TH-400 Automatic Transmission/ NP 203 GRB Doubler
After wheeling my FZJ-80, I decided I wanted an automatic transmission in my FJ-40. I was going to use a 700R4 because they have a deeper first, an overdrive and a locking torque converter. They are also a little shorter than the 400. I first wanted to go from the 700 to the Land Cruiser 1974 transition transfer case. It is a 3 speed case with the 16 spline 4-speed input. I found the adapter is very long and expensive. In fact I could use a NP203 gear reduction box as a doubler and only gain about an inch over the straight adapter. The cost of the doubler adapter was also $100 less than the 700R4 to LC case adapter, which would pay for the NP203. Extra 2:1 gear reduction for no additional cost? No brainer. The deal I got on a NP 203 came with a 400 which has a different spline count on the 203 input shaft, so I am going to start with a TH400. If I decide later I need a slightly lower first or an overdrive I can switch them out then. The 400 is a heavy duty trans, although it does suck more hp than others. I have read it can drop the sprag clutch if used with 500 hp or more and a trans brake. Otherwise it's pretty indestructable.
The first thing I did was disassemble the NP 203. Most of the transfer case is junk. I tossed most everything. I kept the drive chain to make entry steps from for my son’s truck and the differential assembly for show-and-tell parts for my classroom instruction. Complete disassembly was needed to get the input shaft out.
I started on the GRB first. According to Chiltons, that’s not the way to do it. I reprinted their instructions here.
I pulled the bolts that attach the GRB to the main case. The output shaft and counter shaft assemblies come out, leaving the input shaft and gear with the other case. The center diff lock rod hangs up between the boxes. I just cut it with a hacksaw, but there is probably a more elegant solution. You will need to remove the little snap ring from the shift pivot and push the pivot through to get the rod end out later anyway, so that might be a better way. I found it easier to do that later when I could see better what was going on in the case. 15 little roller bearings will fall out. You will need these, so don’t loose them. They go between the input and output shafts.
I then reassembled the Gear Reduction Box portion of the transfer case with the new output shaft from an Advance Adapters kit. You must re-use the bearing carrier, a thrust washer and the snap ring. The carrier has a tab that must be ground down. There is a picture in the instructions. I put the washer on the wrong side of the carrier bearing at first. Be sure to put it between the low speed gear and the bearing retainer. I used some bearing grease to keep the roller bearings on the input shaft when I dropped the output shaft down. I also engaged the shift fork and sliding clutch gear so they would not interfere with the assembly.
The assembly order is rollers, shaft, gear, thrust washer,
bearing retainer with bearing and bearing retainer ring, (you never take that assembly
apart) and the snap ring. Then the AA adapter plate goes on. Tighten the bolts
and you’ve got a reduction gear box and a GM transmission to LC transfer case
adapter in one piece.
It is a good idea to pull the PTO cover off and cycle the shift fork back and forth to be sure it is engaging the gears properly.
I ordered the TH400 rebuild kit, shift improvement kit and torque converter from Oregon performance transmissions on eBay. I also ordered a Th400 rebuild video as I have never rebuilt an automatic trans before. This is the web site for the video. Oregon Performance also provided the shift kit for harder shifts.
We’ll see how that all turns out in a week or two.
Pretty good so far. The kit for Oregon Performance came within a week and was very complete. It included new steels, forward band and vacuum modulator. The video was very informative. I watched it all the way through first. It was pretty boring, but it was well worth it. I then set up a TV and VCR near where I was working. The 90 minute video took about 10 hours the first day and I still have to assemble the case.
I cleaned the outside of the case as well as I could first.
I didn’t want all that dirt in my rebuilding area. I set up on a large sturdy
bench with another bench handy and a cart for my tools. I knew I wanted a lot
of space to spread out all the parts the first time. I’ve seen guys rebuild
these things out of a few Rubbermaid containers, but I’m very happy to have
spread out. The bad part was that I had to get back to mostly assembled so that
things would not get disturbed before I could get back to them. Disassembly was
easy. The video showed some tricks to make a snap ring tool and for getting the
pump out of the case without a pump puller. I laid out a large sheet of paper
on the bench to lay out the parts from the valve body area. I labeled each part
as I took it off. 
The gears, clutches and bands behind the pump I just took off. I tried to keep them all separated, but caused myself some grief later when I went through the disassembly, cleaning and inspection of the assemblies. I would have been hosed without the video.
I went through 5 cans of brake clean on the internal parts. Each part was cleaned and inspected thoroughly before assembly. I really didn’t need any specialized tools, although I wished I had a lip seal chasing tool and a spring pack compressing tool. I made do without them. I also happened to have to tools necessary to press the bushings in and out that also came with the rebuild kit. A couple of different kinds of snap ring pliers are also necessary. Good ones cause many less bad words to be said. I was going to use a steel bench for cleanliness reasons, but I found it is a really good idea to have a hole in the bench to drop various shafts in while working. A bench vise is helpful for some things, but I didn’t find it all that necessary.
I found most Chevy dealers no longer stock parts for the TH400. In fact many of the parts are discontinued. Both dealers I called said they farm out the non-warranty trans work, so they have no reason to stock the parts.
This is at least a 4+ Banana job on the Howler monkey scale. I would not attempt it without a lot of time, patience, and mechanical understanding. I don’t know that you need to know how an automatic trans works, but you do need to be able to look at things and understand how they come apart.
This web page contains the exploded views of the TH400. It was helpful. Many thanks to the author of the page.
The trans/doubler box is quite a bit longer than the stock trans with a Downey bellhousing. The 400-203-adapter is about 33 inches. The stock unit is a little over 18. The engine is already moved up about as far as it will go, but the rear springs are not yet flipped. Once I get the new set up in I can get and idea of how much I will have to play with the rear to still have an acceptable rear driveshaft. I really don't want to cut my tub as I have a lot of time into rebuilding it. If I do have to move it back a lot, I'll go right to a 4 link with coils to match the front.
