M923 Transfer Case upgrade to Oshkosh 55000

Well ok sports fans. It has come that time to upgrade the transfer case cause I am bored and let's face it. We all like mods. So going to swap in a Oshkosh 55000 from a HEMTT. I acquired this unit from Silversate55 a few months back. Impulse buy and I didn't even have to wait for the EUC to clear. Case is air operated to engage the front axle so the single air switch should suffice.

Ratios for this case are .98:1 High and 2.66:1 Low.
Transfer case dimensions: Width 17". Front to back 28". Top to bottom 30"
U Joints - 5-280X
Power rating : Can handle a C15 Kitty and 8V92 so the big cam I in its future should not hurt it.
Speed range : with the CAT 7155 and 16.00s - 76mph @ 2400rpms

As far as the regular 5 ton case. the Oshkosh 55000 is narrower by 6 inches. Longer by 5 inches and height is about 10 inches more
Installing it should be pretty straight forward. My transfer case was moved back 8 inches, and I continued to use companion flanges on my output on the transfer case on the E brake drum and on the jack shaft side I swapped on a companion flange and did away with the "mechanics" style joints. This is my benefit as by removing the companion flanges on each side and using just the spicer joints I gain 6 inches. a companion flange is 3" from center joint to mating face. So forward to backwards alignment should be the same. It will also lower my drive angle to only a few degrees down than a drastic down and to one side as it sits at the moment. My weak point at the moment of upgrade to a Big Cam will be the companion flanges and the transfer case.

Side to side install should not be an issue if I can locate some actual HEMTT brackets. If not I can always make my own. They bolt on the front and rear of the case on each side. So making 4 independent mounts should not be an issue either.

Height wise is going to be the problem. At 30" high the top of the case would be against the arch on the bed frame as it ran across and would be as low as the drivers step and fuel tank brackets. Simple enough would be to cut the bed and reinforce it. However I will probably canter the transfer case to drivers side on the lower part. This would make my front drive line angle almost straight and not have such a drastic turn around the CAT 7155. With the way the oil pump setup works on the 55000 it should not be a problem for the oil pickup. Also would help with the rear drive angle as to not be so inline with the rear axle.

This will be up and coming. I am doing some measurements on the mounts as I am having a hard time sourcing the HEMTT ones so will see what type of design I can come up with. More to come. I promise snow and cold weather won't hinder my progress.

That is a fantastic project, and we will be following the progress with keen interest. Your project is nudging me into a similar project with my 5 ton camper.

Do you know the NSNs or PNs of the mounts you need?

Wes you told me this was your next 5 ton upgrade, YOU waste NO time my friend! I sure wish you lived closer or I still had my plane, I would love to assist on some of your upgrades, they are WAY to COOL!!!

Talk soon Wes
Karl

Swamp Donkey said:

Do you know the NSNs or PNs of the mounts you need?

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https://www.steelsoldiers.com/showthread.php?t=175925

I may end up making them to look the same but wider in their style to reach the frame rails. I should have measured the m977 at John's place but i forgot

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I'm excited to see this done. Been looking at tcase upgrades for a while so would love to see your results.

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I'll post some pictures tomorrow evening of the factory set-up from my M984A4 that's sitting in the motorpool at work. I'll even lay tape on the framerails. Please let me know if you need a close-up picture of any particular part of the mounting arrangement.

This is my benefit as by removing the companion flanges on each side and using just the spicer joints I gain 6 inches.

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If you still needed any HEMTT driveshaft parts (U-joint; slip yoke, etc...) I would gladly donate them, for the cause of this interesting project.

M977 pics

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Woo-hoo! Can't wait to see how this comes out, great job Wes!!

Sure wish we still had a popcorn emoji....

I'm wondering how this transfer case would hold up against the stock M939 allisons super low 10-1 reverse ratio?

bluehorse said:

I'm wondering how this transfer case would hold up against the stock M939 allisons super low 10-1 reverse ratio?

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The allison reverse is not super low at 10:1 . This case comes behind 550/600hp 1500ft lbs.

Any torque from an 8.3 or nhc250 thru that 600series allison would be like a tickle.

I run a 14.77:1 reverse 1 thru a stock 5ton case. In high range it has no issues even backing up 75000lbs.

This transfer case with an Allison is not a wise choice as you would have no overdrive so about a 55mph max on 1600s
This is the big boy of cases. It is just as big as that 600series allison

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Is this going in the truck you are trying to sell?

I was thinking of putting 1600's on my truck and losing the overdrive might be good to get the gearing back close to where it was with 1400's. I dont plan on driving over 55mph but Id like it to be about 1800rpm at 55mph. I need to do the math and see what it comes out to.

I just want a t-case that doesn't split in half if you get on it hard in low range reverse. I know the stock case is pretty tough in high range, it seems low range reverse is what kills it.

Is the Oshkosh 55000 transfer case high and low range air shifted?

bluehorse said:

I just want a t-case that doesn't split in half if you get on it hard in low range reverse. I know the stock case is pretty tough in high range, it seems low range reverse is what kills it.

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It will blow in high and low range. In 6x6 and normal operation. There is no pattern.

Uses a lever to shirt high to low. The front drive engagement is air operated.

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Thanks

Alright folks. it is time. No. not time for the GA rally, but time for the swap. Tomorrow morning all the fun starts. I should have the bed slid back and old case out in about an hour or so and then I am going to start the fab work.

From what I have noticed so far for my mounting is this. I am going to put the bed on a 4x4 to raise it up to clear the 1600 tires anyways. This will give me an extra 3 inches of room overhead than what I have now. So that helps some. The fitment of the bed will be that the front support cross beam will be in front of the transfer case upper section so it will go up inside of it and still allow for enough movement/flex. The lowest part of the transfer case will be 2.5" lower than the bottom of the T1138 is at now. So some drop but still more ground clearance than the fuel tank brackets. I may look at adding a skid to it if i decided to go play offroad.

The case will sit more to the drivers side to help with the angle on the driveline from the transmission and to the rear axle as well as the front axle since I have the larger CAT transmission. This will help alot. My driveshaft sits about even with a normal spare tire carrier. My carrier is raised 3" at the moment. Transfer case is 17" wide total with the center of the input driveshaft being at 9.5". So It will be closer to the drivers side frame rail. I will use the stock 939 double mount bases that are on the truck now. I will have to make a different braced box mount for the passengers side I am sure.

Each transfer case mount point has 3 bolt holes. the inner bolt hole goes thru to the inside of the case. I don't really like that at all. Seems like a nice leak point. but oh well. I will be using 1/2" plate to make all the mounts and L9 hardware to hold it all together. I don't know how much this case weighs but I think the T1138 is around 750lbs. I will post some pics of what i have done and or made tomorrow. I am anticipating about 48hours to have this done. depending on the rain Friday. I will have to drill some bolt holes into the frame i know. Have a nice frame reamer bit for that.

Front to rear should be pretty good on fitment with the companion flanges removed. My Transfer to front tandem driveshaft is 17.5" center to center and the front short shaft on a 923 front driveshaft is 20.5 at 30" extended so i have some play either way.

Let's get this thing done.

I would like to know the before and after weight on your truck. Im "ass"uming the trans is heaviers and the transfer is heavy plus your other power adders.

Good work for sure.

also that yellow motor in the background... wonder whats up with that little kitty