Any tricks for shift shaft leaks?

[PbFut]

I have a bad leak at the shift shaft on my 72 TH350 tranny.
Bought the seal tool and replaced the seal twice, with no success.
Bought and replaced the shaft and another seal, still leaking.
It has to be something I am not doing. What’s the trick?

[pjmoreland]

Did you install the seal with the lip facing into the transmission?

[Steeveedee]

I have the same problem after replacing only the seal. Is it possible that the valve body is worn enough that the shaft has a radial offset? At 220k miles, I wouldn't be surprised. I'd not like going into a trans that otherwise is working fine. Tired of throwing absorbent under the truck, though.

[geezer#99]

I had the same on an 84 350C.
Wanna be cheap fix mechanics like to use sharp tools to pry the old seal out and usually create small burs on the shaft while digging around in there.
The shaft needs to be pulled out and machined smooth. Gotta drop the pan to get it out. Let a trans specialist do it.

[PbFut]

At third attempt I swapped the shaft and put in a third new seal. Old shaft had some sign of wear and I was sure it would fix, but it did not change the leak at all. I am thinking that I or previous hack may have scored the case where the seal is effectively metal to metal at the machined bore of the case. I used a specific shifter shaft seal removal tool but it is possible that a previous mechanic may have scored the bore in the case. Its difficult to get a good look at the condition due to location. Unless someone sees a glaring issue not to, I think I will today drain again and drop the pan. Shift to various gears to allow more fluid out. Remove new shaft and seal. Clean bore of all fluid. Lightly coat bore with RTV and then install seal. Verify rubber area of seal is clean and let RTV cure. Reinstall another new shaft and hope for the best.
If there is a flaw in this process, please let me know. I will let things drain today and order parts. Seal tomorrow, and reassemble Wednesday. Fingers crossed.

[Steeveedee]

Not having done something like this in over 40 years, does the selector shaft slide out easily? The more I think about it, the more I like Geezer#99's idea of taking it to a trans shop. This laying on my back in the driveway got old a long time ago.

[sick472]

I'm not sure if the TH350 is like the TH400 in this case, but...if so, there may be a second seal hiding in your transmission that needs to be removed. Get a mirror and make sure. I had a leaking shaft seal in my TH400, changed the seal with no luck, bought a new shaft and seal, then found the hidden seal upon installation of the shaft. It's a trick that mechanics would do...the seal is leaking and they'd slap another on top of the old one. Easier than swapping the shaft or taking the chance that a new seal alone wouldn't fix the problem due to a worn shaft at the first seal.

[geezer#99]

I was told they trued it up on a lathe.
And my trans had 2 seals sandwiched into the trans to stop the leak. Didn’t work.

[Steeveedee]

A shop I worked at right out of the Navy used to knock the rear bushing on a trans further forward and put in a new one, then install a new seal. Seems like that may be the case with some of these selector shaft seals.

I have a transmission guy I can trust. I should just take it to him.

[Accelo]

This seal isn't very problematic as the shaft doesn't rotate.

I see you used this tool that lets you pull the seal without pulling the pan. I am including a picture of the tool. It also includes the driver. The vendor is NAPA.

You got me on this one. Just isn't typically an issue. I would pull the pan and the shaft for a closer look.
Be sure to file the flats near the rooster comb flat before removing the shaft.
Maybe the last re-builder didn't and damaged the bore when forcing the shaft through the hole?
Magnification is a must if your are close to my age. Inspect the bore as best you can.
Maybe some sealant on the case bore before driving the seal home?
Best of luck

[PbFut]

Todays update, and apparent success.
Correct on the tool. It is worth every penny. Just be sure the area is very clean or you could score the shaft and then no seal will fix it.
The shaft does rotate in use when selecting gears. Rotation is done via the shift linkage to your column.
The Shaft is available at Summit and Jeg's if you find the shaft to be scored or marred in some way. It's 28-30 bucks and you don't risk going too small turning it down at a machine shop to clear a mar.
The issue is probably more on me then anything but....
The end of the shaft is machined on both ends. Threaded as well as a 2 sided notch allowing for the internal linkage as well as the external linkage to connect to the shaft and rotate it. The machined edges are rather sharp. I believe every time I attempted to install the seal the rubber would catch on the edge of the machined notches and damage the new seal. Yesterday I installed the seal and then slid the shaft in thinking the direction being opposite sliding the seal on and less against the way the seal is designed might work. It did not. Today looking at things I think I was on the right path yesterday. Using a new seal every time, I removed the shaft. It's actually quite easy the third time. I slid the seal on effectively from the back side onto the shaft. Doing this out of the tranny also allows for close inspection of the seal on the shaft on both sides. All looked good and no nicks. I slid the shaft into the tranny getting the internal linkage onto the shaft but not pushing shaft all the way home. I used the install tool that comes with the kit and tapped in the seal. Completed tightening down the internal and external linkage arms and being careful not to slide the shaft more than necessary. Installed the pan, new gasket again. The tranny is a little overfull as I have not started the truck so the extra fluid is not pumped into some of the other areas of the tranny putting the fluid level up to the seal. This is sort of a over stress test. It's been a few hours and the outer area around the seal is bone dry. SO My take-away from all this is that the seal does not like being pushed against the grain so to speak past the machined edges of the shaft. The job seems very simple. Literally a couple hours if you can do all from the exterior of the tranny. In my case that little seal could not slide past the edges of the shaft no matter how lubed up it was without nicking. You double or triple the time needed dropping the pan but it was the only way I got a satisfactory result.

[Accelo]

I did mean the shaft doesn't rotate all the way around. I have to be a little more careful how I describe functions.
So if I get this correct, you pulled the shaft. Installed the seal and then installed the shaft. Because the angle of the seal it went right in sans any damage.
I hate leaks for sure. Glad you got it!

[PbFut]

Sorry if not clear enough. I pulled the shaft, pushed the seal on the shaft from backside. Partially slid the shaft with seal on shaft in side of tranny. Tamped seal into place on the case with shaft still in the seal.
This method allowed me to push seal onto shaft the opposite direction as a traditional method. The seal is more tapered on the leading edge of the push this way and slips past the sharp machined edges of a newly manufactured shaft.