When I first started looking at putting a Cummins into the M37, it became very obvious that I would have to change the gearing in the truck to make full use of the power available from the engine. In talking to other folks who had installed different diesels into their rigs, it seemed like I had to do at least two of the following: go to taller tires, use an overdrive transmission, or change differential ratios, in order to have an acceptable road speed while maintaining off road performance. Keep in mind the the Cummins, like many diesels, has a lot of low end torque and is a relatively slow revving engine (2500rpm redline for my 4BT).
Since I like the looks of the truck with the stock tires and I found that it is not difficult to get good tires that fit the stock wheels, so I decided to leave them alone.
Since one of my differentials had to be replaced with a take out unit when I first purchased the truck, it seemed like a good idea to change the ratios as it not only would give me a higher road speed, but it would also give me an excuse to go through the pumpkins and replace any worn out bearings or races and to install lockers. I found out that not only was Paul at MARS getting ready to have a run of 4.89 gears made, but that Charles Talbert's shop not only was located relatively close by to mine and that Charles had done a lot of gear swaps in M37's with a good reputation for the quality of his work.
That left me with figuring out how to stuff an overdrive into the truck... It would seem like an easy bit to do, but once again, things can get interesting.
I've been thinking of the Ranger OD. By placing the speed boost in > > front of the transmission, doesn't it increase the wear on parts by > > making the transmission and transfer case work at higher speeds? Maybe > > I'm missing something, but wouldn't it be better to allow the > > transmission and transfer case to work at a lower speed and boost the > > speed only to the rear drive wheels after the transfer case? (When > > you're in 4WD you aren't going to be doing that high a speed anyway.) > > I agree, with a faster RPM in the transmission and transfer case, you > will get a bit more wear. But consider a couple points.
Ranger Overdrive Modifications
What I had to do to fit a Ranger OD between a Cummins with a Chevy bellhousing and the early series New Process transmission in the M37. This is very similar to what one would have to do to put a Ranger between a Chevy/GM bellhousing and a NP420 or similar transmission in a later model M37 or Power Wagon.
I had the Ranger made with a Chevy front end that will mate to any early Chevy/GM bell housing with a 4.686" diameter index hole and a clutch set up for a generic Chevy 1-1/8"x10 spline input shaft. The rear face of the Ranger has a Chevy index hole and transmission bolt pattern, but with an output gear that is splined for the old style Dodge transmission 1"x10 spline input shaft. I had to send an input shaft from one of my New Process transmissions to Advance Adapters so that they could correctly spline the output pocket in the Ranger, they were not interested in making a rear face plate of the overdrive that would mate to the transmission that I had.
The machine work that I had to do to my NP420 was to slightly turn down the outside of the input shaft bearing retainer from 4.75" to 4.86". I also bobbed off an inch or so of the neck on the retainer where the throw out bearing assembly in the original clutch setup rode so that the total depth of the bearing retainer was about 2.5". For the input shaft I bobbed off a little more than an inch off the tip (almost all of the portion that rode in the pilot bearing of the flathead), and turned down the neck of the input shaft where it rode inside the bearing retainer snout until its shoulder was once again in line with the face of the bearing retainer that I had previously trimmed.
I then slid the New Process transmission into the Ranger and marked the locations for the mounting bolts in the rear face of the Ranger OD. It turns out that I had to drill two new holes for the bottom transmission bolts, enlarge one of the upper bolt holes, and slot the other upper bolt hole in order to get everything bolted together. After marking the holes, I drilled them out and reassembled the combination to insure that everything would line up and turn without binding.
When installing the transmission combination in the M37, I found out that I had to bolt the Ranger OD to the bellhousing in order to be able to install the hydraulic throw out bearing (see the page on my hydraulic clutch setup). I then lined up the bellhousing/Ranger combination with the clutch and the boltholes and pins in the engines' flywheel adapter and bolted the bellhousing to the engine. I then ran a thick bead of sealant around the input bearing retainer on the New Process transmission and then bolted the transmission to the back of the overdrive. Application of the sealant is critical otherwise the gear lube in the Ranger OD that is lubricating the output bearing in the overdrive and the input shaft of original transmission will leak out between the units.
Why not use a five or six speed?
I went with the original transmission for a couple of reasons. I have a winch equipped
truck. The Cummins 4BT with Chevy bell housing is five inches shorter than the original
230 with bell housing. The Ranger OD is seven inches long. By using an electric thin line
Hayden fan mounted between the radiator and the motor, I can have the stock NP420 and PTO
assembly in the same position that they were originally. From an altruistic standpoint, I
figured it would be nice to get a setup where other people didn't have to source a new
which drive shaft or PTO to match a new transmission or have to worry about moving and
fabricating mounts for the transfer and PTO shift linkages. I am not using an adapter
plate since my Cummins came with a Chevy bell housing, the front of my Ranger OD is set up
to match a Chevy SM465 transmission, and the rear is set up to match the stock NP420. In
essence, I am using the OD as an adapter plate... Another feature of using the Ranger OD
is that, since it fits between the bell housing on the motor and the transmission in the
truck, I can spit gears. Effectively, I will have
eight forward speeds in 4x2 operation and sixteen in 4x4 mode), two reverse in 4x2, and two for the PTO (both in forward and reverse).
Since the 4BT is a very common motor, you can put just about any transmission behind it that you want to, you just have to get a bell housing/clutch combination that will work. Since the Cummins can be outfitted with an SAE bellhousing, it can be mated to the Spicer 3053A transmissions from 2.5 ton trucks (military deuce) which are available from many surplus dealers, but the Spicer is a big beast and I understand that it requires a new crossmember for support and some surgery to the floorpan to fit. Another option is to use a Clark 280VO. It is a five speed fully synchronized box, but they are getting a little dated, and one set up to mate to the Cummins will run you around $2000 without the PTO set up either with a GM or SAE front face. Since the Clark 280VO has a fifth gear ratio of 0.80:1, so you will top out just over 65mph with a cruise speed closer to 60mph with 4.89 gears and 900x16 tires.
The NV4500 has a fifth gear ratio of 0.73:1 which is the same as the Ranger overdrive, but they are long. You can get one built to your specifications for $2000, but you will need a wide NV4500 specific bell housing that you can get from a Dodge dealer or your friendly junk yard since the transmission is used in the recent Dodge Ram trucks with the Cummins B5.9 engine. You could also use the NV5600 six speed transmission as is is similar to the five speed with the same overdrive ratio and is also used in Ram trucks.
Eaton-Fuller has a nice five speed out that should fit since it is the updated version of the Clark 280 series. You would have to order it from Eaton and it will be expensive...
In general, five speeds are a little tricky in that they are either too big, or too rare. A really nice deal would be to use a NP540 since they are really short, but they are very hard to find in the overdrive version (and thus are pricey).
Photographs of my Ranger OD and of some other transmissions may be found on photo page eighteen.
Transmission gear ratios and other information for units that are popular for M37 engine swaps can be found on my transmission gear ratio page.