Swapping an NP435 Transmission & Early Bronco Dana 20 into a Jeep With a 4.0Lby Andrew Horvath
April 6, 2002
Why an NP435 and a EB Dana 20?
When fourwheeling over technical terrain, it is common knowledge that slower is better. By having the control to idle up and down obstacles instead of gassing it or riding the brakes, you save parts and minimize body damage. When I was looking into a bulletproof transmission I wanted a really low granny first gear, simple operation, a good reputation and easy availability of spare parts. For a t-case a was looking for a gear-driven, cast metal case that had a good aftermarket following (in case I ever wanted to put in lower gears). My original choice was a Dana 300 out of a late CJ mated to an NV4500 (6.3:1 1st gear with an overdrive) but the only two problems with this set-up are the front output is on the passenger side (I needed a driver side drop) and the expense of getting an NV4500 out of a new Dodge truck. So in the end I chose an EB Dana 20 and a NP435 out of a ¾-1 ton Ford. The EB Dana 20 and NP435 are the perfect combination mainly because there is no need to replace output or input shafts on either of the units because the NP435 has a 28 spline output and the EB Dana 20 has a 28 spline input. How perfect is this!?
The NP435 has a first gear of 6.69:1 and the EB D20 has a low range of 2.46:1. With my 4.10 gears that equates to around a 70:1 crawl ratio without any further mods. In order to mate the two you will end up with an adapter about 8 inches long. The 435/D20 combo is about 1 inch or a little more SHORTER (even with the 7.5 inch adapter) than the AX15/NP231 SYE combo. This was a very important aspect of my swap because of the I have a SOA with 3.5" springs which doesn't yield a good rear driveshaft angle. On top of that I have a Ford 9" in the rear, which makes the angle even worse. After all was said and done I ended up with a rear driveshaft that runs at 24 degrees and is 4-5"s longer than my old one.
Transmission - NP435
When you make your way out to the wreckers you need to keep in mind that there are more than one type of NP435 transmission. Some of them came with a 32 spline output and some came with a 28 spline output. You also need to keep in mind that there is a difference between the 2wd and 4wd version of the transmission. To make things easy for you look for one that came out of a 82ish or earlier Ford truck that was 2wd. When you find the transmission it should have a 6.5" 10 spline input, a 28 spline output. If you do find one with a different spline count or input shaft length pass it by. Also in the early 80s Ford switched to a T-18 transmission. This is a good transmission but not the one you are looking for. The main difference between the 435 and the T-18 is the T-18 has BORG WARNER cast into the side of it, it has a cast cover instead of an aluminum one like the NP435, and has a screw in shifter instead of a spring loaded pop-in one like the NP435. The easiest way to find out which transmission you are looking at is to simply bring a magnet. If the magnet is not attracted to the cover then you have a NP435 since aluminum is not magnetic.
Now that you have your transmission at home it is time to take it all apart and replace all the bearings, seals and gaskets. I went to Bert's to get all the bearings and such. I wouldn't recommend not replacing all the replaceable items since doing the bearings in a year after you put the transmission in will be a long job. I found Bert's very helpful in that they had all the stuff I needed in stock and it only cost me around 80 bucks for a complete rebuild kit. I am not going to take you through the rebuild of the transmission as there are very well written manuals on it at large libraries. The shop manual breaks every step down very well and makes the job quite easy. The rebuild is not very hard to do and the only thing I can recommend is to place everything in a row the same way it came out.
With the transmission all painted and the rear output clear of the speedo gear and snap rings, set it aside to start on the t-case.
Transfer case - Early Bronco Dana 20
After I picked up my D20 I made another trip to Bert's for the rebuild kit. It turns out that the parts, mainly the bearings, are not the most popular bearings. They had the seals in stock but had to order in the bearings. The other thing about the D20 is unless you order a gasket kit from a Bronco place in the 'States, you are going to be making your own gaskets. It was very easy to make my own gaskets because Lordco sells rolls or sheets of gasket paper that I cut up to whatever shape I needed.
I will give you a very good idea to get all the roller bearings back in place on the counter shaft. Without this technique that Chris Waterman told me about I don't know how I would have got them all in. On the counter shaft there is a spacer, then a set of roller bearings, another spacer, another set of roller bearings and finally the last spacer. You will need to go to a hardware store and get a piece of wood dowel that is the same diameter as the counter shaft hole. Then cut the dowel so it is the same length as the gear set and coat the wood with lots of grease. Then just stick all the bearings in the right spot with the spacers and washers on each end. Then slide the gear over the bearings and install the dowel/gear in the case. Now take the counter shaft and start tapping it in the same way it came out and this will cause the dowel to be pushed out the other side at the same time the countershaft is being installed. Great idea hey? For the 10 or so roller bearings that the input shaft run on the best way to install them is to get a socket (either 5/8th or 11/16, can't remember) and place it in the middle of the hole. Then just drop the bearings in one at a time and with some tweezers stand them up. After they are all installed they actually lock themselves in the perfect position. Put a little oil on them and slide the input shaft in and bolt it down.
Now that you are done with the transmission and t-case it is time to make the adapter to mate them together. You can either get one from Advance Adapters or make your own like I did. When you are planning out your adapter you need to take into consideration such things as making sure the output shaft of the NP435 runs perfectly in-line with the Dana 20 input shaft. There are a few ways to do this but using the input shaft as a jig is by far the easiest. Another consideration that needs to be addressed is how to seal the two units from each other. The last thing you need to figure out is how much to clock the t-case. After I took some measurements I decided to clock the t-case so the front output yoke measures 5"s lower than the rear output yoke when the transmission is sitting perfectly flat. This turned out to be the perfect clocking for what I needed.
