TH400 tail seal replacement
 

Hi,
I've got a small leak at the tail seal (where it bolts to the tranny) bottom area and would like to replace it while the chassis is being restored. I think it should be replaced anyway, maybe it just needs to be driven. I replaced the speedo, modulator, and dipstick seals.

It appears it's straight forward; remove the bolts, remove the tail/cover, replace the seal, bolt everything back together. Are there any alignment things I need to consider etc? Before I bolt everything down, should I slip the yoke on to help keep centered while bolts are torqued down? I'll have to double check but I think the spec is 25ftlbs.

Any guidance I can get would be much appreciated.

Attachment 48285114Attachment 48285115

The tail housing is located into the trans case by a lip that's deep enough alignment is not a problem. The seal/gasket is a square cut rubber ring. A little bit of Vaseline on it will lube so it will slide in.
The rear tail shaft seal, just pop that one out, pack the cavity on the back of the seal with grease so that little spring can't pop out and use a big socket to drive it back in.

These were the parts I was going to get. I don't see a spring?
Attachment 48285303Attachment 48285304

If you flip the seal over, it's around the inside of the lip. And I see that one has a paper gasket between the tail housing and the case. Some transmissions use that square ring. Either one works well. Very unusual to see a leak there unless a bolt gets loose or there is a rough spot.

The other issue I may be contending with on this is the lower bolt (side you can't see in the photo) is the threads on the trans case are very soft and I think it's really only biting on the last bit of threads. There's room for a longer bolt...but ugg. I would think twice about just running a heli-coil but it's right on an 'ear' so it's thin - I worry that the new bore side will blow out or penetrate into the wall of the trans. Has anyone done this, or is there a weak thread solution?

A heli-coil will work just fine. Just make sure the drill is straight and use the tap gently. It won't be so thin that it will break on it's own, and the heli-coil will be stronger than the tapped threads in the case were.
There is really no "stress" there, other than the rear mount and just the weight of the trans on it. And the lip on the housing should bear that weight anyway.

What about the case wall? Not sure how thick it is there. Then of course the speedo wall/area starts there too. :shrug:

Consider replacing the bushing while you have the housing out for seal replacement. The reason for this is many times, a seal leak is caused by radial runout due to a worn bushing.

Just an update: waiting on the end seal before I put everything together but I did put helicoils in the bottom two holes. One needed it and I did the other just because its seemed a little sloppy. Came out very good I think. I was pleased anyway. I have the paper gasket and Im planning on using a very light skim- coat of RTV. Im not sure if I'll replace the bushing as it looks pretty good. Both groves are there and it doesn't show any obvious or uneven wear. When you slide the yoke in the bushing (end cover removed) and there is zero slop or movement. So I'll likely just replace the gasket and end seal.

:thumbs::thumbs:

When you get the seal, look at the back of it. You'll see the spring around the lip.

So, I just pop the old one out and push in the new one? Any RTV? I've never replaced one if those seals. Not sure how easily it comes out etc. how about the bushing, whats the process for removing old and installing new?

gerry72 is absolutely correct. The rear seal begins to leak as the tailshaft bushing begins to wear. You need to replace that bushing when you replace the seal.

I just got the bushing and seal set last night. I took a look at the bushing that's in there and it still looks pretty good, almost new - none of the lubricating flutes show that they're warn in any way. There's no slop when the yoke is slid in either. I also noticed that there are what appear to be two notches that the new bushing does not have. These are not part of the lubricating grooves. Is the new bushing missing something?

I do want to replace it even though it looks okay to me. I've never taken one out, and I don't the proper method to take it out. I called the local tranny shop (who does have a good reputation) to see if they could put the new bushing in and they said "if it's still tight and shows no wear,and I can't fit a .01 gauge in it, to just to leave it." They also said that the new ones never fit like the older ones did, and are often to tight and need to be honed. Now, I have no idea if this is the original, but based on it looking fairly new, I would say it's not. The yoke has a snug slip fit in both. The set I bought is Timken and I often find Timken being the standard and not inferior in any way.

So I may just drop it off with them to fix anyway, but if they send me on my merry way.... I'll need to do it myself.

1. Do I need to be worried or concerned about the notch/hole thats in the old, but not the new?

2. How do I remove the old bushing and install the new bushing?

3. Should I use a bit of RTV on the end seal (not a lot, just a skim coat)?

Thanks, for all the feedback. I don't think it looks difficult to do, but I want to make sure I don't screw something up inadvertently thinking I know what I'm doing... done that a few times. I'll try and post some photos of what's in there now just so you guys can see what's actually in there.

Nick

The only way to get a GOOD read on the yoke journal-to-output bushing clearance is to measure the yoke journal and the bushing I.D. to determine actual clearance. Unless some failure trash circulated through that transmission, the bushing will NOT appear worn. But, in fact, it could be 'gently' worn oversize with many years of use.

Use dial calipers to measure the yoke journal (fore & aft at 0* & 90*) and the bushing I.D. (in the same manner) to determine what the real clearance is for that bushing. If the diametric clearance between yoke journal and bushing is .003" or less, the old bushing is adequate. Otherwise, install the new bushing.

[Note: You can measure the I.D. of the new bushing to determine clearance, BUT the actual clearance will reduce a bit due to the compression forces from the bore in which it is installed. The "free" measurement size is only to verify that the bushing is the correct one for this application.]

Not sure about the 'notches' you mentioned; but if there is an oiling path into that bushing, you don't want it to be sealed off with the new bushing.

The picture Im including shows the two holes I was referring to. They're on exact opposite sides from each other. The other major difference is the length. The new one is 1" and the one installed is 1.5", which seems to correct rather than than the 1".

Attachment 48287172Attachment 48287173Attachment 48287174

Again, you need to check for size of the existing bushing and compare it to the yoke journal. I agree that a 1.5" long bushing is better than a 1" long bushing. Those two holes are oiling holes. Aftermarket probably went after shorter bushing without holes (figuring that sump splash would keep it lubricated) only to save money. Makes me wonder what quality of seal you got in that kit?

If the existing bushing is not worn excessively, reuse it.

P.S. That bushing is worn, but it has worn evenly and without any debris mixed with the tranny oil. :thumbs: