in the TMs but it doesn't give me a NSN or description in figure 183. If its an OD unit for the 10x10 trucks I would be more than happy to go for it. The Axles are the least concern of anything as the 5 ton rockwells are well built.

The .98 in high range would allow me to run the 445/80/25s with the big cam and not loose any road speed hardly but still have strength.

As far as how I feel about my current setup. On the way to the rally 2 weeks ago I pulled up this 2lane mountain grade near signal mountain in Chattanooga. I was in 8th gear pulling up the hill at around 43,000lbs. it is an 8% grade for 5 miles from Dunlap thru to signal mountain. pulling not too bad actually. but you can feel the torque and jerk when shifting 8 to 9 or 7 to 8 while climbing a hill. its power to the ground. The 700 ft lbs of torque or maybe a little more from the little 250 is nothing compared to a big cam @ 1000 to 1250ft lbs. If you are going to up the power you gotta get everything in the class behind it. The HEMTT case stands up to the C15's 515hp and 1650ftlbs so it may be better game.
 

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simp5782

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Big Cams don't like regularly sustained over 2100rpm either (if wanting a long life out of it). Realistically no engine of that size likes being over 2000-2100rpm for long. The smaller engines like the 8.3L will last longer at those rpm's.

Supposedly the tcase from the m916-m920 has other gearing options available. Think it was Soni posting about it years ago.

Max big cam torque is 1250 and hp is 2150 and they split at 1550/1600. at 2150 I can still run 68 or so which is nice when you don't bog on hills.

I wouldn't say last long on high rpms. Mine has ran 2000 or higher up until 2 months ago and she has no longevity issues.
 

red

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Max big cam torque is 1250 and hp is 2150 and they split at 1550/1600. at 2150 I can still run 68 or so which is nice when you don't bog on hills.

I wouldn't say last long on high rpms. Mine has ran 2000 or higher up until 2 months ago and she has no longevity issues.
The 855 series engines in commercial rigs regularly go 700k-1 mil miles, or 15k hours in stationary units. That's the longevity I'm talking about and want yours to reach. They don't go over 2100rpm for long though in commercial use. Can the 855 series spin over 2100rpm? Yep, but they are known to throw a rod doing it all the time. Plus they inhale fuel fast at those high (for them) rpm's.

Agreed that a tcase upgrade is needed to use the first gear on your CAT trans, but so is a overdrive gear in the tcase that's better than .98 if you want to cruise around 70mph.
 

simp5782

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Trying to find out the PLS specs of the drum on it first. Hopefully its an od unit. As far as case options. They are limited. The Oshkosh 30000 series that the MTVR has actually is the same as the HET apparently so that is out of the question. Not sure of what the MK48s use. Other option would be to drop to a 5.81 ratio in the axles but those aren't easy to come by. 4.88s are more common actually. Really make it a morf for sure. I can check with oshkosh on the internals on the 55000A and see if it can be changed to a higher ratio.

For what I have into the HEMTT case is 1/3 of what the T1138s sell for.
 

quickfarms

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Going to bring this one back as I am looking at going to a stronger case but wanting to keep some road speed but still be able to utilize low range if I need to with the 16 speed.

The T1138 would blow apart in low range. I shouldn't need it but still. They are getting low on supply and no sense on blowing one up when someone can use it. The T138 I don't think would survive either. Plus the 1:1 high and low range is 1.98:1

I scored a HEMTT transfer case from Warren. Oshkosh 55000 High range .98:1 and 2.66:1 low range. Should not have the blow apart issues in low range especially when I go to a big cam with a lil more umph. Transmission should be fine. I see that is uses the standard spicer joint flanges so its easy on U joints to be mated. Even finding a companion flange should be simple.

I am looking to utilize it due to it being a cockeyed mounting on it to bring the front driveline angle down a little more so that my driveline angle around the CAT isn't so extreme. It is 140lbs heavier than the T1138 so just fabbing up a different mount system is easy. may have to lengthen a shaft or shorten one. Should not be an issue. It does not appear to be much thicker than the T1138.

with the .83 over drive on the CAT with the .98 high range and 16.00s I would be turning 70 @ 2250rpms. with the big cam it should have no issue holding up the top end.

Looked at the specs on the MTVR 7 ton case and unless you upgraded to the 7 speed Allison 4070P its a killer on speed. It is a single speed ratio at 1.271:1 it would limit my .83 to 52mph.
The problem with the transfer case blowing apart in low range is due the particular model of Allison transmission they put in the truck. The mt654 Reverse has a ridiculous reduction.

