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GMC 550 Hydramatic

Posted By 75gmc454 Tuesday, November 15, 2011 3:52 PM
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75gmc454
 Posted Tuesday, November 15, 2011 3:52 PM
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I have got an old '55 550 gmc that was originally a hydramatic.  When the truck was new it was used as a lowboy truck for the local caterpillar dealership.  Well my dad bought the truck in the 60's for his land clearing business and the hyramatic kept blowing seals.  They installed a manual trans and got rid of the hydramatic.  He thinks they kept the trans but I have not been able to find it.  I have been looking for an original model 245g trans for this truck with no success.  Does anyone know anything about these transmissions?  Here is a pic of the service parts id in the glovebox and a few other pics.  This truck has been sitting for a long time, but I still want to find the right trans for it.



The engine is a 324 olds.  The model # for the truck is M551.  Here is a pic of the shifter and hood emblem:



The truck was caterpillar yellow when it was new:



Thanks,  Clint
junkmandan
 Posted Tuesday, November 15, 2011 4:38 PM
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Clint-------Don't have a Hydromatic..................But I do have a 2 speed auxilliary that went behind them . I believe it was an option and is direct and 2.59 underdrive  .It would be a smart option for a truck that size .
Tuesday, November 15, 2011 4:39 PM by junkmandan
75gmc454
 Posted Tuesday, November 15, 2011 4:53 PM
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Sorry Dan I forgot to mention that this truck still has the 2 speed auxiliary in it.  Thanks for the post though.  Here is a pic of the intructions on the dash for shifting the auxiliary:

Tony Bullard
 Posted Wednesday, November 16, 2011 1:51 AM
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What transmission was used in the 2.5 ton M135, M211, M221 etc. military trucks of the '50s? That one had the two speed built in the tail.

Tony Bullard, Chelsea Vermont
''''''''34 Ford BB restored,
''''''''62 Autocar DC870H restored
junkmandan
 Posted Wednesday, November 16, 2011 6:16 AM
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Tony------From what I recall that setup had a married box direct connected to the special adapter castings on the Hydromatic . Mine is a remote mount with input and output shafts and a single shifter rail .
Dennis Wells AKA Smiley
 Posted Wednesday, November 16, 2011 10:51 AM
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James with the origanal hydros on display.



TWIN HYDOS = DOUBLE TROUBLE

 

THIS WAS THE FIRST YEAR THAT GMC PRODUCED A HEAVY TRUCK WITH AN AUTOMATIC TRANSMISSION  TWO 4-SPEED HYDROMATIC UNITS FROM THE MEDIUM TRUCK LINE WERE STACKED ONE ON TOP OF THE OTHER, AND THE POWER INPUT ROUTED IN SERIES THROUGH THEM THIS RESULTED IN 7 FORWARD SPEEDS (1-1, 1-2, 2-1, 2-2 ETC.} THE LAST TWO RANGES ARE OVERDRIVES.

 

THESE ARE THE TRANSMISSIONS THAT CAME WITH THE TRUCK WHEN IT WAS NEW.

I HAVE BEEN TOLD THESE HYDROS WERE GOOD FOR ABOUT A YEAR OF SERVICE BEFORE REQUIRING REBUILDING DUE TO THE HEAVY LOADS THE TRUCK WAS CAPABLE OF HAULING.

 


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Wednesday, November 16, 2011 10:55 AM by Dennis Wells AKA Smiley
75gmc454
 Posted Wednesday, November 16, 2011 3:03 PM
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Thanks for the information.  So my 550 would have had the twin hydramatic in it when new?  The trans code is 245G?  I'm not sure what the GVWR would be on the truck.   It is a tandem axle tractor though.  It does have the single shifter left in the floor with some kind of electronic assist on it for shifting.  It also has hi and low lights on the dash. 
75gmc454
 Posted Wednesday, November 16, 2011 3:31 PM
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I have just found that the 245G trans is just a 4 speed trans, not the twin hydramatic. 
Dennis Wells AKA Smiley
 Posted Wednesday, November 16, 2011 4:32 PM
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Sorry to add confusion to the tread. Most GMC would have had the standard 4 speed hydro not the duel hydros James was equipped with.

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75gmc454
 Posted Wednesday, November 16, 2011 4:58 PM
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There is no confusion.  Any information is appreciated.  Thanks,  Clint

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