FWIW it doesn't feel a lot different than engine building, as long as you replace worn parts and do things in the right order and check your clearances you should be OK. The shop manuals are very detailed, most measurements are done with a set of feeler gauges and the expensive measurement tools can generally be worked around. At least you don't need a machine shop for anything!
How a conventional transmission functions is actually pretty logical once you start getting into it, it's basically just a series of planetary gears that are activated in sequence (via clutches or bands) to give the desired drive ratios. The real wizardry is in the valve body.
But first, a brief history of Torqueflite transmissions.
In 1962, the A727 Torqueflite was introduced. It was the "big" transmission Chrysler designed to handle the ever-increasing power appearing in passenger cars in the 60s, alongside the smaller A904. It was a very stout unit, with far stronger clutches than older 3-speed designs and beefed-up internals. It was also very popular in drag racing circles as it could be built to handle huge power outputs - even on stock form it would live happily behind a Hemi so long as you kept it well-cooled. (Hemi 727s did have a few additional internal upgrades from the factory as well, but they were pretty minor). It was also a lot lighter - the previous A466 3-speed had a full cast-iron case, which weighed an absolute ton compared to the aluminum A727.
Eventually (ie the 80s), people figured out fuel economy was a thing and Chrysler (along with most other manufacturers) knew that 3 speeds wouldn't cut it anymore - spacing out the ratios more would just lead to poor performance, but adding an extra planetary/clutch unit to the transmission would require a costly complete ground-up redesign. Instead of designing a brand-new unit at massive expense, the OEMs realized you could add a 4th gear by basically bolting a second simplified two-speed transmission to the output of the main unit - what we call an Overdrive. Due to this, the Overdrive is built separately from the main case and is only bolted on in the final assembly. The downside is that being a bolt-together, you need to do a good amount of clearance measuring to set the apply piston and intermediate shaft clearances. You'll see what I mean later.
The addition of the Overdrive unit created the A518 Torqueflite, later renamed the 46RH (4-speed, 6 on torque capacity scale of 2-8, Rear (longitudinal) output, Hydraulically-controlled). As time went on, the Torqueflites got electronically-controlled shifting and bigger guts - 46RE, 47RE, and 48RE. 48REs built to handle 2000+lb-ft are commonly found behind 4x4 pulling trucks - and most of their standard internals can be used to upgrade all of the other models based on the 727.
The last of the new bushings get installed in the OD planetary/output housing. These were a pain in the dick to get out with a slide hammer.
The OD sun gear gets its fancy new upgraded spring seat and new Torrington bearing.
The OD double-roller sprag bearing and its new Torrington bearing go in next. The old one looks fine but you know, "while I'm in there"...
Got a great deal on an NOS 47RE 5-pinion OD planetary, that's a nice upgrade and goes in next. Coat liberally in green assembly goo.
With all the planetary guts in, it's time to head to the vise. Since the OD section is only activated by a one-way apply piston, there's no way to hydraulically engage the OD direct clutch. Enter the Big Fucking Scary Spring! This meaty unit applies something like 800-1000lbs of pressure into the direct clutch and is only disengaged by the OD apply piston when overdrive engages. Remember how far this sticks out - we'll be compressing it nearly flat in a later step.
Stack 'em! The late 46RH OD unit is already a high-capacity drum from the factory with 8 friction packs. I could have replaced the reaction/apply plates with a thinner set to fit a 9th pack, but I couldn't find the right thickness plate set anywhere and it wouldn't be much of an upgrade anyway. The direct clutch hub sits on top of that big spring and the clutches/steels slide down around it.
Into the press! Here you can see all the clutch packs disappear into the drum - so the spring has to be compressed that much to get everything together. It's all held together with two snap rings, big and small. The big one is always replaced when rebuilding these units as the stock ones have a tendency to fail eventually. The snap ring isn't flat - it's wavy and acts like a spring to cushion OD disengagement.
The cut-down intermediate shaft stays in place here to keep all the internal splines aligned; once the unit is compressed there's no way to turn them by hand.
OD clutch section assembled and back on the bench. Try not to think about the giant spring in there, ready to send this into the stratosphere! Don't point directly at your face.
(OK, it's actually quite safe at this point, you would have had to have really screwed up the assembly for this to come apart.)
Add the big ol' OD bearing, secure with snap ring. Install hydraulic governor assembly, secure with snap ring. Install new governor seals, secure with shitloads of green assembly goo. (The goo is synthetic transmission assembly lube and will quickly dissolve into the trans fluid.)
The OD direct clutch section is then installed into the OD housing.
Now for the first of our measurements. On the end of the intermediate shaft is a small spacer available in 4 different thicknesses that sets the shaft depth, so a tool is used to determine which one you need. The tool is just a cylindrical rod 5.500" long and a flat bar that's 0.500" thick and you use a set of calipers to measure from the top of the bar to the top of the rod. I built my alignment tool to be 6.000" in the required dimension and used a piece of 1.000" square tube stock, so 0.500" longer on both so the final measurement will be exactly the same as the real tool. Getting the dimensions down to a thousand of an inch required some delicate sanding because I don't have a mill or lathe.
