I sent the original 17.5"x38" exploded-parts diagram of the 21-speed Twin Hydra-Matic to Gene (with a GMC 1955 Operation and maintenance manual). He kindly sent me one of the copies he had made.
Simply described (as if I know anything about Hydra-Matics), the Twin Hydra-Matic is two 4-speed hydra-matic transmissions mounted above/below each other. They each have a front and a rear input and output shaft. Together, they have 7 forward speeds and one reverse speed.
"The rear transfer gears/case connect power through a common output shaft to a two or three-speed reduction unit."(14 forward speeds or 21 forward speeds) A two-speed reduction unit was standard, and the three-speed reduction unit was optional equipment. The transmissions were available on 600-900 series GMC trucks.
