The Stovebolt Site Forums The Parking Lot Big Bolts 1963 gmc 4000

anybody know what the small gearbox attached to the speedo cable under the dash of my gmc is for?it has a vacuum line attached to a diaphragm and it looks to control the speedo but i dunno what its for.

It's for the 2 speed rear. It shifts when the rear shifts so the speedometer is correct.

Anyone know where to find a new 1

well the gears and shift lever works but the diaphragm is busted so it wont shift so i just ran it to the upper side and it seems to work ok when the axle is in the high side.but i would like to repair it so it works properly but havent found any parts as of yet

The vacuum became obsolete in the mid 60's. Ever once in a while the adapters appear on eBay, but I don't know if they are any good either. Electric has been available since the 50's was used many years. I have a vacuum and don't worry about. It would be possible to wire a micro switch in to operate an electric adapter.

It does depend on the distance the mechanism needs to travel, but I've repaired similar items with a chunk of bicycle inner tube cut to fit. If it's too thick for that unit there are other items available that can be used for material, various types of chemical gloves come to mind. There are also rubberized fabrics available that are a bit harder to get the fit right, but are more durable over time.
Not meant to be permanent fixes, but may get you operational until replacements can be located.

Ten years ago I bought a new diaphram for my 1956 2-speed from my local BIG GMC Truck Dealer...have you tried them?

Mike B

The scary part about that is that they don't make big GM trucks any more. I guess the dealers who previously sold them will still service them and handle parts, but I think the Medium Duty Truck business was sold to Isuzu or someone.

As long as there is a part number, and if it is still available,
any GM dealer can get it for you.
That part # should be listed in the 46-72 Medium Duty Truck Parts Catalog.

ok thanks for the input folks greatly appreicated on this end

I found a vacuum actuator from an eighties carburetor that was close in size and fabricated a mount for it work great on my 56. Just another idea.

I have made antique outboard motor fuel pump diaphragms out of rubberized gasket material. Check with any industrial rubber and gasket supplier, and they should be able to point you in the right direction. I've been given scraps of thin rubber gasket material big enough to make dozens of small diaphragms from these places.
Jerry