Measurement of 0.765" means I need a 0.175" spacer. I only have a 0.195" spacer. Crap. Hopefully my transmission guy has one and is willing to do some horse-trading.
The second measurement is very similar to the first, except you measure down to the face of the hub instead. This measurement determines what thickness of piston apply spacer you need. There are 10 spacers available, ranging from .110" to .245" - I had a .215 and a .200 on hand, but I needed a .185 based on my measurements. Fortunately, those smart guys at Sonnax offer a $7 set of spacers that consist of one thick one and a set of thin spacer shims that you add up to your final required thickness. Easy and cheap! The spacer sits in a recessed groove on the piston so the shims are in no danger of escaping. This spacer thickness sets the OD 3-4 shift timing and is quite important! Too thin and you'll try to engage both gears at once, too thick and the direct clutch won't enage all the way and will burn up.
The OD clutch hub gets its new Torrington bearing installed with some goo to hold it in place. The OD 4th gear clutches are also installed into the case - I upgraded from the 46RH 4-friction set to the diesel 47RH style of 5 frictions by replacing the frontmost pressure plate with an extra friction/steel set, just like they did in the factory for the 47RH.
Finally, the OD apply piston get new lip seals and more goo, and is installed into the main case piston retainer. You can see the two holes for the governor lines on the bottom right of the retainer - these are plugged off on the RE models as they are electronically goverened instead of having the governor unit we saw earlier. This makes for a significantly shorter OD unit!
That's it for now, need to pick up a few last upgrade parts from my trans guy and see if I can trade some of my extra parts for a better front planetary.
Hello, and welcome back to Too Many Pictures of Transmission Guts Theater after that short commercial break.
The forward direct clutch drum is back from the machine shop and has a nice new stepped reaction plate that allowed me to run an extra clutch. The waved snap ring has been swapped out for a flat one that allowed the milled plate to be as thick as possible to avoid breakage. This will make for firmer shifts but I'm quite happy with that. Endplay spec on this drum is very wide so it's pretty easy to hit.
Time for the main planetary array endplay check. Throw your intermediate shaft in a set of softjaws and add the rear planetary shell and thin splined 0.030" thrust washer.
Time for the upgraded rear planetary! It's got moar pinion gears.
Add the rear planetary gearset and its new thrust washer. Note that I'm not adding any green goo during this process as I don't want it to interfere with the measurements, it's really thick stuff.
Sun shell and thrust washer is up next.
And finally the front planetary assembly with its new thrust washer and end snap ring.
Flip the assembled array over and move stuff around to get it all settled into place and check the clearance to the intermediate shaft shoulder - this is the assembly endplay. Spec is pretty wide (0.006" to 0.0048") but I was very much on the loose end and it's much better to have this tight. My trans guy won't let one go out the door over 0.010". So the factory way to adjust endplay of this assembly is just the snap ring installed in the last assembly step - it comes in a standard thickness and one that's 0.010" thicker. I happen to have both (which is lucky as you basically cannot buy them these days and most builders have resorted to pricey thrust washer shims) but an extra 10 thou wasn't enough to get me where I wanted to be. Fortunately, I'd learned a neat trick - a second thin splined thrust washer can be added to the rear planetary to take another 0.030" out of the end play, which is exactly what I needed. With that in place, it took the endplay to 0.008" which is bang-on.
The final selective on the intermediate shaft is the snap ring spacer that rides on the inboard end of the rear splines, and sets how deep in the main case it sits. My measurements earlier indicated I needed a 0.175" thick spacer instead of the 0.195" I had, and I was fortunate to get my hands on the right one.
With that in place, the whole assembly is complete. It can now be taken apart again as it gets installed into the main case later after I've given it some goo.
Back to the main case! This has been cleaned out and pre-lubed a bit in preparation for the new sprag.
Sprags are magic.
And now the reverse drum and band can be installed with its thrust washer and snap ring.
Going to stop here for a bit as I need to figure out the transmission vent relocation next and that may require drilling the case body, so I'd like it to be pretty empty when I do that. The current vent location means that if you are fording deep water, you will flood your transmission long before you get water into your diffs or transfer case - it's kind of a dumb design but water crossings were probably not what they had in mind when they designed it in the 60s. At any rate, it's fixable, I just have to decide where to put it.
Took a weekend off from transmission work and took a trip out to Waiparous Falls. Usually a very easy, popular winter trail but blowing snow had caused some pretty deep drifts. So many stock Tacomas on street tires buried to the frame with eight people trying to dig them out. The river crossing is usually frozen solid but the melt has come a bit early this year due to the mild winter which turned the approach into a 15"+ sheer ice shelf. Tougher to handle on the way out, as climbing it required a good approach angle and either 35s or a front locker to get through, as well as judicious application of throttle to make the bump. We also found the time to take turns burying ourselves in a huge snowbank until we finally plowed enough snow out to break trail.