6066 (1960-1966) GMC Truck Club

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-   -   Transmission identification (https://6066gmcclub.com/showthread.php?t=50948)

Onthefly October 4th, 2020 12:52 AM

Transmission identification
1 Attachment(s)
so now that the motor is running strong my next step is the transmission. Just trying to see exactly what I have before I drop it and replace the clutch. Any help would be great!!! Attached picture of drivers side serial numbers, maybe you guys can school me in deciphering these.

George Bongert October 4th, 2020 11:40 AM

Re: Transmission identification
Greetings Onthefly!

Those numbers mean nothing to me. Does your Transmission have a shift lever on the top, or two short shift levers on the drivers side of the Transmission? Shift lever on top, you have an SM420 4 Speed Transmission. Two short shift levers on the side, you have an SM318 3 Speed Three On The Tree column shift Transmission.

Onthefly October 4th, 2020 04:45 PM

Re: Transmission identification
Thanks for the reply!!! The shifter is on top, I know its a sm420 more trying to find the manufacture date. My truck was origi ally a Yakima school district truck and I *** I me had a 3 on the tree originally , it still has the steering column for that anyway. At some point a 4 speed was put in and I'm trying to keep everything as original as possible at least year wise.

BillT October 4th, 2020 06:35 PM

Re: Transmission identification
If it's an SM 420 for sure, the Date Code reads April, 22, 1958 (possibly 1948 if they made them that early, or 1968 if they made them that late).

Also curious. If it got changed from a 3-Speed to a Truck 4-Speed, did they change the Floor Hump or does it still have a Low Hump?

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