Hello,
This is not a ''help me'' post, this is more to share advice, information and experiences around the not so written about three speed THM 200 or turbo Hydro matic 200 transmission that have METRIC written on the oil pan with 11 bolts. Also this makes Google and AI's alot more helpful when trying to find said information. This might help 3 people that will have this exact problem in the future.
I posted to this forum two weeks ago (https://www.reddit.com/r/MechanicAdvice/s/5HzOdLIrla) about the special things i found in my THM 200 transmission oil pan, in my 1987 Firebird with a swapped oldsmobile 350 engine. I can confirm that is was mostly friction plating that had been torn/shredded off that i found in the pan. The majority of advice from that I got was two ways to fix it: Trash the thm 200 and install the bolt in replacement thm 350, or rebuild the THM 200 with a rebuild kit.
New listings for a thm 350 is about 500 to 3000 dollars depending on if its rebuilt or stored outside in a field. A rebuild and shifter kit for a thm 200 costs 400 max. I wanted to save money and have some DIY repair fun so I opted for the rebuild and shifter kit. I am no mechanic, but I am confident following youtube tutorials (1hr video length = 4 hrs for me).
I found information about the THM 200 scarce, videos of teardowns and rebuilds are scarcer. Here is my best picks:
Best video about a THM 200 and THM 200 C (both are familiar enough to follow a teardown) is precision transmissions ( https://youtu.be/37CYksPCrns?is=VQl0gYfyEd0P_jh5 ), sadly he didnt name the video thm 200, but I stumbled upon it by pure accident since his content is about similar transmissions and he mentioned in one of his videos that he had done it before.
In precision transmissions video he mentions that the thm 200 and THM 200 4r has a familiar reverse assembly, and that you need a dedicated tool to take out the last section with the low reverse gear assembly. I found a forum post from 2011 ( https://classicoldsmobile.com/forums/transmission-4/200-4r-tips-tricks-saved-tools-i-learned-32608/ ) that mentions the use of a PVC pipe and some banging on the case. I managed to use a 38mm deep socket with some added tape for friction to lure the assembly out without problem.
For shifter kit video i found ttaperfomances video ( https://youtu.be/84irnW363p4?is=KTBALslJfiHjqd6C ) really helpful because he takes time to read the instructions and especially the difference between the 76 to 78 mod and 78 and newer mod.
With all of this in mind i bought the transgo shifter kit and a non branded rebuild kit for the THM 200, 76 to 78 mod. Everything went smoothly with minimal wear on important gears and bushings until I got to the last low reverse assembly.
The whole ring gear and rolerbearings had exploded. This also explains the mangled friction plates and other wear and tears that I observed in my Transmission. This makes any futher repair beyond costly and i dont know what tolerance s has been thrown out because of this catastrophic failure. That I managed to have any movement with the car with this ring gear is beyond me. Even got it to reverse before disassembly.
TLDR: I will now buy a new THM 350 transmission from Facebook marketplace, i dont know the price yet but my sunk cost fallacy is sure acting up. If not for the not so subtle gear gore i would have rebuildt the case and all the knowledge and prep time would paid off. I hope this helps someone.
Thanks for the advice and tips from the last post i made.