318 V8 swap lessons learned

[wvcharlie]

After two years, I finally have my 318 Poly V8 installed in my M37 and running good. One thing that took so long was finding a bellhousing to matchup with the stock NP420 transmission. I was assured before I started that the 318 bell would bolt right up to it. I had seen some swaps myself. After amassing quite a collection of bellhousings, none of which would work, I discovered that there were early and late model NP420s. Even though my truck is a 65, it must have gotten an early transmission swapped in at some point in time. The small block V8 bellhousing will only match up to the late model transmission. I eventually found an NP435 transmission that I knew wouild bolt up to the smalll block . That ended that problem.
After I got everything bolted up and in, when I took it out for a drive, I got this horrible gear grinding sound. First I thought I might not have my tranfer case linkage adjusted right and it was in between high and low range or something.
I checked everything and could find no problems. I attended the Power Wagon rally held here in West Virginia and there I got to talking with Powerwagon Tim. He told me he had the same problem when he swapped in a 318 Poly into one of his Power Wagons. Come to find out, the drive line angle for the intermediate shaft can't be any more than about five degrees or one gets this terrible noise. I shimmed up the transmission about 3/4" and behold! No more noise! Thanks Tim! I still don't understand what causes the noise, as my drive line angle was only about 15 degrees. It pretty much has to be straight in line. I had never seen this information before. Maybe everyone else knew this but I thought I would put it in writing. Maybe my hard learned lessons will help the next guy.

[M376X6]

The early and late M37 transmissions would interchange without any modifications. The NP420 used in the M37 is the B series or later transmission. There is also the civilian NP420 transmission. The input shafts used on these transmissions are of different lengths. And you will find find different input shafts on the civilian 420's. Also the bearing retainer or throw out bearing sleeve will have a different outer diameter. The good news is that the internals between the two transmissions are interchangeable. A civilian NP420's input shaft and retainer will bolt into a M37 NP420. You will find the mounting ears between the two 420's are different. One hole on the M37 will line up on the civilian bellhousings. Two holes on the M37 420 will have to be ground or milled to align with the civilian bellhousing as they are close but not close enough and the fourth will need a new hole just next to the original, This ear is wide enough to allow a whole separate hole to be drilled. Civilian 420's are getting scarce. Internals between the 420's and the 435's are not interchangeable at all.

Making the modifications for an M37 420 to a civilian bellhousing is fairly easy. The downside is the clearance between the PTO and the bellhousing. The bellhousing will have to slightly ground in noncritical areas to allow the marriage and I use a double row roller chain coupling on the pto output shaft and the shaft running to the winch for a coupling. It is a reliable and fairly simple modification to drop a Mopar V8 into a M37/M43 and it is really rugged and powerful. It has worked hard and well for me for over 20 years.

[powerwagontim]

Hi Brian,
The Power Wagons that had the crashbox trannys, have the same bolt pattern as the M37 trannys. But you are absolutely correct that the Power Wagon NP420 bolt pattern is different than the M37 NP420. Go figure!
Tim

[Josh]

From a manufacturing perspective, my guess is that the government ordered some extra features or parts on the M transmission that were not present in the civvy PW transmission, and so to keep the wrong transmission from going in the wrong truck they changed the bolt pattern to make it impossible to put the wrong one in. It's pretty common in mass production. This is one of the ways where I work where we keep the wrong parts from engine up on the wrong engine, we have different bolt patterns for similar parts that cannot be used on certain engines for emissions or durability reasons.

[wvcharlie]

Yea, I'm not sure why I couldn't bolt the 318 bell housing to my stock NP420. I recently acquired another NP420 right out of a stock 52 M37 and it is exactly the same bolt pattern as the one that was in my truck. But. no matter, the NP435 out of the donor truck was an easy swap. The only modification needed was to make a plate to bolt the e brake and transfer case levers to. It's all good now. I will be in the Christmas parade with it and my M725 ambulance on Friday.

[M376X6]

The bellhousings used on the flat head sixes were a different bolt pattern on both sides of the housing. The M37 NP420 case was designed to bolt up to a flat head bellhousing while the civilian NP420 was not. Or at least all civilian NP420's I have found were bolted up to either V8's or the slant six, the slant six used a bellhousing that would bolt up to a V8. I've never messed with the civilian Power Wagons but since they use a flat head engine and the bellhousing for it I have no idea if the bolt pattern for the M37's trannies would match or if the input shaft is long enough. If the input shaft is long enough and the bolt pattern is the same then civie Power Wagon owners could drop in an M37 transmission and have third and fourth synchronized with helical gears. From what I've experienced, I wouldn't bother with early series M37 transmissions as they had one major weakness that would make them undesirable for even the M37. Most uses won't bother the early transmissions but under hard use the mainshaft would give up the lock ring holding everything together. While the NP420 wasn't the greatest transmission, it was a definite improvement over the 88950. I understand there were some civilian NP420's that had the PTO opening on the drivers side, but I've never seen one. Hence it was easier for me to make a few simple mods to the M37 420 and bolt it to a V8/Slant 6 bellhousing and keep the winch set up on the trucks. I have a few post 72 W300's and W350's that have the NP435 and NP445 with pto openings on both sides of the case. But these 435's aren't very common either. I'm collecting parts for my next project which is to drop a NV4500/NP203front/NP205 into my M37 4x4 with the 318. I like the idea of 15 forward speeds and 3 reverse speeds. I'm a bit stumped as to how I can get an outside rear output on a NP205 for my M37 6X6. I've not seen a 205 with anything but a centered rear output. The M37 transfer is a good unit, but pushing a 4X4 with a 318 or a 6X6 with a 360 is very hard on them and heat is real life shortener for the cases.

[powerwagontim]

Hi Brian,
Yes, swapping an M37 tranny into a Power Wagon that had a crashbox is a viable option, and one many have done.
In about 56, Dodge switched over to the NP420 in the Power Wagon, and they were all dual PTO windows. Power Wagon PTO's are all on the drivers side.
Tim