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Old 09-05-2014, 11:14 AM   #1
gmebey
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TH400 pan still leaking!

Here is the short version of the long story of the leaky TH400, look at my past post and you will get the whole story.

1) Replace pan gasket on OEM pan with a cork type. However the pan lip had seen better days. I did flatten it and reuse the OEM pan.

2) Replace pan with cheap cast aluminum (eBay thing) using the rubber gasket that came with it

3) Switched to the to the Moroso gasket (93103)

4) Had the transmission totally rebuilt and found out that front bushing and seal were shot, needless to say this was most likely my source of the major leak.

Now that I have freshly rebuilt transmission I still have a leak and at this time I suspect that the cast pan is my source of the problem. I did check the flatness of the pan, and it seems to be OK. But it is leaking at the rear around a couple bolt holes. This makes me thing that cheap cast pan has insufficient area around bolt holes….I think.

Anyway here are my questions:

1) Has any one tried a cheap cast aluminum pan with success of NOT leaking? Digging on the internet I have found other blogs, post…etc that claim that the surfaces of the cheap pans are to smooth, they are poorly designed…etc.

2) Since I’m considering replacing the pan with a better quality one. Suggestions please?
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Old 09-05-2014, 11:42 AM   #2
geezer#99
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Re: TH400 pan still leaking!

Try using some spreader bars on the bolts that you suspect are leaking.
Like the spreader bars used on valve covers.
Or make some from some flat stock.
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Old 09-05-2014, 12:21 PM   #3
gmebey
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Re: TH400 pan still leaking!

I have consider using the spreader bars before.

Sad to say but the cast pan has bosses around the bolt heads.
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Old 09-05-2014, 12:28 PM   #4
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Re: TH400 pan still leaking!

Toss the aftermarket pan ,use a good old style cork gasket and Permatex sealant and it will never leak again ...
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Old 09-05-2014, 12:47 PM   #5
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Re: TH400 pan still leaking!

NAPA "The Right Stuff". Will seal gap up to 0.090. Part # 765-2401. Be careful not to glob a lot of extra on the inside. Sealants floating in the pan would be bad. Good luck, Bruce
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Old 09-05-2014, 01:12 PM   #6
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Re: TH400 pan still leaking!

I had a similair problem with the th350 make sure they are torqued to spec not to tight not to loose
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Old 09-05-2014, 01:31 PM   #7
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Re: TH400 pan still leaking!

Quote:
Originally Posted by HEADTURNINGC10 View Post
I had a similair problem with the th350 make sure they are torqued to spec not to tight not to loose
X2^^^^, overtighten and you will squeeze the gasket as if it were not there. Some of the better gaskets have a spacer to prevent this from happening
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Old 09-05-2014, 03:24 PM   #8
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Re: TH400 pan still leaking!

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Originally Posted by BLT gmc View Post
NAPA "The Right Stuff". Will seal gap up to 0.090. Part # 765-2401. Be careful not to glob a lot of extra on the inside. Sealants floating in the pan would be bad. Good luck, Bruce
X2 I use the same sealer I found "The Right Stuff" in grey color cleans up pretty easy if you have to disassemble in comes in a cheese wiz type can.
I used two or three of those pans on turbo 350's and all of them needed sealer or would leak bought them all off ebay.

Brian
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Old 09-05-2014, 03:02 PM   #9
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Re: TH400 pan still leaking!

BLT_gmc,

"The Right Stuff"? is that the sealant used by Dodge?

I was considering using Loctite 518 (not permatex) ,which is what SAAB switched to for flange to flange sealing without gaskets.

My only concern is removal of the pan and cleanup. How tough is it to separate parts when using the "Right Stuff"?

Jim,

Just for the record the service support tech at Moroso informed me that over torquing their gasket will cause the gasket to split. Which is what I think may have happen to mine since the torque spec is only 70 in-lbs (5.83 ft-lbs).
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Old 09-05-2014, 03:55 PM   #10
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Re: TH400 pan still leaking!

I do not know if Dodge uses it. The right stuff is packaged in Napa packaging. the number I posted is for 2 tubes and the short chaulking gun. I no longer use gaskets on my 4x4 differentials. It seperates well/reasonable. This is my preferred sealant. Tons better than crappy silicone. Bruce
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Old 09-05-2014, 04:38 PM   #11
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Re: TH400 pan still leaking!

