Speedo Gearbox Cleaning Pictures, and New Vacuum ActuatorWhile going over the dash on my 1959 GMC 9600, I had to look at this odd box attached to the back of the speedometer.Larger views for all the embedded images are at the end of the thread, as Attachments.
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See Images 1 and 2 below for overall pictures before disassembly. : : :
Some searching on the Stovebolt forum revealed
it was used on two-speed axle models to adjust the speedometer output depending on which range you are in. When you shift the axle range, vacuum from the shifter switch is routed behind the dash to shift this gearbox as well.
Well, this guy needed some cleaning to get the dirt off of it -- so let's look inside!
The cleaning
First things revealed was that the gearbox was made by Stewart-Warner, who also likely made the speedo for the truck. There were these stampings on the housing, the 675 likely being the ratio of the gearbox. Image 3
Inside the gearbox is a pretty simple mechanism. A shifting fork moves a collar back and forth engaging a dog clutch on one of two gears. A spoonful of grease inside provided lubrication.(4,5)
To put this guy back together, I dabbed some RTV around the housing flanges to make the gearbox a bit more sealed up, and filled the gearbox with some John Deere Auto-Lube oil. (Basically jellified oil - perhaps a little more liquid than corn-head grease.)
Now, to address a common age-related issue on theseOne of the problems is the vacuum diaphragm rots away on these, and because replacements are no longer available, these gearboxes are either plugged off or removed because of the leaking. One old thread had suggested you could substitute a vacuum actuator from an older emissions carburetor, but no specific part numbers were given.
The inspiration for my solution happened to be a 8HP Honda-Clone generator I picked up. I had noticed It happened to have a vacuum actuator to pull the choke back open once the engine was running, and both the stroke and size of the actuator were pretty darn close to the original actuator! A quick search on Amazon quickly found me a replacement. (6)
The OEM part number for this actuator is Honda 16400-ZE2-704. If you search for 'GX240 choke assembly' on a online retail space like Amazon or Aliexpress, you can find clones that are cheaper if you strip the extra parts off.
I sat down with Fusion360 and created a new bracket to hold the actuator (7):
I got the part CNC-routered out of some 1/8" aluminum. I opted to drill and tap the hole on the shift lever to 4-40 and use a small bushing to address the slop left over on the actuator arm. to ensure that the screw was not going anywhere, I put an extra jam nut onto the screw. (8,9)
ADDMENDUM MAY 1st,2025
At the Admin's suggestion, I created a PDF of the adapter plate outline. To use it, print it from your computer (ensure your print settings do not "scale to fit" the image to maintain accuracy!), cut the outline out, and glue it to a sheet of 1/8" aluminum. Use a scroll saw or a coping saw to cut the shape out.
I also discovered the gears in this gearbox can be assembled backwards! I noticed this spring that the speedometer was reading high while in low, and low while in high. If you are in this situation, you will need to take the gearbox back apart to flip the gears around.
for thinking way outside the box. Great solution to a common problem.