Overdrive section? Overdrive section!
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.