The Stovebolt Site Forums The Shop Area Driveline Speedometer Drive Gear questions

I have a ‘47 AD Chev 3100 with a 3speed floor shift. PO replaced the original engine. Not sure if the tranny is original. Discovered my speedometer wasn’t working. Attached a drill to the cable and it showed it worked properly. Took off the speedometer driven gear and probed the inside. Cannot see/feel the speedometer drive gear. Thinking it is missing…left out when the engine was replaced, I found a replacement drive gear. ( see photos). I “think” it is press fit to the Universal Joint Spacer. Cogs are compatible with the driven gear.

Since I have not opened the tranny and inspected it, I’m trying to understand the issue before I tear down.

I’m wondering if other internal parts are missing ? Looking at the ‘48-‘53 Truck Shop Manual, listed are Main Shaft Rear Bearing Snap Ring, Speedometer Drive Gear Spacer and might they also be missing? If so, where, how do I find these specific parts?

I am also confused why the Universal Joint Spacer is a smooth shaft without any splines? Is it somehow pressure fit? Seems to me it should be splined so it wouldn’t slip on the main shaft?.

I wonder if the AD model, with the 3 speed floor shift was simply an unused up ‘46 3 speed tranny that the factory installed…or didn’t want to retool for the new model?

I’m trying to understand this before I tear it down, search for parts and lose summer driving while locating/installing parts. Seems to be more of a winter project.

Many questions and little understanding, so any help you could provide would be greatly appreciated!

Thanks,
Jim

I'm not sure you got the right gear, unless your truck is a 47.1 year. Here's the cross section from the 48-51 shop manual. The gear looks narrower than yours. The sleeve between the speedo gear and the u-joint is not splined, but just holds the speedo gear in place when the front u-joint yoke is secured by the bolt into the shaft. As long as the sleeve is the right length and properly tightened, it will keep the speedo gear turning, as everything in contact with the gear rotates (shaft, inner bearing race, and sleeve). If there's too much load from the speedo driven gear, then it might slip, but that's highly unlikely.

Here's the 1947 transmission cross section also, The same thing applies to the sleeve and how it keeps the speedo gear from slipping.

Jim ,
Look very carefully into the end of the driven gear that the cable connects to. Often the very tip of the cable will break and be stuck in the end of the gear. If you look into the end of the gear and can't see the square hole wear the cable slides into then a piece of a cable is stuck in the end. I believe i have pictures in my project journal of what I encountered.

If there is a piece of cable stuck in the end of the gear it can be removed. What I did was spray PB Blaster, or any other penetrative Lubricant, in the end where the cable piece is and let it sit over night.
Then I struck the end of the gear straight down on the plate of my bench vise. It took a couple of strikes but the piece popped right out.

Thanks for the pointers guys. I was afraid the drive gear might be the wrong one….I’ll have to check that out real careful.

Never thought of a broken speedometer cable. I will first check that out as it appears to be the least invasive and easiest!

Back to the drawing board!

Crummy, your pictures were very helpful. The driven gear was clearly eye opening. Also found your detailed u-joint breakdown helped me to better understand what lies before me. Your photos will be a benefit to many.

The pictures also give support to klhansen questioning if the drive gear is correct for my transmission. (I hope I’m right, but I’m afraid he is. He’s got far more experience than I!!!!). 🤞🙏

Glad they were helpful to you Jim.
There is a chart in the shop manual that gives teeth count for the drive and driven gears. Tells you what matches up with what. I have a few extras gears around if you get in a bind. I will check on what I have and get back to you.

Your ‘47 AD was the first year and only year with a floor shift 3speed. ‘48- went to 3 on the column. I would imagine that the 47 and 46 are the same transmission.

Jim
To clarify for you my pics are of a sm420 four speed up joint and speedometer gears. Not to confuse them. They are different.

I’m thinking my 3 speed floor shift was a left over ‘46 stock transmission....or a use them until we design an update......but.....I can’t find ant numbers etc. that help me identify the transmission. Anyone have an idea how I can identify the origin of or the date of manufacture?
Seems the more I learn, the more questions I have! Lol

Jim - A little digging through the web and the GM Heritage Vehicle Information Kits provided what I think are the best pointers to help ID your transmission. This on-topic post found in the old VCCA forum provides descriptions and pictures of where you can find the transmission casting number, casting date codes, and serial number locations on your truck top mounted shifter 3 speed. The info is the same for 1938-47 truck 3 speeds.

Also, the below pictures and extracts from the GM Heritage PDFs for 1946 and 1947 trucks confirm where you can find the stamped 3 speed serial number and info on how to decode where it was built. Once you find your information on the transmission, perhaps you can post pictures so we can help confirm what it is?