The Stovebolt Site Forums The Shop Area Engine Shop 55 GMC Automatic trany

I have a 1955 GMC truck with the rare automatic trany,and my original 6 cyl is shot, [cracked block] and I have a good 1956 chevy 235 engine, will it bolt up to the original 1955 GMC automatic trany?? thanks for any info.

Yes it should. You would use the H-M "flywheel".

The only problem/trick might be aligning the block to the transmission. This originally was done at the factory for Hydra-Matic equipped trucks, allowing-for/requiring "matching numbers" for block and hydra-matic (an engine ID plate should be on the H-M transmission).

I think HrL has knowledge of these transmissions - maybe he will post advice/comments on the necessity of the "matching".

The H-M "flywheel" is actually the front part of the fluid coupling, and I'm not sure if GMC and Chevy crankshaft hub flanges have the same bolt pattern and/or size. I do know the design of the starter ring gear is slightly different. That might become a sticking point during a swap. You'll also need to relocate the front motor mount because the Chevy engine is about 2" shorter than the GMC. Where is the block cracked, and how badly? If it's not in an area exposed to combustion gases or cylinder pressures, it might be better to explore the possibility of getting the crack repaired. I think the dial-indicated adapter ring between the engine and trans was a bit of overkill- - - -the Hydra-Matic division of General Motors was a picky bunch back in the 1950's!
Jerry

The block has a crack as long as the block and has been tried to be fixed several times, still leaks.Thanks for your info.

GMC and Chevy flywheels do not exchange. Find a good 270 or 302.

For the life of me, I can't figure out where the dial-indicated adapter ring is located.
Are you talking about the 1st part on the left in the Illustration
Group # 0.683 Part of housing assembly used on six cylinders engines only.
If it is the part, by dial-indicated, do you mean timing mark indicating pointer?

The part on the far left of the illustration which bolted onto the block to adapt the transmission housing to the engine was individually fitted to each engine by a set of factory-installed alignment pins to minimize runout. If that adapter ring was attached to a different block, it was a crapshoot whether the block and the transmission would align properly. A dial indicator mounted on the crankshaft hub was used to check the alignment between the engine block and the adapter.
Jerry

An H-M aficionado could probably do the adjustments, but most of us are mere mortals (I know two places, but would only trust one of them - near Boston, MA).

Hotrod Lincoln,
Got it, Thanks for the info.
I think the 54 shop manual calls it Face Runout and Bore Runout.
Flywheel Housing Alignment check
Page 2
Page 3
Ether way we're on the same wave length.

You will have a long drive if you need repairs now.
Maybe you could fly in a mechanic.
Just kidding.

So, every Hydra-matric was essentially custom-fitted to its engine? I sure learned something today!

What were the real-world consequences of not following this alignment procedure? Vibration? Shortened service life of the transmission?

Mostly binding and premature wear of some bushings up front, along with seal leaks. My father was the first man to work on automatic transmissions in Nashville Tennessee in the late 1940's, and I attended a class he was teaching for some of the mechanics in town on Dual Range Hydramatics in 1958, at age 12. GM certified me as a factory-qualified Dual Range technician at the end of the course! I haven't rebuilt one in many years, however!
Jerry

A similar check with a dial indicator for the clutch housing is described for manual transmission setups for ADs in the Factory Shop Manual.
Anyone ever do that?

Nope........I figured the last 66 years of use proof it was close enough when setup back in the day.

Not the Hydra-matric transmission itself.
Just the rear main Flywheel Housing and it's Front 3rd half that bolts to the engine block.
Those two flywheel housing pieces were supposed to remain with the engine block that they were dial indicated too. Pic 1 , Pic 2 , Pic 3
I don't have a picture of the front housing but it is the far left part in this Illustration.
Once the housings where aligned to the crankshaft, the bolts were locked down, then the rear housing
was separated from front housing and holes were drilled into the Front housing and engine block.
Dowel pins where driven into the drilled holes to insure the housings stayed aligned.

The idea behind it is blue printing.
As in a drawing on blue paper.
The actual engine and transmission measurements should match the measurements on the blue print.
The GM engineer would draw the engine and transmission on paper.
The engineering drawing indicated a straight line from the front end of
the engine crankshaft all the way to the end of the rear main transmission output shaft.
One straight line front to back.
The break in the straight line is at the engine block to flywheel housing connection.
When the flywheel housings are dial indicated to the crankshaft, it insures that the straight line
continues on all the way to the end of the transmission output shaft. Straight as a arrow.
The straighter the line, front to back, the smoother everything runs.

Lets say the engine block has to be replaced.
The 54 shop manual says to align the bore runout, remove the old dowel pins, dial indicate housing,
drill new larger holes into the front housing & engine block then install oversize dowel pins.
Today instead of using oversize dowel pins you can use offset dowel pins. (no need to drill new larger holes)
There is a screw diver slot to rotate the dowel pin to align the housing.
Some offset dowel pins have a screw diver slot to rotate the dowel pin. Google offset dowel pin search
Some have a Allen wrench set screw. Video

The face runout is usually good, the 54 shop manual says to use shims to align for housing tilt, up and down, side to side squareness to the crankshaft.
That also can be corrected by making sure the front and rear of the housings were machined parallel.
Assuming the back of the engine block is square to the crankshaft.
And making sure the mating surfaces of the engine block and the housings are free from debris.

The consequences of not following this alignment procedure I fear the most is vibration, shake.
Like something is out of balance, a whole truck Shake when auto shifting up or down.

I've heard story's from a couple of people that have replaced engines from Hydramatic trucks.
They couldn't get the truck to run like it did before they put in the new engine.
Me thinks the reason is, is because of misaligned flywheel housings.

The manual shift transmissions are a little more forgiving.
Unless you are driving at 200 MPH. NASCAR