Jackfit

Hi ,

Just when you think you done, you're not. As I try to finish all mechanical work on my 66 , prior to paint, I have decided to tackle the speedometer problem again. Rebuilt my dash speedo, and put in new driven gear (22 teeth) and fitting for 3.70 rear. Tranny is M-20 wide ratio, original to car along with motor.

I had speedometer working for first time in years..... for 90 miles !!!!

Dash speedo works........Driven gear chewed up at transmission fitting.

L79 M-20 and original rear was 3.36 Changed rear 35 years ago to 3.70

Question

Does the speedo drive gear on the tranny shaft have to be changed to be right for the 3.70 rear end or is the one on it OK for 3.36 and 3.70

Also, the spot where the driven gear (plastic) rests on the transmission housing is excessively worn. The plastic gear has vertical motion in fitting more than I think you should.

Can this be part of problem?


I was on my back all day to take out tranny,, Boy, a lift would really make that job easier. I only want to put this car back together once , before my trip this summer.

Comments on possible fixes please........


Jack

vetrod62

There are large dia. and small dia. driven gears and there are large dia. and small dia. drive gears . you should check to see if you have a mismatch. Click below.

http://www.stl-vettes.com/65Vette/GearCalc.htm

Jackfit

Now to get the proper driven gear

My question about the worn spot on transmission housing resulting in excessive end play?


Jack

66BlkBB

Jack,

Are you saying that the opening that holds the speedometer gear and its housing in place is worn??? I am not getting any idea of what your worn spot may be from. Maybe you could post a photo of the area. Just a thought.

By the way, go to LIC and check out their speedo gears. They give you which gear has to be used with which gear on the rear shaft.

Steve

Jackfit

Hi Steve,

I think after you look at the photos, this problem I am having may be a problem with many others .

If you look at the photos of the driven gear photos you will see that the plastic gear moves in and out of the fitting toward end spot in transmission. The fitting is set in housing by and lock key the puts the fitting in the same spot (depth in housing) when inserted in housing each time.

The photo shows the ware spot on my housing. I am sure the engineers at GM machined the end spot that the gear rests on, to put the plastic gear in line with the drive gear on shaft. If the plastic gear can move further in because of ware on end shelf, it will no longer be in line with drive gear. I think,

Could this cause the plastic gear to be chewed even if gear (drive and driven) are matched.

What is the allowable amount of end play of driven plastic gear before it gets chewed up by drive gear?


What ever it is , I have more I think

Jack

66BlkBB

Jack,

Are you sure these are matched. Did these come with the tranny or have you replaced the fiber gear with one that you put in? When I had my speedometer gear out along with the gear housing, my tail shaft housing looked the same as yours. Also, I don't know how a fiber gear would wear on aluminum. My guess is that the old fiber gear is worn and needs to be replaced. You may have to check the drive gear on the shaft to see if it has any problems. Everything else you've shown in the photos seem to be usable as is.

Steve

MiguelsC2

I believe the green gear is for the 3.70

rayvaflav

Hey there Jack, as a long time GM mechanic I have replaced dozens of these speedometer driven gear components, but not for at least a dozen years ! As a rule, the speedo drive gear on the transmission output shaft is likely to never wear out whereas the driven gear always does and in your photo, it absolutely is a problem. The OD of the driven gear sleeve usually does not wear but the ID of the sleeve does and therefore wears the driven gear. With the sleeve out of the trans, see if the driven gear will move up and down in the bore (I bet it will). Try a new sleeve (which should come with the inner seal) a new o-ring and driven gear. And then take that thing for a long trip around the USA !

Ray

Jackfit

I did change the fitting. I just took some pictures of my drive gear and it appears that it has been distorted. see photos. Some members of the other forum saw the pictures and agree. Tranny is out, 3 hours on my back (I just hate that job) and I will change drive gear.

Jack

Jack

66BlkBB

Jack,

You may want to check for something in the tranny while you have it out. That is a metal gear and no fiber gear should damage it like that. Has to be something hard to damage the drive gear on the shaft.

Steve

Jackfit

Thanks everyone for their input. I going to have the drive gear replaced. It has been a problem for 35 or more years. It may have been damaged when tranny was rebuilt in 73 and 91

Jack

TheSaint

The number on the gearbox on the pictures posted of Jackfit is that the number to see if the transmission is the one the car came with?
Sorry for taking a litlle off your thread here Jackfit

66BlkBB

NO!!! The number will be stamped on the side/rear flange of the main case. I am not sure which side but it will be stamped vertically so you will have to turn your head to read it.

Jackfit

No, the vin # number of my car is stamped (not cast) on the case, it matches the number stamped on my engine pad and vin plate under glove box.

The number you see on photo is GM #s for transmission case. (extension housing) It is a M-20 wide ratio , you count the rings on front shaft of transmission.

Main case #3885010
Ext case #3857584
side cover # 3884685
build date P0321 March 21 1966

Jack

count

So is this gear stem (the part that goes into the fitting hole) is sticking out of the trani housing or is it internal? Im at this point too where I am hooking up the fitting but when under the car I see nothing protruding out of the case but everything is flush and no where near a hole big enough to insert the fitting in...that's why i thought maybe i was missing the gear? Sorry for the hijack but this seems pretty close to my issue without opening a new thread.

Jackfit

Hi,

There are two parts to the speedo fitting. The fitting (shown in above post) and the plastic driven gear, ( shown in the fitting)

The fitting with the plastic gear in it ( teeth of plastic gear facing tranny) is placed into tranny housing . The fitting is locked into place by the tab which goes into slot in fitting, and is tighten down against housing .

If you can't see the housing opening, it is because you still have a fitting and plastic gear in opening.

The drive gear, attached to shaft inside of rear of tranny can only be seen if fitting is not in opening. (photo of my housing showing drive gear)

Jack

PS

I took the tranny to the transmission guy and he has the tranny apart. I will have the new drive gear tomorrow, with a new 22 tooth silver gear. The new gear is same as stock with M-20 wide ratio. 3.08 to 3.55 , but with my 3.70 the silver plastic 22 tooth one should work.

The leak from the front is probably coming from the counter-shaft opening on front of tranny. We hope silicone will help.

count

Thanks for the info, the only thing that doesn’t make sense is why I don’t see the external threads of the fitting sticking out of the train case……if you are correct and the trani already has the assembly installed I would just need to screw the cable end with the internal thread end onto the fitting….I’ll get under there again tonight.
Thanks

good to hear you got your issue solved!

Jackfit

Hi, You should see threads of fitting (about 1/2 " ) sticking out. The cable screws on to those threads, If you can;t see the threads, the fitting has been pushed it too far, but that can only happen if there is no plastic gear in fitting. Does the fitting have the tab and screw attached to housing? See photo in post, with opening showing, see the tab and screw. You should see that tab holding the fitting in housing. That sets the right place for fitting to rest in housing so you can screw in cable.

Jack

count

Hi Jack
I’ll check for the tab too…you might be right and there is no gear in there…
Maybe I’ll be 2 for 2 and have another awesome day!!!

count

Jack thanks for the info, I indeed did have the fitting already installed. I removed my other fitting to expose the "square" steel cable but noticed this did not spin, it was a fixed wire.
I proceeded to install the cable as you showed me and tightened the cable onto the fitting by hand since i could not get any tools up in there.....well after starting the car, the rev needle did not move on the dash. I would think that steel cable would spin freely in order to transmit the revolutions...maybe something is seized

Thanks for letting me tag along on this thread!! lol