Put a mt643 in the truck and the transfer case won't have the issue
 

simp5782

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The problem with the transfer case blowing apart in low range is due the particular model of Allison transmission they put in the truck. The mt654 Reverse has a ridiculous reduction.

Put a mt643 in the truck and the transfer case won't have the issue
Transfer cases blow apart in high and low range. There is no pattern. Ive blown several in Low forward, and High forward. Not in low reverse actually. I can blow the transfer case in half with my 1st gear on my current setup in high range if I jumped on it. Silverstate55 rode with me the other day and she did not like taking off in 1st. it was a leap and not really a take off. The internals of the T1138 are not well thought out compared to the T138. Especially when you cant even get a replacement case.

The Allison transmissions in the 600 series don't hold up if you put em to work. Especially with a good bit of heat involved. Period. I averaged 1 every 30,000 miles. After the setup I have with the CAT I would never downgrade it to a 600 series Allison. a 45 or 4600 series may be an alternative but its electronic and is another nightmare. Plus the 7155 is fun to drive.

7155 Reverse ratios are R1 14.77 and R2 8.33
 
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juanprado

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Trying to find out the PLS specs of the drum on it first. Hopefully its an od unit. As far as case options. They are limited. The Oshkosh 30000 series that the MTVR has actually is the same as the HET apparently so that is out of the question. Not sure of what the MK48s use. Other option would be to drop to a 5.81 ratio in the axles but those aren't easy to come by. 4.88s are more common actually. Really make it a morf for sure. I can check with oshkosh on the internals on the 55000A and see if it can be changed to a higher ratio.

For what I have into the HEMTT case is 1/3 of what the T1138s sell for.
A few years ago I picked up a hemtt transfer case in the can for min bid $25 at Ft. Polk. Sold it to one of the few hemtt owners at the time (before GL sales) in Colorado I think for a couple Hundred as no one was interested. Supply and demand plus timing.....
 

red

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Trying to find out the PLS specs of the drum on it first. Hopefully its an od unit. As far as case options. They are limited. The Oshkosh 30000 series that the MTVR has actually is the same as the HET apparently so that is out of the question. Not sure of what the MK48s use. Other option would be to drop to a 5.81 ratio in the axles but those aren't easy to come by. 4.88s are more common actually. Really make it a morf for sure. I can check with oshkosh on the internals on the 55000A and see if it can be changed to a higher ratio.

For what I have into the HEMTT case is 1/3 of what the T1138s sell for.
How much are those axle gearsets running with the 4.88's?

quckfarms he has a CAT 7155 trans in his m923 instead of the Allison. 1st gear is even lower than the Allison's reverse gear.
 

simp5782

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How much are those axle gearsets running with the 4.88's?

quckfarms he has a CAT 7155 trans in his m923 instead of the Allison. 1st gear is even lower than the Allison's reverse gear.
180 set for takeouts

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74M35A2

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Some good options spelled out here. As always, each their own. I drove my 1999 Jeep Cherokee 4.0L 4x4 5 speed manual for 17 years as a daily driver, never regretted the third pedal for a single second, especially since I ordered it that way. Looking forward to the 13 speed Road Ranger in my 925a2, and thanks for the transfer case info in case mine goes out to lunch and does not return.
 

silverstate55

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I lost the front output gear in my 931A2 case, trying to pull my D4-5T onto a set of wood blocks so I could get underneath it to do some work. Moving forward in low range when I felt it slip & rear tandems began to hop in soft dirt. Case is fine but a big chunk of gear is lying in bottom of case (with magnets on outside of case in hopes of keeping loose chunk in place for time being). No way to get pieces out without separating case halves.
 

simp5782

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Bump with some ratios and information alternatives.

Oshkosh 30000 from the 7 ton is a single speed case @ 1.27:1 ratio. You would need a double over drive transmission to get any speed out of it. Speed of 41mph on 1600s at 2150rpms.

Oshkosh 55000 from the HEMTT/HET/M916FLs is a .98 high range and 2.66:1 low range. Would need an overdrive transmission. On 1600s at 2150rpms you would be around 55mph. An A2 truck may be better suited to run faster down the highway with its higher rpms.

Spicer 738 thru 792 models. 738 or 784 would be better suited for a 5ton. Direct 1:1 ratio. Air shift front axle. Torque rating is 3725ft lbs. Weighs around 450lbs.

These spicers are more common than people would think. Little pricey but a lot stronger than a T1138

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