The Stovebolt Site Forums The Shop Area Driveline 1956 GMC Hydramatic

Hello Everybody,

Recently picked up a GMC 270 with a Hydramatic transmission.

It was removed from a '56 250. The guy I purchased it from said he bought it from a local farmer. He pulled the engine and trans from the truck when he performed a frame swap - placing the old body on more modern running gear.

I know less than nothing about these transmissions, except:

1) The case is cast iron, so it is extremely heavy.

2) The torque converter to flywheel interface is wet - so it pukes ATF all over the floor when you separate it from the engine.

3) It is a 4 speed - I think ? (That's what the guy said, at least)

First, I'm surprised that a farm truck from that era would even have an auto trans at all - especially a 250. Were these common in that era? Just for my own edification, if someone could point me to an online manual that covers these, it would be interesting to learn more about them.

Also, as I purchased this package for the engine, the trans is totally useless to me. Are these things at all desirable? Or, do people typically just install more modern designs, and ditch these? There is a lot of iron in that thing, so I know the scrap man would be interested...

Any info would be greatly appreciated.

Best Regards...

It is a 4 speed, dual coupling transmission. There is no torque converter on these units. It is a fluid coupling.

My '56 Coupe de Ville has one. Different versions with different valve bodies were made for GM's other makes. It's a strong, durable transmission with lots of torque. I wouldn't scrap it because it is certainly rare on a pickup truck.

The truck version is the heaviest of all the versions and weighs 435 lbs. in a pickup and 655 lbs in a bus.

With regard to swapping it out, at least with the car versions, there is no direct swap due to bell housing bolt patterns, etc. People who switch to a turbo 400, for example, have to fabricate everything.

Otto: The '56 GMC HydraMatic transmission (and up thru '60 at least, I think '62 but I don't have documentation) is the same design as used in '40 through '54 Cadillac transmissions. The Dual-Coupling HydraMatic in your '56 was first used in '55 Cadillac until it was replaced by the Turbo Hydramatic 400 in most Caddies in 1964.

It's certainly worth something more than scrap value. I'm sure someone on this site would be interested in it.
Most importantly it has all the linkage, bellhousing adaptor, and flywheel.
It would be a good listing for the Swap Meet.
Fred

Thx to all.

Appreciate the input.

Best Regards...

4 speed yes- - - - -Dual (actually "controlled") coupling, NO! The proper name is "Dual Range" which indicates there is a 1-3 speed selector position which locks the trans out of 4th. gear until the RPM gets extremely high. There is no "torque converter"- - - - -it's a fluid coupling which does not multiply torque. The coupling is two pieces, with the forward part also serving as the engine flywheel. It also has the starter ring gear. There is a BIG O ring that seals the fluid when the coupling is bolted together. Since the back of the coupling is held onto the input shaft by a big hex nut, it's necessary to split the coupling to remove the transmission from the engine. One final bit of trivia- - - -the original fluid for those transmissions was "Type A-Suffix A", which contained a small percentage of whale oil. No longer available, obviously. The current substitute is Dexron, I believe.

If you want to get something more than scrap value for the transmission, contact FATSCO Transmission Parts www.fatsco.net . They sell rebuilding kits, parts, and possibly complete vintage transmissions. They're located somewhere in Noo Joisey- - - -which means I'll never actually visit their business!
Jerry

Here's an interesting fact about transmission fluid that no one has been able to answer for me.

The Hydramatic transmission originally used type A fluid. It's possible to get it but extremely difficult.

The original Ford-O-Matic and Cruise-O-Matic transmissions also used Type A fluid.

Ford Recommends Type F fluid as a replacement for Type A fluid in its transmissions.

GM recommends Dexron for it's Type A transmissions.

If both Type F and Dexron are recommended substitutes for Type A, why can't the two be mixed? Would anyone use Type F in a Hydramatic or Dexron in an old Ford transmission?

