Shift shaft seal replacement on THM425

[TJ Hopland]

Anyone know how long the shift shaft is in a THM425 transmission?  Or if it will clear the frame in a 73 eldorado?

The shift shaft seal on my THM425 in my 73 eldo is leaking pretty bad.  I should have replaced it when I had the motor out.

[Joe G 12138]

It's my understanding that you can't replace that seal in-car on a '73 Eldorado, due to frame interference. Someone I know tried it on a '73....No dice!  I replaced all the seals on my trans when I had had the Engine/ trans out a few years back after hearing of his failed attempt. The shop manual doesn't give a procedure for doing it.....that's a clue or warning right there!

[TJ Hopland]

Joe,  you remember that shaft being fairly long then? 

I was hoping someone had a 425 apart and could tell me for sure.

[TJ Hopland]

Turns out it is possible.   I didn't see them do it but the shop said that you can lift the engine/trans in such a way that the shaft will just sneak between the torsion bar and frame. 

Wow I have apparently been tinkering with this problem for a long time.   Finally gave up and took it to a shop that is good at old cars and they told me they got the easy stuff fixed and are now left with the shift shaft as the last leak.  I told them I didn't think it would come out in the car and was prepared to say go ahead and pull it if you have to.  Just for fun I hit the old Google to see what others said and was so excited to get a hit from someone with the exact same car and problem as me.   Too bad it turned out to be me and I never got it fixed till now. 

I'm hoping for a bulk discount from these guys.  They got the leak fixed on the wife's Mustang, now my 73, and they are currently working on my dads 57's Hydramatic which is likely going to get a full rebuild.  I actually went to them to see when thy could get to the 57 and they said right now which would have been great if it was at my house 5 miles away rather than my dads place 30 miles away on the other side of town.  I asked if they wanted to do a mustang or eldorado first and they said sure.  I wonder what I have spent on trans fluid and oil dry since 2007?   I have been buying the trans fluid in 2 gallon jugs. 

[bcroe]

I have a drip pan with the edges bent up under some of my cars.  A lot of TH400 shift shafts are starting
to be a problem, speedo outputs too.  But the TH425 seems to be OK.  They used a couple different type
seals, an outside press in and a groved for an O ring.  The first fails when rust comes onto the outer shaft
and ruins the seal; not sure about the O ring type (used on my TH425s).  Bruce Roe

[The Tassie Devil(le)]

The seal is replaceable in the TH425 whilst the trans is in the car and is explained in the Shop Manual.   Not a big job.

In the TH400 in the RWD vehicles, the Trans has to come out.

Bruce. 

[TJ Hopland]

I saw that in the book but it pretty much just says remove shaft like it will just slide out and not hit anything.   Problem is its so long that it does hit.   I don't remember if its the frame or torsion bar but I do remember trying it back around 07 and giving up because it would not quite come all the way out.  Maybe the earlier cars didn't have that problem?   Frames changed a bit for the 71 redesign and at least slight changes for again in 73 I assume related to the impact bumpers? 

[bcroe]

These old cars just love to develop leaks.  The problem on my TH400 shifter shafts was rust on
the external end, so that had to be fixed before changing the seal.  My fix (for now) is find a
shifter shaft without the rust, and swap them.  Did manage to do that in the car, the main
problem was that nail they drop in to locate the shaft.  I pulled the nail out till it hit the front
pump, then bent it over and pulled it the rest of the way out.  After the shafts were swapped,
I put in a long skinny cotter pin, think its the same one I used for governor weights. 

Its been fine, no more cleaning up a mess and adding trans fluid.  Bruce Roe

[SaltLife97]

10 year old topic but still want to check in, we still haven't found a way to remove the shift shaft to replace the o'ring on a TH425? Annoying that the book just tells you to slide it out but hits the frame.

[The Tassie Devil(le)]

10 year old topic but still want to check in, we still haven't found a way to remove the shift shaft to replace the o'ring on a TH425? Annoying that the book just tells you to slide it out but hits the frame.

As I posted back then, the Shop Manual for the TH425 deals with this, and it is easy to replace the seal, as I have done it myself.

Bruce.

[TJ Hopland]

Ya like was said earlier in the thread, if you have a 71/72 apparently its easy.  73-78 its still possible but must require loosening mounts (which are quite different too) and other things to allow the whole engine and trans assembly to move enough to get the clearance past the frame and or torsion bar. 67-70 is a different design also, not sure what the deal is on those. 

[The Tassie Devil(le)]

Ya like was said earlier in the thread, if you have a 71/72 apparently its easy.  73-78 its still possible but must require loosening mounts (which are quite different too) and other things to allow the whole engine and trans assembly to move enough to get the clearance past the frame and or torsion bar. 67-70 is a different design also, not sure what the deal is on those. 

No wonder I wanted a '72.   The best of all of them.   Class with style.  

Bruce.