Almost all US overdrive transmissions used Borg Warner R10 or R11 units. These were in separate housings that bolted to the regular 3 speed case. They do require a special longer main shaft. Pre '56 or so the solenoids are 6v. They are good units and fun to drive. They provide about 30% OD at the tail shaft. There is a lot of interchange of internal parts.
They can be wired so that the driver can engage OD in all 3 (4) forward gears. Normally a governor kicks in the OD at about 27 mph. The R10 has 3 planetary gears. The R11 has 4 and is a stronger unit used in some heavier cars and pickups.
The Saginaw 3 speed OD from the late 60s-early '70s (I think) is a good and reasonably strong trans though not as strong as the Borg Warner T85 OD. The 3 speed Saginaw uses the same case as the 4 speed. Many people have put the OD on their 4 speeds. The only real advantage is 4th gear OD because the closeness of other ratios are so close in most of the 4 speeds. The Saginaws use a unique solenoid that is getting hard to find. NEVER throw any BW OD solenoids away. All can be repaired most with a simple drop of solder. Hot Rod Magazine wrote on Saginaw 4 speed OD conversion In May or April 1980, maybe?
