Which Tranny do I have

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Mattrob

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New owner of 77’ GMC Sierra 25. Which transmission do I have in it. Label in glovebox does not say 400 or 350 but Turbo Hydramatic 1M40.
 

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bucket

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I *believe* that the standard automatic for 3/4 ton was the TH400, starting in '77.
 

nvrenuf

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That RPO code with “M40” in it, that’s a th400.
 

84c10Tony

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Trans, trans, tranny is a whole nuther' thing.
Uh, escuzie!...just had to click lol.
Oh, wait...never mind.
 

Bextreme04

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M40 is usually the code for TH400. MX1 would be the baseline trans, which would be the TH350. As others have said, the only way to know what you actually have is to climb under the truck and check the pan shape. If its a square shape its a TH350, if it looks a bit like Texas, its a TH400. A previous owner could have put anything in there in the last 45 years.
 

CalSgt

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Does it look like this?
 

oldretiredafguy

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75gmck25

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I don’t know for sure about a ‘77, but on my ‘75 K25 the M40 code is a TH350. Kind of confusing, since on later years ( I think it started in ‘77) the M40 option was a Th400.
 

mtbadbob

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Trans, trans, tranny is a whole nuther' thing.
Uh, escuzie!...just had to click lol.
Oh, wait...never mind.
The term "blowin' a tranny" has a whole nother meaning these days!
 

oldretiredafguy

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LMMFAO!!!!!
Keep laughing you sarcastic SOB'S. As soon as I can get a GPS installed on the wheelchair, I'm going to visit each one of you, and have a come to Jesus meeting!
 

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