The Stovebolt Site Forums The Shop Area Driveline 1957 3100 Speedo drive gear issues

I am working on a 1957 3100 with a 4 speed on the floor. The P.O. replaced the 57 tranny with what appears to be one from a '59. The case is very clean and painted black. There is '59' painted on the side in yellow paint, which I assume means it is from a 59.
The speedo drive gear shaft, square hole for the speedo cable is round. It will not drive the cable. So here are my questions.
1. Is the speedo drive gear the same for a for a '57 as for a '59?
2. If so, is the speedo drive gear replaceable? (Just the gear and shaft?)
3. How do I separate the shaft and gear from the housing?
4. Any suggested sources for a replacement shaft and gear or the complete assembly?
Thanks for your guidance.

If your driven gear is a 20 tooth unit, that would match up with GM# 3707598 which is a 7 tooth drive & 20 tooth driven gear complete set listed in my 1933-62 Chevy Master Parts book that fits 1955-58 1/2 Tons with the 4 speed and the tire sizes 15" or 6.50" X 16" tire sizes. Finding just the driven gear unit in your picture is probably going to require finding a donor transmission with the same piece as I couldn't find anything currently being offered around the web.

The later 1958-59 4 speeds with the standard tire sizes 6.70" X 15" or 6.50" X 16, used gear set GM# 3752392, which is a 7 tooth drive & 19 tooth driven gear set. It sound like the gear you have pictured is not from a 1959 4 speed. Perhaps that transmission was mis-marked by someone along the way?

Before you go further in search of another driven gear, I would suggest you double check that the shaft hole in the gear doesn't have a broken off stub stuck down inside the whole. Those cables have been known to shear off and leave just enough of a stub in the drive hole to prevent a new cable from setting down deep enough to engage the squared portion of the hole. Soak it in Kroil or something similar and dig down in the whole with a very small sharp tool like a dental pic. You may be surprised to find and remove an offending stub and get your drive gear working again.

FWIW, below is a pic from an NOS GM# 3752392 gear set currently offered on ePay. If you look at the cable hole in the driven gear, it looks round to start. The squared hole portion doesn't start until a good bit down into the hole.

3100 in Santa fee give me a hour and I will take a picture of mine I think it is the same .they should be the same from trucks with a 390 rear .

Looks the same the square portion is about 1/4 inch inwards of the round end I put a piece of zip tie in the hole and it goes in about a inch.They don’t make these anymore and the older ones have a different tooth count for the 411 rears.

Thanks to all for your help! I have checked for bits stuck in the drive hole and found nothing. I am able to put a new cable deep into the gear and it still does not spin the cable. I may have found one from a donor. We will see. Thanks again! george

pretty silly question, but did you pull the old cable out of the sheath to make sure its intact? The transmission speedo gear seldom breaks but is easy to replace if it does, on the other hand one of the first things I replaced in my '57 with a sm420 was the cable. clean it up and lube before reinstalling and make sure it spins freely in the sheath as they get abused, crushed or kinked with PO's changing out motors and trannys.