Hey gang!  I have changed my rear gears from 3.36 to 3.70 ratio.  I have a M-21 4 speed.  Can anyone give me a starting point for selecting a new speedometer drive gear?  Color & number of teeth?

I am a new/old guy.  I just rejoined.  Its been about three years.  I see alot of the old names and several new ones.  It's good to be back online with the group.

Thanks,

Steve

The Woodlands, TX

 

You won't be able to change the drive gear without removing the tailhousing. You can change the driven gear, tho. I think you should have a 19t(dark brown) gear there now. Let me check and see what you need for a 3.70, and I can get back with ya...

Thanks Joel!  I haven't even pulled the old one out yet.  I had a chart at one time, but I can't find it now.

Thanks,

Steve

Hey, Steve, here it is!! You should have a brown, 18 tooth driven gear in it now. You need to get a blue, 20 tooth gear(GM #3987920) for the 3.70 diff. The speedo housing and the adaptor are the same, so there's no need to change either of them, just the gear. If you can't locate one, holler at me, and I'll get one for ya.

And, for anyone else that's interested, here's a Speedo Gear/Diff. Gear chart!

Thanks Joel, Your da MAN!!!!  I appreciate the help.  I have a couple of local sources for the gear.

Steve

    

I have just finished pulling the white 19 tooth gear out of my auto transmission. reinstalled. not wore or any missing teeth. have driven cable from transmission to speedo head with drill motor and works through the transducer. Still not working now. Red drive gear that i can see inside the opening looks fine.

What holds the white gear in the housing? It slides into housing from inside and just stays there? Did not see anything to hold it in place!

The bolt and slide plate hold the housing to the transmission.

1978  C-3

Thanks

Bick

Bick, on the inside of the trans tailhousing, there is a machined boss that the speedo gear butts against. This is what holds it into the housing.
If the speedo works when you drive the cable with the drill motor, there is either a problem with the driven gear(white), the drive gear(red), or the cable end.
Check the square end of the cable, and make sure it is still square.

Then, look at the hole in the end of the gear that the cable fits into, and make sure it is not rounded out, but square. Also look into the housing("bullet") from the gear side, and see if it is ovaled, instead of round. This is very common on the aluminuminum bullets, but has never been a problem with the steel bullets. This would allow the gear to not make contact with the drive gear.

Then, with the speedo gear/housing off, reach into the tailhousing with a thinnish screwdriver, and see if you can move the drive gear(red) back & forth on the shaft. If the gear in the trans moves, that is your problem. There is a spring-steel "clip" that holds the gear to the tailshaft, and it can break, allowing the gear to spin freely on the shaft. The gear itself can crack, and create the same symptom.

Joel,

Thanks, I will get into it again when return from Carlisle this weekend!

Tach works so know estimated speed anyway!

Bick

Bick, jest tuh gives ya an idear of whit tuh look fo, check out THIS PIC.

And DIS ONE too!

Sorry they be a wee bit blurry, but...I'm an ole fart wid shaky hands! It be hard tuh takes a close-up wid one hand, and not mess it up. Adams' Apple38954.8366550926