Transmission History: GM’s 8-Speed Hydra-Matic and 21-Speed Twin Hydra-Matic For Big Trucks – Complex Dead End

GM pioneered the automatic transmission with their Hydra-Matic back in 1939, a four speed unit with a fluid coupling (no torque converter). It was rather complex, but it worked well enough, and soon proved itself in automobiles as well as military armored vehicles, tanks and other applications during the war. After the war, the GMC M-135 2.5 ton 6×6 truck proved its capabilities in that application, with a two-speed auxiliary transmission for a total of eight speeds.

But it wasn’t until 1953 that GM began to offer the H-M in civilian trucks, starting with the basic four-speed unit in 1/2 – 1 ton light trucks. Later that year, GM expanded the availability of H-M in both the medium and heavy duty lines: the 8-speed Hydra-Matic, which was a 4-speed unit with a two-speed reduction unit to yield the eight speeds in 4-5 ton trucks—essentially the same unit used in the M-135 trucks—and the much more complex Twin Hydra-Matic, which was essentially two 4-speed H-M units in one case yielding 7 speeds, backed by a three auxiliary transmission for a total of 21 speeds, 13 of them non-duplicated.

The Twin Hydra-Matic turned out to be overly complex, expensive and unreliable, and were withdrawn within just a couple of years, with GM offering to replace them with conventional transmissions. Sound familiar?

This ad from 1954 references the basic 4-speed unit for ½, ¾ and 1 ton units as well as the 8-speed unit for 4-5 ton trucks, although no real specifics.

This is from a brochure for the Twin H-M for sale on eBay, and decided to buy it, as I couldn’t make out the print on the ad for it.

The Twin H-M came standard on the DFM660-47 version of the “cannonball” GMC HD COE unit. It was teamed only with the 150 hp DD 4-71, which sounds a bit modest in power, but 150 hp in the early-mid 50s was common and typical. Trucks back then rarely exceeded 45-55 mph in the pre-interstate era, except for certain big West Coast units.

This gives some basic information on the unit, including its gear ratios.

Here’s a cross section. Note that this is a composite of two scans, and the shaft appears as being offset between the two units.

This cross section shows how the two 4-speed units interact to create 7 speeds.

Here’s a table of all of the available 21 speeds. Note that a number of the duplicate.

This refers to the column below that shows all of the possible gear steps with the unit utilizing the three speed auxiliary, in “creeper” and “low” range as well as direct.

Here’s that table. It shows various rpm on the bottom, and the respective range of road speeds in each gear.


I couldn’t find much detailed in on the 8-speed H-M, but essentially it worked similarly, with two ranges and four speeds.


By 1955, a revised Twin H-M appeared with a lighter 2-speed reduction unit, which was deemed adequate for most applications. The three-speed reduction unit was still available too.

The Twin H-M proved to be unreliable. Most of them were later replaced by manual transmissions without cost to the owners.

In 1957, wide ratio range twospeed rear axles were teamed with HydraMatic transmissions in medium duty 4x2s, replacing the unreliable 2speed reduction boxes. Meanwhile the 7-speed Twin Hydra-Matic transmission was discontinued because of poor reliability and low sales.

It was replaced by the Allison Torqmatic (called Powermatic by Chevrolet)  6speed automatic transmission, which could of course also be teamed with a two-speed rear axle.

 The Allisons included torque converter lockup clutches and hydraulic retarders were available, providing superior performance and reliability. They were made in several version covering a wide range of applications from medium duty trucks to giant oil drilling rigs. These were widely used for many years. I drove a big Ford Super Duty dump truck and some others with this transmission. It banged out the gears with a bit of a nudge in the back each time.

It’s difficult finding more information on the 8-speed and Twin Hydra-Matics, as they were only built briefly, and not in large numbers. But clearly there were limits to what could be done with the H-M, which continued to be built in several evolutionary steps for passenger cars through at least 1964.


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