So I spent all weekend crawling through junkyards with no luck until I ultimately ended up finding a transfer case on craigslist (no bellhousing yet though). Only caveat was I had to pull it myself. The truck I was pulling it out of must've been a farm truck because it was covered in mud and smelled like cow pie (probably wasn't all mud I was digging away from the TCase ). In the end it probably saved me $500 or more off getting one on ebay though so I can't complain too much.
The good news is that it turns out it's an np241
DHD, the heavy duty one. I was expecting a DLD the light duty one, so that's great. However the bad news is that it has a 29 spline input and since I have a nv4500 out of a v8 truck the output on the tranny is 23 spline. This happened because I had a mixup on my part numbers, apparently in 98 they changed the DHD's ending in 7 to be a 29 spline not a 23 and I wasn't aware of this change (should have noticed it was 7AC on the # though not 7AB like I was expecting). Not too big a deal, just need to put a new input shaft into the transfer case, worth doing that and having the better case IMO (HD not LD). Also means I'll be forced to go through it and freshen it up as well which is good since I don't know the mileage.
Anyways, I think I should address the frame issues because they keep being brought up (not only here but especially IRL haha, I want to have some numbers on hand).
WARNING: the following post contains math
. Yall didn't think I was just going to YOLO this and hope it worked did you? Oh you did, well fair enough
.
On a more serious note none of this is really necessary. It would be perfectly fine to just box the whole frame and call it good, don't need to get this technical especially not for a build like this, but like I said I want to have actual numbers in mind for my own sake.
Here are the measurements on the explorer and cummins frames. The cummins frame is boxed on the entire front up to about the firewall then followed by C channel, whereas the explorer frame is boxed up to the engine crossmember and no further, C channel after that. Notice the explorer frame uses .167" plate whereas the cummins frame uses 1/4" plate.
To compare the frames I drew them up in CAD (computer aided design) and did some quick and dirty finite element analysis (FEA), note I'm not using a legit simulation suite or anything like that, just the back of the napkin "trial" sort of version - simulation express for solidworks. What I wanted to do was compare the amount of twisting force, torsion, on the two frames. The way I did this was I drew up 1 of the framerails and added a beam which represents the engine crossmember, I then added half the weight of the engine to this beam and fixed the end of the frame to get a simple beam in torsion. Note the actual values and setup aren't that important here, I just want to compare numbers between the two frames. The goal here is to get an idea of one, if boxing the explorer frame could make it adequately stiff/strong enough compared to the ram frame, and two how much boxing is necessary to do so (can I get away with only boxing part of the frame).
Here is the stock explorer frame. The safety factor was .445 which implies it's about half as strong as it needs to be not to break (note this is mostly due to the loading conditions, this doesn't imply the frame would break if I put the cummins in it unless I found a way to hold the whole vehicle horizontally cantilevered by it's rear frame rail, numbers are for comparison purposes only). Additionally I just picked a generic alloy steel, for all I know Ford used something much better then that and it'd be fine - comparison purposes only haha. The maximum stress in the center of the beam was approximately 6-9MPa. Note the bending is exaggerated for illustration purposes only.
Here is the rams frame. The safety factor was 1.25 making it much stronger then the explorer's .44, which is what we expected. We have some strengthening to do
. The max stress in the center of the beam is on the order of 2.5 MPa.
Here is the fully boxed explorer frame. The difference is
outrageous. Notice that there's no noticeable twisting of the frame, fully boxing makes a HUGE difference. This safety factor is 3.85 compared to the rams 1.25 and the stress is between 0.8-1.2MPa. A fully boxed explorer frame would resist twisting way more then even the ram's frame. Part of the reason why is due to the ram frame being longer, but the biggest reason is just that using thicker plate makes much less of a difference compared to having a closed cross sectional geometry. This is a similar reason to why a solid driveshaft wouldn't be THAT much stronger then a hollow driveshaft and why noone uses a solid driveshaft. In fact with say a 2" diameter driveshaft if you drill a 1/4" hole or bigger in the center and look at the torsion, you'll find no depreciable loss in strength (for a huge increase in material cost and weight). Note this isn't the same for all failure modes, this may resist twisting better but may not necessarily carry large compressive loads as well.
The last analysis I ran was boxing only sections of the frame. There are parts of the frame that have crossmembers or would just be impractical to box. Additionally fully boxing the frame removes all my options for running fuel or brake lines inside the frame rails and fully boxed frames can have serious corrosion issues since its hard to protect the inside of them (hard to paint adequately). The last analysis showed I can make the frame as strong as I'd need it to be with boxing, and the next analysis shows how much boxing I can do and still have a frame on the same order of strength as the ram's frame.
I cut 3 12-18" holes in the boxing and achieved a safety factor of 1.35, still slightly stronger then the ram's frame. The stress was 2-2.5 MPa. This is the frame geometry that I will use, boxing the areas that I can get to easily and won't be in the way, and leaving gaps where it's impractical not to or where I want to add things like fuel and brake lines or fuel filter access. These gaps will also make it much easier to paint the interior of the frame.
As always if you want more detail I went into it on my youtube page, here's the playlist:
Cummins Explorer Offroader Build! - YouTube
So that's the plan on the frame! Box the sections I can do so easily, leave gaps where they serve a practical purpose, and I should still be able to come up with an explorer frame as stiff as the ram frame. I ignored things like how much actual compressive load the frame can take but keep in mind this is an explorer, it'll never pull a gooseneck or be loaded with a ton in the bed, for my application I think this will work just great
.
If my parts truck had been a manual 4wd then like someone else said, it probably would have been much easier to just use the trucks frame and the explorer's body (mostly considering the suspension and drivetrain, the actual boxing of the frame shouldn't be too hard), however people think THOSE trucks are made of gold where I'm at and noone wants the cheapy 2wd haha, so that's what I ended up with.
EDIT: screwed up all the pictures again, should be fixed now.