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**'68OrangeSunshine** · 09-04-2003, 06:27 AM

SynchroMesh is what the SM stands for. The last year for U.S. production SM 420s was 1967. They continued making trucks with the 420 in Canada until they ran out. Muncie is a town in Indiana where they make transmissions for GM. Saginaw is a town in Michigan where GM also has a tranny plant. Both of these plants made more than one style of 4-speed and/or 3-speed, thus the confusion.I believe the RPO code on my '68 C/10's glovebox states "M20."
I used to think this was the famed "Muncie Rockcrusher," because I have gone out in the boonies and literally crushed rocks with it. I had a difference of opinion with some musclecar dude, who was sure the "Rockcrusher Muncie" was a car tranny so-called because it sounded like a box of rocks. I guess it all depends on if you're sitting around drinking beers with car guys or truckers.

09-04-2003, 06:27 AM	#8
'68OrangeSunshine Senior Member Join Date: Feb 2002 Location: Tucson, AZ USA Posts: 7,622	SynchroMesh SynchroMesh is what the SM stands for. The last year for U.S. production SM 420s was 1967. They continued making trucks with the 420 in Canada until they ran out. Muncie is a town in Indiana where they make transmissions for GM. Saginaw is a town in Michigan where GM also has a tranny plant. Both of these plants made more than one style of 4-speed and/or 3-speed, thus the confusion.I believe the RPO code on my '68 C/10's glovebox states "M20." I used to think this was the famed "Muncie Rockcrusher," because I have gone out in the boonies and literally crushed rocks with it. I had a difference of opinion with some musclecar dude, who was sure the "Rockcrusher Muncie" was a car tranny so-called because it sounded like a box of rocks. I guess it all depends on if you're sitting around drinking beers with car guys or truckers. Last edited by '68OrangeSunshine; 09-04-2003 at 06:45 AM.