BUSY BOLTERS Are you one?  The Shop Area
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Jul 21st, 2025 | | | | Joined: May 2009 Posts: 41 New Guy | | New Guy Joined: May 2009 Posts: 41 | I recently picked up a 1947 towtruck,it had a newer 261 and that was all I was really after. The tranny I believe is a sm420 but it has a little lever on the shifter with linkage that runs right down along shifter into the top of the tranny.Is it some kind of tranny brake for engaging PTO? The tow truck winch would run in reverse according to the last owner, could have something to do with that? | | | | | Joined: Oct 2003 Posts: 5,152 Cruising in the Passing Lane | | Cruising in the Passing Lane Joined: Oct 2003 Posts: 5,152 | You don't have an SM420, that's an earlier trans with a reverse lockout.
1955 1st GMC Suburban | 1954 GMC 250 trailer puller project | 1954 GMC 250 Hydra-Matic | 1954 Chevy 3100 . 1947 Chevy COE | and more...
It's true. I really don't do anything but browse the Internet looking for trouble... | | | | | Joined: Dec 2002 Posts: 2,538 'Bolter | | 'Bolter Joined: Dec 2002 Posts: 2,538 | What kind of tow unit does it have on it. Factory, or Home-made? | | | | | Joined: May 2005 Posts: 8,877 . | | . Joined: May 2005 Posts: 8,877 | You don't have an SM420, that's an earlier trans with a reverse lockout. I think an early AD truck could have had that before the SM420 came out, so could it be the original transmission for the truck?? Grigg | | | | | Joined: Mar 2005 Posts: 684 Shop Shark | | Shop Shark Joined: Mar 2005 Posts: 684 | I belive Grigg is right. That would be the correct transmisson for a '47. | | | | | Joined: Oct 2003 Posts: 5,152 Cruising in the Passing Lane | | Cruising in the Passing Lane Joined: Oct 2003 Posts: 5,152 | I don't see that he says if its an Art Deco or Advance Design truck, but my experience is '47 trucks of both types have the crash box with the reverse lock release lever.
I've bought a couple of trucks called 48's that had that transmission, and the serial number always resolves to a '47.
1955 1st GMC Suburban | 1954 GMC 250 trailer puller project | 1954 GMC 250 Hydra-Matic | 1954 Chevy 3100 . 1947 Chevy COE | and more...
It's true. I really don't do anything but browse the Internet looking for trouble... | | | | | Joined: May 2009 Posts: 41 New Guy | | New Guy Joined: May 2009 Posts: 41 | I think that it is the original tranny.It is a home made wrecker. By reverse lockout do you mean the switch is used to put it in reverse and avoid doing so while driving?If it's not a sm420 does anyone know what it is? I've got a buddy that wanted it for his brothers jeep- maybe not if it isn't the 420 or is it just as good? | | | | | Joined: May 2009 Posts: 41 New Guy | | New Guy Joined: May 2009 Posts: 41 | By the way thanks for all the previous info, it's nice to be able to find help these days | | | | | Joined: Oct 2003 Posts: 5,152 Cruising in the Passing Lane | | Cruising in the Passing Lane Joined: Oct 2003 Posts: 5,152 | The reverse lockout prevents going into reverse unless you use the release, hopefully meaning you won't go there unless its intentional.
The SM in SM420 stands for Synchro Mesh. I don't know what the earlier transmission was called, I just call it crash box because it has no synchros and is hard to shift unless you double clutch.
It's not what your buddy wants for a Jeep.
1955 1st GMC Suburban | 1954 GMC 250 trailer puller project | 1954 GMC 250 Hydra-Matic | 1954 Chevy 3100 . 1947 Chevy COE | and more...
It's true. I really don't do anything but browse the Internet looking for trouble... | | | | | Joined: May 2009 Posts: 41 New Guy | | New Guy Joined: May 2009 Posts: 41 | | | | | | Joined: Mar 2002 Posts: 9,112 'Bolter | | 'Bolter Joined: Mar 2002 Posts: 9,112 | I have a 1947 shop manual for the AD style truck and it shows the old style tranny. | | |
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