What transmission is in my 1988 3500?

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rose359

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My RPO codes: MM4: Transmission, Manual, 4-Speed (Merchandising Option); M20: Transmission, Manual, 4-Speed (Muncie).
The transmission in the truck is a 5-speed; first gear does not feel nor sound like granny gear. My reference is the SM420 in my last truck (1966 C10) - first gear was a crawler (7.05:1). I don’t see any part or model number on the transmission case, which is aluminum. Do these photos provide any identity clues?
 

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someotherguy

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Definitely not the original transmission. That appears to be an NV3500 or one of its variants (it changed names and design over the years.) It's the light duty non-granny gear 5 speed that a lighter truck would get. In GMT400's they only came original in 1500's and 2500 "light duty" (6 lug 2500) trucks. Never in a 2500 (8 lug) or 3500.

Richard
 
packer0440

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It was swapped out. The M20 code means the truck had an SM465, which is the descendant of the SM420 and has a granny gear (and is a cast iron case).

By your description, what you have now is likely a light duty 5 speed from a GMT400, but they are classified differently based on year, so hard to pinpoint what it is.
 
618 Syndicate

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packer0440 said:
It was swapped out. The M20 code means the truck had an SM465, which is the descendant of the SM420 and has a granny gear (and is a cast iron case).

By your description, what you have now is likely a light duty 5 speed from a GMT400, but they are classified differently based on year, so hard to pinpoint what it is.
Click to expand...
He's asked this in another thread and got a similar response.

@df2x4 @kennythewelder can you merge the threads?
 
rose359

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I did some more digging for the answer. A person in a Facebook group said that the Getrag transmissions had external hex head fill and drain plugs, while an NV3500 has 17mm internal hex (Allen) plugs. My transmission has the latter.
 

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