m818 repower

I am interested in the possibilities of putting any type of turbo cummins into my 5 ton. I would like to reuse the same bellhousing transmission and transfer case. I have been tossing around the idea of turboing the n h c 250. But for the money putting in a turbo kit and then getting the injector pump rebuild and turned up I might as well look into the possibility of a complete repower.

I did start a thread about a week ago about putting a 400 ntc cummins into a m818. But that thread went way south way too fast.

FYI I am not interested in my truck going any faster and there will not be any gear ratios change in the rpm range will be very close to what it is now. I am wanting this truck to pull better for handling loads only.

I am asking that you please respond with factual information pertaining only to the repower of a 5 ton truck. If you have any safety concerns please direct those to the safety part of the military truck forum.

Let's try to keep it civilized this time. And hopefully I don't end up getting banned again. Lol

Thanks guys I know ya will help me out!

The easiest transplant would be another Cummins 855 block motor, like a Cummins 290 or 350. A 400 Cummins would be to powerful for a spicer 5 speed transmission. I am putting a 290 in my M813 this week. I know that others here have put in the smaller cummins motors with some success. It all depends on how you will use the truck. You can add a turbo to your Cummins 250 and gain about 20 hp, but you will need to watch your pyrometer to keep the pistons from melting . I have found that Cummins 290 can be easy to find and inexpense. You just may need to buy a whole truck. Take the parts you can use and scrap the rest!

Thanks vintage iron! Yea i have been thanking about the 290 I just don't know if the power would be enough of a payoff for the money and work. I am very interested to see how it turns out for you. I am a bit new to the big diesels as far as working on them. The 290 and 350 are small cam motors, right? You are going to post up some pictures for us to drool over, right?

290's and 350's are big cams.

ODdave said:

290's and 350's are big cams.

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That depends on year. Small cams were built till something like the late 70's or early 80's. The 1974 IH 4300 Transtar I had (the one in my current avatar) had a small cam 350. 290's and 350's can be either small cam or big cam.

They made 290 small cam motors too. There are formula 290's too. I haven't looked at the spec for the formula motors yet. I hear that they were more light duty? But can't say for sure

Can someone explain the differences between a small cam motor and big cam motor? Other than I'm guessing the size of the cam. Lol

Small cams are rare around here. I had a 315 big cam formula, I guess they had a lighter (cast) crank shaft and peak power was at a lower rpm for fuel milage. Mine had the low flow cooling on it as well.

To bad someone around here isnt doing a re-power. I have a nice 3406 cat but no 5-ton.

So any 855 cummins motor will fit up to my bill housing?
what about the motor mounts are they in the same place?

pay attention to the front mount on your replacement motor.
1st photo is of a stock 250
2nd photo is of the BC 4 that went in.

Suscribed

There are other options. A trans swap to a 9 or 10 speed fuller with the stock engine would help a bunch. With less jump between gears you can utilize the 250 power much better.

There are some clearance issues with a turbo engine in a M818. They can be overcome but it's something you have to think about.

Im not really that worried about the Turbo clearance or the motor mounts. They seem like small problems to me.

Good info guys! And getting it all down in one spot for everyone that's interested.

m16ty said:

There are other options. A trans swap to a 9 or 10 speed fuller with the stock engine would help a bunch. With less jump between gears you can utilize the 250 power much better.

There are some clearance issues with a turbo engine in a M818. They can be overcome but it's something you have to think about.

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Has anyone done this or a similar transmission swap? Or seen it done before?

tiny110 said:

Im not really that worried about the Turbo clearance or the motor mounts. They seem like small problems to me.

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I posted the 2 photos so you could see the difference between the 2 mount types. they may be small problems but they still have to be dealt with and clearance is must be created for the bigger harmonic.

I found it kinda funny that a truck this large really has a lot of tight areas.

the 13 speed clears the brake booster by 3/4" and would not have fit if the engine remained in the original off center location.

You can put any 855 Cummins in but you have to dail in the bell housing in to make sore that it is in the .005 to .010 so you do not have a problem with the operation of the clutch

DaveP... So will the transfer case will steal fit with the 13 speed? And if i reminder right the 8-10-13 speed are all in the same transmission case, right?

If they are all basically the same block can you not just swap motor mounts? If not why?

Small Cam...2" cam, Big Cam...2 1/2" cam. Cam diameter was increased to change metering. You can put a SC flywheel hsg on a BC but you need to modify the upper bolt hole to make it match up to the BC block...it was moved up and out to make room for the larger cam bore. There was also a 350 and 440 HP SC made. IIRC the front of the NHC is a trunoin mount, mounted to the front cover around the crank. The front covers are different so you'll need to make a custom one. You can't take SC performance parts and put on a BC or vice versa.