The Stovebolt Site Forums The Parking Lot Big Bolts 1954 GMC 350 Hydromatic

I ran across a GMC 350 with a hydromatic. Are they rare? I don't know much about the automatics but never saw one in a bigger truck. Is the tranny the same as the 1/2 tons. It is on the Spokane Craigslist.

It would be the same Hydramatic.

The 1954 GMC/Chevrolet 1/2 ton Hydra-Matic transmission is slightly different from the 3/4 and 1 ton Hydra-Matic transmission (that year). The 1954 1/2 ton H-M had an enclosed drive-shaft fitting and the various HM transmissions for larger trucks were made for fully open drive-shafts.

The GMC 350 transmission (2-ton truck) had a different transmission than the one used on 1954 GMC light-trucks.

1954 GMC Hydra-Matic transmissions

Esoterica: larger GMC truck H-M transmissions were available 14 forward gears (this option was available on a 350 in 1954) and with 21 forward gears (in 1955) - if anyone has seen or driven one, please let us know.

i knew they werent the same because i saw one twice. they are bigger but my master parts book only goes to 53 and only cover up to the 250's. thanks tim for the link. never know what youll find on the old car project site

Mike,

I am still waiting for a ride in a 1955/later GMC big truck with a 21 forward gear Hydra-Matic. Since it has a triple-reduction unit, it would really help me with my lack of double-clutching shifting skills.

I've seen two TF Tandems with the Powermatic (Allison) 6-speed automatic with the 3-speed Power divider that gives double under drive and direct for 18-speeds forward gears. Is the H-M trans a 7-speed?

Mike B

The Hydra-Matic transmissions in this thread are 4 forward speed transmissions. The 1954 GMC Hydra-Matic shown at the link above had an optional 2-speed reduction unit, resulting in 8 forward speeds.

The GMC Twin Hydra-Matic (available in 1955 and maybe later) has 7 forward gears/speeds, because it is two 4-speed hydra-matics sort of mounted on top of each other and sharing one output shaft (it should have been called the Siamese-Twin Hydra-Matic).

The 21 speed GMC Twin Hydra-Matic is a 7 speed Twin Hydra-Matic with a 3-speed reduction unit, providing 21 forward speeds (a 2-speed reduction unit was also available). I have a GMC Shop Manual for these transmissions.

It would be very interesting to see one, and even more interesting drive a truck with one.

Thanks Tim!

I've hever heard of this set-up.

Mike B

i saw one of these a couple of times also. theyre as big as brownie three speed if not bigger.

The twin hydromatic you describe sounds very much like the 6 speed Powermatic that Chevy used in heavy trucks from '54 on. It was described as 2 3 speed automatics on top of each other. Made by Allison.
I'm looking for one that bolted to a 235/261.

Still the basic transmission is the same. I would sure like to know where to find one of the tiwn hydramatics.
I was under the impression that just like with the manual transmissions ie the SM420 it was the same from 1/2 ton up through the largest truck offering. And the reason for them not going too crazy with engine torque and power, and the use of very low rear axle ratios as to not over stress the transmission offerings of that day and so they remained upward and downward compatable.

Jim,

I believe 1956 was the first year for the Powermatic (Allison) in big Chevrolet trucks.

Mike B

The Powermatics and Hydra-Matics were different transmissions made by different GM transmission divisions/plants.

Yes, the Allison 6-speed Powermatic (Torqmatic) was available in 1956 GMC and Chevrolet trucks.
Here is a 1956 newspaper article describing the availability of both transmissions in 1956 (Powermatic available in the Chevrolet 2-ton group)

1955 Twin Hydra-Matic (a Supplement Manual was available in 1956)

1957-1959 Hydra-Matic manual

1956 Powermatic manual cover

1958 Powermatic manual cover ( complete manual )

great news article!

Now if somebody can tell me where to find a Powermatic.