TH400 leak
 

I know this has been beat to death. I have a cast aluminum pan and I have tried a rubber gasket and a Fel-Pro fiber gasket, to no avail. The transmission is a fresh rebuild that I did myself, replaced all bushings, seals, o-rings during the rebuild. It is NOT coming from the speedo gear housing, kickdown connector, shift shaft, modulator, dipstick or cooler lines, or pump or front seal, I have verified this. I have torqued the bolts to factory spec. Before the rebuild the stock pan leaked, the flange was pretty wavy, so I put a chrome pan on it with the Farpak gasket that the kit came with, and it leaked right away, so I put the aluminum pan on and used the rubber gasket that came with it. Leaked almost immediately. After the last gasket that I tried, which is the Fel-Pro, I have leaks and I haven't even filled it with fluid yet! I can see it running down from around the bolts, which are the socket head type and have a flat washer and a lock washer on each of them. Anyone with this same experience that has come up with a solution?

Re: TH400 leak
 

Are you gluing the gasket to the pan or putting it on dry?
I haven’t done a trans but have done an aluminum pan on a sb.
Glued the cork/rubber gasket on with form a gasket, put it gasket side down on a flat table with a pile of books on it and let it cure overnight.
Installed with a light smear of grease on the dry part of the gasket and a little dab on each bolt.
Never leaked a drop.
Your torque pattern is something to consider too.
Aluminum doesn’t flex and bend like a stock pan does.
You could do the 4 corner pattern rotating 1/2 a rotation.

Re: TH400 leak
 

Putting it on dry no sealer, it’s not recommended on transmissions
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Re: TH400 leak
 

It is but that’s a stock pan.
Yours isn’t.
Think outside the box!

Re: TH400 leak
 

I don't want to have a PITA time when it comes to a filter change later on. Gasket sealer should not be necessary
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Re: TH400 leak
 

It’s sealer on only the pan side of the gasket,

Re: TH400 leak
 

Be sure you use the right pan bolts
If the bolts are too long they will crack the case and cause it to leak as if it is coming from the front seal

Re: TH400 leak
 

Take a flat edge and make sure the case side is flat ive seen over torqued pan bolt pull the case a bit higher around the holes and not seal a small flat file will fix and high spots

Re: TH400 leak
 

If the flange on the trans is "wavy"...it is possible that attempting to place and aftermarket flat pan would actually leak more than a "flexible" metal pan....

Place an original pan bolt thru the metal pan and measure how far the bolt sticks thru...make sure the new socket head screws are no longer than the original bolts....

There are some blind holes in the pan rail and even if the bolts are torqued to spec, if they are bottoming out before providing the crush needed to seal...it will leak...

Grab the old metal pan and check each of the bolt holes in the rail...over time the pan deflects from tightening creating small hills around each bolt hole....again, the bolt may be tightened to spec, but the raised metal area can bottom out before the gasket is sealed...(the old cork gaskets would actually split around a raised bolt hole and leak like a sieve).

Suggestion....lay the metal pan flange on the edge of a solid bench edge, and tap the area around the bolt hole down flat on all bolt holes.

Clean the surface with brake clean and allow to dry.

Grab yourself one of these gaskets and install the metal pan using original bolts....do not glue the gasket to either the pan or the trans flange.

https://www.summitracing.com/parts/l...yABEgIzzPD_BwE


Hope this helps

Image courtesy of AGCOAuto

:chevy::chevy::chevy:

Re: TH400 leak
 

The original stamped steel pan is beat up pretty bad, I attempted to straighten it before the trans was removed for rebuild, but it leaked after the trans was back in the truck. That is why I went with the chrome pan originally, as I got a good deal on it from the parts board. I've read good and bad reviews about the Lubelocker gasket, have you personally used it on a persistent leak and had good results?

Re: TH400 leak
 

I never....repeat never...use a chrome pan...I used to but they leaked badly, despite the use of a variety of gaskets.

I have used the Lubelocker gasket on many builds and have not had one leak yet....

Source yourself a standard metal pan...make sure the pan rail is stright and use the Lubelocker gasket...

Make sure the pan bolts are the correct length....

:chevy::chevy::chevy:

Re: TH400 leak
 

The bolts that came with the aluminum pan are the same length as the stock bolts, so that is not the issue. I checked the pan for flatness before installing and had to file a couple of high spots. I have not had time to address the issue again yet, but will this weekend. Thanks for all your suggestions.

Re: TH400 leak
 

Could you please post a pic of the "aluminum pan"....

Thanks

Bill

Re: TH400 leak
 

Here is a pic of the exact pan I bought

Re: TH400 leak
 

That looks like a cast aluminum pan....if so, it should not leak....

Double check the bolt holes along the front of the trans pan flange are not cracked thru into the inside of the trans...those bolts can also bottom out and flex the pan rail and cause leaks.

If the pan flange of the trans is warped it will need some work to get it to seal...

For me, the number one task when starting to reassemble a TH400 is to make sure the pan rails are straight...if not, file them flat and rewash the case....

I know you have said these were checked...but here goes....

modulator has the O ring in place
dipstick tube has the O ring in place and the tube is fully inserted in the case
trans vent pipe is clear
pump to case O ring in place
pump to case gasket in is place and correctly aligned....

Let us know how you go...

Bill

Re: TH400 leak
 

I should have specified, it is a cast pan. I agree it should not leak. I am going to have to pull the trans and check everything out, it wasn’t leaking this bad before I removed it. I must have screwed something up during the rebuild without realizing it.

Re: TH400 leak
 

I didn't reply because I can't believe a cast pan would leak. I was waiting for you to tell us it was coming from somewhere else but you say you can see it leaking around the bolts. It's a stumper. I never use a torque wrench on such light torque applications; wrist tight has always done it for me.

Re: TH400 leak
 

Same here, I don't own an in/lb torque wrench and always snug them up same as valve cover bolts. I am going to remove the pan again and lay a straight edge on the case to check for warpage. This is a 50+ year old case so it could have issues. I will try the Lubelocker gasket if I can't fiind anything wrong with the case.

Re: TH400 leak
 

I ended up using the Fel-Pro fiber gasket and got it sealed up. Now I have a leak from the governor cover. I replaced the gasket but it still leaks. Any ideas on how to solve this one?

Re: TH400 leak
 

Governor cover leaks are usually due to a warped cover especially around the bolt holes....

Take the cover off, place the ears of the cover where the bolt holes are located on a flat steel vice and tap the flange flat....usually easiest if the the outer face of the cover is down on the vise face.

This thing leaking oil seems to point to a clogged breather valve or tube....

Question...When you fill the trans does is burp back fluid if its being filled a little fast?

:chevy::chevy::chevy:

Re: TH400 leak
 

Fluid went in ok. The vent is not blocked, I thoroughly cleaned the case when I rebuilt the trans and blew out all the passages. I checked the gov cover for flatness when it first started to leak, and it appears to be flat. I will remove it and check it again.