The Stovebolt Site Forums The Shop Area Driveline 58 GMC Hydromatic

Hello All,

Would you school me on a 58 GMC hydromatic?

Are they as solid as manual SM420? Would they adapt to a chevy truck of the same vintage relatively easy? If it is an easy swap what type of change is performance is there?

Thanks all!

They were used in military equipment from WW11 into the 60's.
I worked on many vehicles that used them so there is no question in my mind of their durability.
If you were able to find every single component from a donor vehicle to match your application it may be a good conversion.
However, they are old technology and finding someone to work on them can be a challenge. There are so many later versions that perform well, are easier to install, and cheaper.
Fred

You'll have to start by changing the cab, the floorboard insert, and several crossmembers. The rest of the conversion is pretty simple once you remanufacture the basic cab and frame assembly. It would be simpler to buy a Hydramatic-equipped truck to start with.
Jerry

Hydramatic or HydraMatic or Hydra-Matic (depending on the year, shop manual, or advertisement - no "o", however).

Fred & Jerry posted very good advice.

They are rugged (original-equipment) transmissions, but you need experience and special tools to go inside them. Private Message me if you want a name/address of a good H-M shop near Boston, MA.

Thanks for all the info guys!

I had a line on one but it got away. Are these things a rare option?
Tom if I find another one I will PM you.

Thanks again!

They also saw use in many drag cars during the '50s and '60s, mostly Gas Coupe class, though some guys used in Modified Production in '57 Chevies through rule loophole (in other words, they were cheating).

First used in an Oldsmobile ('39) and later in Cadillac, Bentley, etc, etc, . .

If anyone ever finds a mid-1950s large GMC truck with a 21 forward speed Hydramatic, please post photos.

The old B&M Hydro-stick was a modified dual range Hydramatic with a full manual shift valve body and a modified fluid coupling to let the engine rev up tighter without "stalling" where hydraulic pressure inside the coupling prevents any higher RPM. That transmission does not have a torque converter, which essentially doubles engine torque just before the input shaft of the transmission starts to turn, but the fluid coupling achieves an almost 100% RPM lockup, which a torque converter cannot do. 90% RPM efficiency is the absolute max a TC can achieve.

Rather than go to the effort of retrofitting a Hydramatic, why not look into installing a 700R4/4L60 4 speed overdrive transmission? They're much newer, easier and less expensive to rebuild, and adapters are available to bolt up that trans or a Turbo 350 (3 speed non-overdrive) to the 235 engine.
Jerry

As a testament to the durability of even the first of the
hydramatics, I practiced my driving skills over the winter
of '56-'57 in my girlfriends mothers '39 Olds coupe with a
hydramatic. Believer me when I say that car had been thru
the war. At least thru the war 'years' and it didn't live a
very well sheltered life in a large poor family with no
father. All the kids had used it to the hilt but that
transmission was still going strong after 17 years of abuse.

I had two '53 88's, both had solid tight shifting transmissions,
one of which we simply used stiffer springs in the valve body
to extend the shift points.
Good boxes, and I've got a '55 hydramatic which I'm thinking
of using in my '54, if I live long enough to get to that end
of my project list.

Denny Graham
Sandwich, IL