I'm getting my trans cleaned up and ready to go in the Chevelle hopefully soon, (missing the motor) I spun the output shaft and noticed that the speedo output wasn't turning. Took the retainer and speedo adaptor off to have a look inside, and didn't see a gear that I figured would be on the output shaft. I did what any guy would do and stuck my finger in the hole. I found the gear towards the back, but it just spins freely back and forth and around on the shaft. I'm guessing that it somehow has to be attached to the shaft.

What is wrong and how do I fix it?

Thanks for the help,
Roger

No easy fix, you will have to remove the tail to repair the speedo drive gear.

Regards, Jody

That gear is a press fit, and should be on the shaft rock solid. It's need to be pressed on, and be at an exact location to mess correctly with the little plastic gear.

Fred.

It's a plastic gear. I did a search on this site and found out that I have to remove the tail housing and replace the gear and it's retaining clip.
Should I replace with plastic or use a steel one?
If I use a steel gear can I really just hammer it on using a piece of pipe, as is mentioned in one thread, without causing any damage to anything from the beating?
If a tire diameter is changed and I want to dial in my speedometer to read correctly which gear do I change, drive, driven or both?
Where do I buy speedo gears and gaskets. I also need a gasket for the front bearing retainer/throw out bearing collar... whatever it's called.

Thanks again for the help,
Roger

Roger,
I thought for sure Wally would jump on here, but he hasn't.
As already mentioned, that gear is NORMALLY a press fit. But, you say that you have a plastic gear, which tells me that you have a 69 (plastic gear and retaining clip). The other years used a steel gear that is pressed onto the shaft. You will have to remove the tail housing to do this operation. Getting the tail housing off is no big deal, but getting it back on can be! The reverse shifter shaft is retained by a small tapered pin. Use a punch and drive out the pin FROM THE BOTTOM. Then grasp the reverse shifter shaft (if it has a threaded hole, insert a bolt and pull on it) and pull out on it. It will only come out about 3/8in. Remove the five bolts and remove the tail housing. BE CAREFUL THAT YOU DON'T SEPERATE THE MIDPLATE FROM THE MAIN CASE!! ALSO, ONCE YOU REMOVE THE TAIL HOUSING, DON'T PUSH THE REVERSE SHIFTER SHAFT BACK IN. A SMALL BALL WILL FALL OUT AND IT CAN BE A REAL PAIN TO GET IT BACK IN!! After removing the tail housing, take that plastic gear off of the shaft and throw it as far as you can------------------------or incenerate it! Then install a steel gear. There are two sizes with eignt teeth for your tranny, you want the larger gear which is 1.84in diameter. This gear will be compatable with the small plastic speedo gears which go into the fitting on the side. The plastic gears are still available from Chev, the steel gear is not. The plastic gears are 3987917, 3987918, 39879019, 3987920, 3987921, 3987922 (17 to 22 teeth). Check with Wally to see if he has the steel gear. If not, you can get one from Larry Fischer, 631-351-4837.
As mentioned, you can just drive on the steel gear with a length of pipe. ALSO, AS MENTIONED, IT MUST GO ON AT A SPECIFIC POSITION!!! Simply measure from the front surface of the tail houising back to the center of the speedo fitting hole, then measure the same distance from the rear of the mid plate to the location on the tail shaft. Then drive the steel gear onto the shaft the same distance.
To re-install the tail housing, I find it easiest to do by placing the tranny upright (if you have a big enough vice, place it in the vice and clamp around the outer diameter of the front bearing retainer). If the shaft for the reverse idler gear came out with the housing, remove the reverse shaft (it just pulls out of the tail housing) and install it through the idler gear (there should have been a smooth thrust washer that goes up against the reverse idler gear), making sure that the pin in the shaft is in the verticle position because it lines up with slots in the tail housing. Shift the reverse shifter shaft forward. Install a new gasket with sealer on both sides (or if you wish, you can just use RTV, I prefer a gasket). Insert the reverse shifter fork into the shifter shaft. Lift up on the sliding reverse gear and place something like a screwdriver handle between reverse gear and the mid plate. Slip the tail housing down over the tail housing and shine a flashlight up into the housing to see when the fork lines up with the flange on the reverse gear. When it is lined up, push in on the shifter shaft to engage the shifter fork with reverse gear. Remove the screwdriver handle and push the tail housing on down all the way. Sometimes this requires some juggling of the tail housing to get it all the way on. Button it all back together. I don't know what the correct torque is on the five bolts, but for the past 40yrs that I've been building Muncies, I just tighten the crap out of them.
I shouls have mentioned that since you have the tail housing off, now would be a perfect time to replace the rear bushing (if it appears to be worn) and especially the rear seal. The bushing has to be driven out of the tail housing and a new one driven in. Just pry out the old seal and tap in a new one (seal and bushing are available at the local auto parts store, same seal and bushing as a turbo 350).

Geez Tom, Thanks a ton for the detailed how to. It's much appreciated!

Just a little clearification. There is only one steel speedo gear that fits on my shaft inside the trans (1.48)in? The six other gears that you mentioned are for dialing in the speedometer reading and held in with that gizmo on the outside of the transmission?

I should be able to handle this repair just fine using your great detailed explaination.

Thanks again,
Roger

You should also do your homework and determine your SGR (speedometer gear ratio) before you buy a new steel drive gear. There is a 7 and 8 tooth drive gear available. Also take into consideration any future plans for your car. A differential gear change of a ratio or even a tire diameter change could result in a different drive gear selection to dial your speedometer. Who knows, maybe your existing speedo gear combo is not correct.

Regards, Jody

If Chevelles are like Camaros then you'll see the Stewart Warner speedo adaptor on the side of the tranny. See http://ca.geocities.com/1970-z28@rogers.com/images/speedo_adapter_a.jpg

I myself have the plastic gear on my 70 Muncie in my 70 Z/28 held in with a clip on the tail shaft which makes it easier when you need to remove reverse, 1st and 2nd gear/clusters/syncros for repairs.

You can also heat up the new steel speedo gear with a propane torch and it will slide right on the shaft. I did that recently and it went on with no problems and it keeps the gear from getting beat up.

The gasket should be available from a gm dealer or Mr gasket makes a gasket kit that is available at Jegs or Summit or anywhere that sells Mr Gasket. that kit has the new rear seal but does not include the bubhing.

It's a plastic gear. I did a search on this site and found out that I have to remove the tail housing and replace the gear and it's retaining clip.
Should I replace with plastic or use a steel one?
If I use a steel gear can I really just hammer it on using a piece of pipe, as is mentioned in one thread, without causing any damage to anything from the beating?
If a tire diameter is changed and I want to dial in my speedometer to read correctly which gear do I change, drive, driven or both?
Where do I buy speedo gears and gaskets. I also need a gasket for the front bearing retainer/throw out bearing collar... whatever it's called.

Thanks again for the help,
Roger

I sell whatever you need. I use steel gears, never a plastic gear. I also stock all the driven gears that GM still has in stock.

Well the first thing I need to do is take a stab at what I will end up using for rear gears and tires. The trans is an M-20 with a 252 1'st gear. I currently have 456 rear gears and thinking that eventually I will put in 355's. A 275/60/15 tire is about a 28" tire. When I ever get the motor back it will be a 383 with around 450 TQ and HP so i'm thinking 355's would be a good for all around choice
can somebody tell me what speedo gear combination I will need with 355's and a 28" tire then I can order all the gears and gaskets I need at once wally. I come up with a 9-22, but would like a second opinion.

Thanks a ton for all the help,
Roger