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Can the drive gear on the turbo 350 in a 1978 Corvette tailshaft be replaced with the transmission still in the car? If so, what is the procedure and does to transmission fluid have to be drained?
I found a new gear in the glovebox of a recently purchased 78 Corvette and the speedometer does not work. It has less than 40000 miles, the previous owner is deceased. I don't know how he determined this was the problem. What is the likelihood that this gear would be bad since I have never had one bad in all my years of experience.
Thanks in advance.

7T1vette

Yes. You can remove the speedo drive gear from the transmission. Not that familiar with a THM-350, but there should be a single 1/2" bolt and retainer at the location of the speedo cable on the transmission. Remove the bolt and retainer. Loosen the speedo cable 'nut' but don't remove it. Now, grab the speedo cable and pull the speedo drive gear carrier out of the transmission. Now, you can remove the speedo cable. You may get a bit of fluid out, but not a lot. Put some paper towels/rags down under that area and they will sop it up.

There can be several reasons why the speedo doesn't work:

1. No speedo gear installed;

2. Bad speedo cable;

3. Shaft on speedo gear is cracked where the cable gets inserted, so that the cable can't really be driven by the gear;

4. Problem with the speedometer.

It sure won't hurt to try making a fix. Just realize that the gear you found could be a new gear...or it could be the defective gear the prior owner removed from the trans.

409/409

I am not talking about the gear on the end of the cable. I'm talking about the gear on the output shaft. The gear in the glovebox is absolutely brand-new and the hole in the center is the diameter of the output shaft.

TimAT

The drive gear can be replaced in the car- remove the driveshaft and the 4 bolts that hold the rear housing on the case. Once it's off the gear is right there. Use a small screwdriver to press the retainer down and a large open end wrench as a driver to tap it off. Be aware that some tail shafts have 2 holes for the retainer so make sure you get it back in the correct spot.

409/409

Does the drive shaft have to be removed or can it be slid back far enough? Does the transmission have to be drained and is there a gasket there? Thanks.

7T1vette

The only reason to replace the worm gear on the tranny output shaft is if the one in the trans is the wrong part for getting your speedo to read correctly. The internal gears are usually metal, but are made of a harder material than the outboard gear so that the internal one can not wear out! You need to find out if the speedo can actually WORK before you get to fooling with that new gear.

Once you establish that the speedo can function OK, THEN you can worry about calibration. So keep the existing internal gear in place until you get to the 'calibration' question. If it needs to be changed at that point, have at it.

409/409

I disconnected the speedometer cable and ran it with a cordless drill and it worked. The small gear on the end of the cable looks good. The big gear I found in the glove box is plastic and the same color as the one you see in the hole on the output shaft. I can't tell if it is bad or could it slip up the shaft and is no longer meshing with the small gear? Does it look like it is in the right place on the shaft? Could the small gear be too small? What color small gear goes with the orange big gear for a 3.08 ratio? Also, does the transmission have to be drained to remove the tail housing?

409/409

I have read that Muncies have 2 different diameter gears for the output shaft.
Does the THM350 have more than 1 diameter also? Please, I need answers before I take the tail shaft off. Thanks.

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Please, anybody!

SciVette

My 1980 4-speed had a blue drive gear and the natural (19 tooth) driven gear with a 3.08 Dana rearend. Here's a chart for your 1978:

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My driven gear is natural and 19 teeth. All teeth are perfect. I have a short piece of speedometer cable and I can turn the small gear when it is installed. It is obviously not meshing with the orange drive gear. Can you tell if the orange gear is in the proper place or has it slipped on the shaft from where it is supposed to be? Is it possible the silver 22 tooth gear would mesh??? How much of the orange gear should be showing in the hole?

TimAT

From your picture, the gear has slipped forward on the output shaft. The retainer has either broken or it was not installed. The drive gear has to mesh with the driven gear. The shiny flat spot you can see thru the hole is where the end of the driven gear sets. The drive gear should be centered in that hole.

Antz81

Your drive gear is not positioned correctly.

7T1vette

You might reach in there with a finger or screwdriver to see if that gear will slide back & forth on the output shaft. It should have a clip that keeps it locked in position on that shaft. If the gear won't move, it may have been installed incorrectly. If it slides, the clip is broken or was never engaged with the hole in the shaft.

If you cannot get it back in position nor keep it there, you will need to remove the tail housing of the transmission. Not that bad of a job. You'll need a new housing-to-tail shaft gasket and it would be a good time to install a new output seal. But you can diagnose and fix the problem with the speedo gear on the output shaft.

P.S. MAKE SURE you install the correct gear on that shaft. Verify with the above chart or other references. You don't want to do this twice.

409/409

Do you have any idea who sells just the tail shaft gasket and the retainer? Does the transmission have to be drained?

7T1vette

Try a local parts store. If not, stop at a tranny repair shop and see if they will sell you one. You can always buy a sheet of gasket material and make one.

409/409

Does the THM350 tail housing take a gasket or "o" ring?

TimAT

350 uses a square cut o-ring. Chances are it'll be fine, but if there's a trans rebuild shop close by you might stop and see of you can talk them out of one. If the tail housing has never been removed, the 4 bolts may be really stuck in the trans case. A little heat or some of your favorite penetrating oil will help.

409/409

I replaced the gear, clip and "o" ring and all is well. Thanks to all.

7T1vette

What did the problem turn out to be? Just curious....