To make the adapter I used three ½" aluminum plates. In order to make it all work two of the plates need to be bolted to the transmission (put a hole in the middle of the plates to allow the output shaft to pass though).
Keep in mind that the inner plate will be a smaller rectangle than the outer one because of the bearing retainer on the back of the transmission. The 4 bolt holes in the middle plate (2nd plate) need to have a recess in them so when you bolt it down the bolt heads will not interfere when the third plate is up against it. Now that there are two plates bolted to the transmission you need to find a seal that sits nicely over the output shaft. After you find one get the center hole in the outer transmission plate enlarged to accept the new seal (red arrow in pic). Make sure the seal has to be tapped in so it will not be flopping around in the adapter. Now take your 3rd plate and put a hole in the middle of it so the output shaft will slip through. Then sandwich the plate with the 2 plates that are bolted to the transmission and the 6.5" adapter D20 adapter. Now that the transmission output is splined into the t-case input clock the t-case to the 5" yoke difference I explained earlier. Then mark where to drill the holes on the plate so you can send bolts through the t-case adapter to thread into the 3rd plate (blue circles in pic). Take it all apart and drill the holes in the 3rd plate which will need to be threaded to accept 3/8th" bolts. Next, you can bolt the 3rd plate to the t-case adapter and reinstall it back onto the transmission output shaft with the 2 plates still bolted to it. This is the tricky part…how to bolt the 3rd plate to the middle plate? By now you will see that the 1st plate (closest to the transmission) is a lot smaller than the 2nd plate but the 2nd and 3rd plate are about the same size. This is the important part - with the t-case clocked to the position you want it mark where the 2nd and 3rd plate overlap. You need 4 bolts in a square-ish pattern (green circles in pic). The 2 bolts on the bottom will go through the transmission side of the 2nd plate and then thread into the 3rd plate. The top bolts will be the other way around, meaning the bolts will go through the 3rd plate then thread into the second plate. Attaching the 3rd plate to the 2nd plate needs to be done like this because of the clearance problems. All this work needs to be done very precisely so I would recommend taking the transmission and t-case to a machine shop and get them to mate the two together, but don't forget to tell the machine shop about clocking it and having the seal recess machined. After you finish the adapter make some gaskets to seal it all up. You will only need a gasket between the transmission/adapter and the adapter/EB adapter.
All the other things you need
Clutch Slave Cylinder
Bellhousing and CPS
Twin Sticks and Shifter Linkage
Transmission Hump Cover
AX-15/NP231 Removal and NP435/D20 Install
I am not going to give you a very detailed description of how to remove the old AX15 and NP231 because a Haynes manual will do the job just fine. Before you start the removal you will need to go purchase a reversed torx E-12 socket. Jeep used two bolts on top of the bell housing that have a very different bolt head pattern so don't try and use any other sockets because you will strip the heads and then you are…well, screwed. I was able to remove everything by myself because I removed the t-case by its self then I lowered the transmission. It would have been nice and easy to take it all out as a unit but I didn't have anyone there to help at the time and I really wanted to take it all out to replace the pilot bushing.
After a long debate I chose to try and install the NP435 and D20 as one unit. The main reason I was a little apprehensive about doing this is because the two cases weigh a ton together. So with that in mind I filled the two cases with 80W140 gear oil and ran some straps around the cases. The unit was on a trolley so I wheeled it under my Jeep and got out the engine hoist. The put the engine hoist perpendicular to my Jeep so the hook part would lower through the large hole in the transmission hump. Keep in mind that you will need to do a fair bit of trimming of the floor to get the transmission and t-case shifters tucked up nice and high. Do not try and install the transmission shifter because you will not be able to move the transmission around to line up. I was able to line everything up and managed to install the unit in only around 45 minutes as the engine hoist allowed me to move the transmission around in any direction I needed to. I did this step by myself, too, so don't worry if you can't get anyone over to give you a hand. After it was lined up, I installed a few bolts to keep it in place and began to trim the floor to allow everything to be tucked up out of harm's way. I then reinstalled the starter and connected the clutch hydraulic line that needed to be bled. After bleeding the clutch I installed the skid plate and made some measurements for the mount. After making the mount it was installed and everything was double-checked. The last thing I did was install the transmission shifter and adjust the t-case shifters so the perfect amount of throw was achieved. After the shift linkage was the right length I tightened the little lock nuts and started looking for my keys. Then the true test was about to be performed, I put the Jeep in neutral just in case the clutch didn't work and turned the key. It started right up and everything looked okay. I then put it in reverse and drove it out to the street.
Parts Cost (Canadian Dollars)
A special thank you must go out to Chris Waterman, Chad Loyd and Larry Soo for there assistance in choosing the best transmission/t-case combo and for answering my many questions as the swap was taking place. Without them, chances are I would have not figured out half the stuff involved in the swap.
Do you want even a lower crawl ratio?? How about adding another t-case between the NP435 and EB D20. Read about it here http://www.bc4x4.com/tech/waterman/waterman.cfm.
If after reading this write-up you still need some questions answered feel free to email me at email@example.com.
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