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TH 400 to doubler adapter leak

waybad1

waybad1

1/2 ton status
Joined
Mar 30, 2006
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Location
Ventura, CA
So my rig started to leak transmission fluid the other day. I determined it was at the adapter
off the TH400. I removed the doubler and realized I only had a gasket there. I believe
there has to also be a seal? I got the factory part number and ordered the seal. I already had
a gasket on hand. @ seals came in the packing, I think I need just the thin one. I dont see how
the gasket and seal work together. Any advise on how to install and which seal would be
appreciated.

001.JPG
 
Iirc, the adapter to trans bolts have o-rings on them to seal fluid that can creep down the threads.
 
You are probably right, fluid has seeped from the lower bolts since I put it together years ago.
It just started leaking a lot. Maybe the thinner seal in the groove first, then the gasket. I have searched
a lot for a pic or diagram.

th400 seal 001.JPG

th400 seal 002.JPG
 
I did think about o rings due to the bevel under the bolts. I think using a serrated washer is the wrong choice with an o ring.
I used those because the factory bolts to the 205 were that way and I felt I needed a type of lock washer so I would not
have to worry about it coming loose. I dont know if I need a washer at all with orings, but would worry about it coming loose. Maybe
a flat and lock washer. The seal does contact the gasket and flattens right up against the trans housing. I already did remove the
doubler previously to seal the bolts and it did not really work. It would be nice to see a factory diagram of how it went together when new
since this was an original factory setup. Also the leak was definitely on the transfer case side. The transmission side of the gasket was dry.

th400 seal 001.JPG

th400 seal 002.JPG

th400 seal 003.JPG

th400 seal 004.JPG
 
I have a virgin 400 w/ 203 adapter at home. I'll check what gasket is on it and post pics of the bolts used.
 
I used a small bead of RTV around the large O-ring and I put some rtv on each bolt going into the trans. Never even knew there was little o-rings on those trans bolts.
 
Seems like you could accomplish the same thing by using the thread sealer you'd put on head bolts.
 
I actually pulled the doubler and tried to seal the bolts with GM thread sealer, helped for
awhile. Maybe this will work

th 400 seal 001.JPG

th 400 seal 002.JPG

th 400 seal 003.JPG
 
Any chance the 2 washers + bolt head will be taller than the pocket they're in? Maybe drop the lock washer and use Locktite?
 
and like I said before, I never used those little o-rings, just used RTV and the big square o-ring and they never leaked.
 
The bolts will clear the gasket when tightened, and believe there will be liberal amounts of rtv.
I don't want to take this out for a third time. especially with just a floorjack. pita
 
that's a TH350 but it appears to be the same design, less holes obviously....
 
I think you may be right about just the o ring too. When I went to the dealer they gave me the part number
for the o ring and gasket. I just ran the gasket number and it is the gasket for the other side of the adapter. Why somebody
would make a gasket for that location I dont know. I found it weird that the gasket sits on top of the o ring anyways. Just finding out
the bolts use o rings has helped a lot

Either this makes it more confusing or clear. T o ring sits in the trans extension
perfectly haha

th 400 seal 004.JPG
 
Last edited:
Ok, I checked my parts and they're not as virgin as I thought which I guess shouldn't be a surprise. My adapter only has a thin bead of RTV where the o-ring should have been, no paper gasket.

The bolts are all the same and have a smooth flanged head with o-rings.

IMG_1938.JPG
 
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