Sometimes I just have to wonder how us old guys survived without today's expensive 1 product fixes all products ? it's a pan and a gasket it doesn't require $35.00 worth of parts to seal it . Expensive does not always mean best ! Or permanent ! That's why it's called PERMATEX ! OLDER and WISER usually means we have played these games before and offer advice that works ! LIVE AND LEARN ! I'm a hands on guy I don't send trucks out for anything other than powder coating , paint /interior /welding /mechanics/suspension ...I've built hundreds of muscle cars and trucks for a living just because it's new does not make it better .Spend less than $10.00 and fix it once .

Last edited by Grumpy old man; 09-05-2014 at 04:45 PM.
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Old 09-05-2014, 05:24 PM   #12
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Re: TH400 pan still leaking!

If it really is a pan leak I would get a new pan and gasket from GM and be done with it. When it comes to oil & trans pans cheap = leak.
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Old 09-05-2014, 05:50 PM   #13
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Re: TH400 pan still leaking!

Quote:
Originally Posted by gmebey View Post
Here is the short version of the long story of the leaky TH400, look at my past post and you will get the whole story.

Now that I have freshly rebuilt transmission I still have a leak and at this time I suspect that the cast pan is my source of the problem. I did check the flatness of the pan, and it seems to be OK. But it is leaking at the rear around a couple bolt holes. This makes me thing that cheap cast pan has insufficient area around bolt holes….I think.

Anyway here are my questions:

1) Has any one tried a cheap cast aluminum pan with success of NOT leaking? Digging on the internet I have found other blogs, post…etc that claim that the surfaces of the cheap pans are to smooth, they are poorly designed…etc.

2) Since I’m considering replacing the pan with a better quality one. Suggestions please?
I have used a JW Performance Transmissions deep cast aluminum pan on my truck since 2006 without a single leak. It may not be the least expensive but it is tight.
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Old 09-05-2014, 06:15 PM   #14
gmebey
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Re: TH400 pan still leaking!

After talking to mechanic friend. At this point I need to pull off the pan and see what the leak path is, bolts, gasket, pan....etc. All I know each time I dump out the AFT it is $30.

Correction to my comment about Loctite 518 (not Permatex)......I should have said the "NOT the Permatex brand equivalent" to Loctite 518. Without going into details in our manufacturing facility the only approved brand is Henkel (aka Loctite) due to quality issues with Permatex. Which BTW was once owned by Henkel, but as 2001 (?) Permatex broke off to focus on automotive supply.
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Old 09-05-2014, 08:10 PM   #15
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Re: TH400 pan still leaking!

Generally, I like to use my original parts. However, over the years I had deformed the pan flange due to over tightening to the point where it had very slight leakage. I liked the appearance of the aluminum pans, the beefy flange and the convenience of having a drain plug since I'd drenched myself and messed up the garage floor too many times over the years. So, I ordered a cheap pan from Skip White on eBay (http://www.ebay.com/itm/GM-TURBO-400...da829f&vxp=mtr). It looks great and after a year or two, doesn't leak. Just depends on what YOU want.
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Old 09-06-2014, 02:37 AM   #16
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Re: TH400 pan still leaking!

The only advice I can offer is, follow the directions, and I mean that in the nicest way. I always read about people with leaks and others swearing by certain gaskets. Rubber, cork, paper, or just sealant I don't have any leaks, and i don't say that with a chip on my shoulder. I always prep the same: clean the surface with a plastic scraper and a soft brass wire brush, wipe with mineral spirits, wipe with acetone, apply gasket per manufacturer recommendations and finger tighten all bolts followed by recommended torque specs. Re-check torque specs in the tightening sequence after running up to operating temp. Fwiw- I do use stock pans, but have the same results with stock or aluminum valve covers.
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Old 09-06-2014, 04:46 PM   #17
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Re: TH400 pan still leaking!

Look really hard for a leak above the oil pan. Spray brake clean all around the modulator, filler tube, shift-shaft and electrical connector. Use foot powder (any spray powder) and spray all those areas after they dry then drive it around and see if the leak shows up above the pan.

Regardless if it's been rebuilt, the front pump and/or torque converter seal might still be leaking or even a leaky torque converter (cracked).
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Old 09-06-2014, 09:14 PM   #18
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Re: TH400 pan still leaking!

Hi gmebey,

I went through this last summer and thought I'd share my experience in case it helps. I expended a great deal of energy trying to fix what I believed was a pan leak, that was not a pan leak at all.