For the record I use Dexron in the Cadillac and Type F in the Fords but I suspect they're pretty much the same thing.

Whale oil was used as a friction modifier in transmission fluids until it was banned in 1972. Mechanics at the time swore up and down that no automatic transmission ever shifted properly again. They were right, at least for a time.

Transmission failures went through the roof by 1975 topping 8 million. They were fairly uncommon prior to that.

This led to rapid advances in synthetic friction modifiers which.

The next time you take one apart, note that there is a drain plug in the torus cover (what you called the torque converter) that will help keep the ATF off of the floor.

Type F has a little more aggressive friction enhancer in it- - - -using it in a transmission designed for DEXRON will give a much firmer shift. Ford used a lot harder clutch material (black instead of the light brown color of GM clutches) and those clutches needed more friction to engage properly without slippage. The "racing" fluid B&M used to sell was nothing but Type F with a special label and 3 times the price!
Jerry

I was told that for that transmission to operate correctly, there is some linkage related to the carburetor that requires skilled adjustment. Is that correct?

Yes, that's true. It's called a TV rod (throttle valve). It has to be adjusted properly for the transmission to shift at the correct time. They're adjusted with jam nuts on a threaded rod which passes through a fitting on the carburetor linkage. One or two threads either way can make the difference in perfectly smooth shifting or rough/late shifting.

Yes. My daily driver from 1990 until it was T-boned in 2012 was the '57 GMC 102 V8 HydraMatic shown below and I can tell you that both the band adjustments (better done with the pan off using the internal gauge tool) and especially the throttle linkage adjustments (factory gauge is made of un-obtainium) are critical to proper operation of the HM transmission. The specs for the settings of the throttle linkage adjustment is in 64ths of an inch.

I have an original "Hydra-Matic Transmission Operation and Maintenance" Manual for GMC trucks X-5706 (1957) that has a detailed "Diagnosis and Tests" section. If you'd like I could copy that section and post it here.

In the mean time, you could get a feel for non-model specific adjustments by reading through this manual.

I've also attached FatCo's (probably your best bet for HydraMatic parts) "shift speed chart" which has some basic throttle linkage related trouble shooting hints. The speeds in the chart are probably a bit high for your 3/4 ton truck's rear axle ratio, but you could probably reduce the speeds by 30% or so and be close.

Look here for the 1st page of the HydraMatic linkage adjustment procedure from the GMC Maintenance Manual. "Next" and "Previous" page links near the bottom of the page.

That is correct. I had a 57 Pontiac that came with a Roto Hydromatic. I replaced that with a dual-range B&M copy out of a 55 Pontiac that a local guy modified. I couldn't get the carburetor linkage off the 55 in the bone yard as someone had beat me to it. So I attempted to fabricate something that would work. It never shifted right and the abrupt shifts would chirp the cheater slicks and the U-joints lasted only weeks.

55 was the last year for the dual-range hydomatic on Pontiacs. I have believed that was true across the GM line including Olds and Cad. I don't know when Buick abandoned the DynaFlow. The Roto was used until the turbo hydromatic came along in 63 or so. I also had a 62 Olds with a Roto Hydromatic.

Anyhow don't scrap the dual-range. It'll be worth a lot to somebody.

Low end '56 Pontiac and Olds used the older dual-range HydraMatics while upper end models used the newer dual coupling HydraMatic. Pontiac named it "Strato-Flight". Olds and Cadillac had different names.

The "56-28" models are the Pontiac Star Chief models. Document scanned from a '56 Dealer's handbook.

The dual coupling was a mechanic's nightmare, especially the first couple of years of production. My father's shop in Nashville Tennessee was the first independent garage to offer automatic transmission repair. He spent about 5 years as the only place other than the dealerships to get an automatic fixed, and even took some business away from the dealers because he could fix them right the first time. Automatic transmissions and precision tuneup and carburetor work kept me fed, clothed, and housed comfortably from the late 1940s through the early 60's!