After changing several gaskets and lots of cleaning, I discovered it was the shift shaft seal that was leaking down to the pan lip, which in turn leaked to the gasket and made it "appear" the gasket was leaking.

Replaced the shift shaft seal and it STILL leaked! Replaced it again, it still leaked at the shaft seal. Doing some research here on the forum, came across a post where someone else had the same issue and turned out to be a small bur on the shaft, where the seal slid over.

I took a small fine file and sure enough, there was a bur. After a cleaning it up and replacing the shaft seal for the third time, it sealed, no more leaks.

Tip: Clean up around the shaft area and get completely dry and clean. Tape a piece of paper towel so that it sits under the shaft seal, let it sit over night. Inspect the next day. If it's red, you've found the source of the leak. It can leak right down to the pan, then onto the gasket itself making it "appear" it's a gasket leak.

Hope that helps and hope you get it resolved. Best wishes.

Wade
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Old 09-06-2014, 09:43 PM   #19
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Re: TH400 pan still leaking!

I went through two pans and four gaskets, never fixed the leak. Ultimately had the trans rebuilt and it's not leaking now. I suspect it was actually the front pump or seal.

Buy a quality gasket ideally with crush stops (so you can't overtighten them).. You should probably get the manual and use a torque wrench. See if there's a torque sequence listed, but I haven't seen them for pans, so alternate to next-furthest-away and do it in two or more torque steps. If you do that and it still leaks, then its got to be the pump or pump seak or shift shaft seal or something else running down.

And even if the old boys hassle you, use Right Stuff RTV. It's not the 60s when everyone just assumed it was normal that cars should leak.
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Old 09-06-2014, 09:58 PM   #20
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Re: TH400 pan still leaking!

The NAPA # listed above is for 2 tubes and the gun. It is enough to do about 5 or 6 diff covers to give you an idea of how far it will go for ~$27. Great advice above. But if a sealant is needed this is very good. Coming from someone whose livelihood depends on his work. Good luck, Bruce
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Old 09-08-2014, 04:14 PM   #21
gmebey
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Re: TH400 pan still leaking!

First of all thank you for all the advice.

I had some time yesterday and started looking around for a possible leak source above the pan…..and guess what. I think there are one or maybe two; first being the governor cover and the other being the gasket between the main housing and tail housing.

Both of these really bother me since I had the transmission rebuilt about 3 months ago! I’m in contact with the rebuilder to see what my options are. After a few conversations with the rebuilder I suspensions that his torque wrench(s) are a bit out of calibration, if used at all.

So at this time my patience over the constant leaking has worn a bit thin……haven’t decided if I’m going to fix the one (or two) leakers on myself and hope for the best when it comes to the internals of the transmission.


With that can anyone point me to a GM document of the torque specs for the whole transmission?
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Old 09-08-2014, 04:56 PM   #22
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Re: TH400 pan still leaking!

BTW, NEVER use RTV on a trans pan gasket or any other trans gasket unless you can make 100% sure the trans fluid will not seep onto the silicone. The trans fluid will permeate the silicone before it can set up and destroy any sealing properties it has. RTV sealer is good for filling low spots and a substitute for proper prep work. You can use it if you keep the oil away from it until it completely sets up. I've taken apart hundreds of leaky engines/transmissions to find failed silicone sealer. It might seal a little while, but if the oil gets to it before it completely cures it's an oil leak time-bomb.

Or, take the advice of someone else who has went through 4 gaskets and 2 pans and never fixed the leak and had to hire someone to fix it for him.
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Old 09-08-2014, 09:38 PM   #23
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Re: TH400 pan still leaking!

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Or, take the advice of someone else who has went through 4 gaskets and 2 pans and never fixed the leak and had to hire someone to fix it for him.
Is he trainable?
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Old 09-09-2014, 09:55 PM   #24
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Re: TH400 pan still leaking!

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Or, take the advice of someone else who has went through 4 gaskets and 2 pans and never fixed the leak and had to hire someone to fix it for him.
Post a picture of your truck, I'll post one of mine. We'll see who the real mechanic is.
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Old 09-09-2014, 10:44 PM   #25
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Re: TH400 pan still leaking!

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Post a picture of your truck, I'll post one of mine. We'll see who the real mechanic is.
Well...There goes the neighborhood !
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