TH400 pan still leaking!

[mechanicalman]

Most important issue on oil leaks is gasket/material quality and surface prep. I would fix the governor cover first as it might be blowing back onto the tail housing. I recommend using a cork governor cover gasket, factory GM is cork. Now the prep-remove the cover and clean it with a wire wheel (safety glasses). Make sure the cover is not distorted, usually if they have been over-torqued the metal under the bolts will protrude towards the trans and the cover will not crush the gasket enough to seal. Straighten the cover, then get a drill attachment that takes a sanding disk, a really small one. Put sanding/grind marks across all the bolt holes on the gasket side to put grip (friction) or "gription" on the gasket to keep it from splitting.

I know, that sounds like a lot that you should not have to go through, but that is how you put that cover on and not worry about leaks. I used to rebuild transmissions in a GM Cadillac dealership, and I had to make them leak-free at whatever effort necessary. You will not find anything like this in the GM service manual, but then again, the manual is for newish, non-damaged parts and the T400 is very old and that cover has likely been tweaked.

Even if I was using a new cover, I'd still prep it. It's a really common leak, and it's a poor design (leak-wise). Aftermarket gaskets for that cover are usually hard, thin cardboard that is not appropriate especially on a worn cover.

I do not recommend using sealer on that.

[motorcritter]

I found also that proper prep and
making sure the pan sealing surfaces were straight and true went a long way toward eliminating leaks before they started. The only OEM that I remember to use or recommend sealant was Chrysler on their FWD units. I use composite cork gaskets (usually OEM) and take it easy on the bolt torque. If I've for a transmission out of the vehicle, I reseal it as a matter of course. Shift shafts, governor covers, servo covers, speedo outlets all leak, of they aren't already, they're going to. Also, those rubber clad metal framed gaskets (I know Frod uses them, I've seen them on recent GM units, too) are probably the best evolution of trans pan sealing. If they've got any frets or damage, put another one in- ain't cheap, but pretty much set and forget.

[davepl]

Everybody knows I'm the toughest stranger on the Internet :-)

[gmebey]

Update.

I cleaned off the tranny and look closer for the leak. There is two, governor cover and the tranny pan along the front. I removed the pan and checked the flatness...... It is not flat and has about a 1/64 to 1/32 gap between a couple of the bolt holes.

Hmm a cheap cast pan = leak!

Any thoughts on Moroso 42020 pan or similar?

[leddzepp]

Weld the pan on there. It will never leak again.

[gmebey]

Wow. That may work. Lol

[cwilkie]

Mine would slowly leak out a lot of fluid if parked for a long time. I changed the gasket and pan numerous times. It turns out the shaft of the speedo hear was hollow all the way through. That little pin hold caused it drop out of the cable and onto the pan. After I changed speedo gears it didn't leak again. That was my th409 leak experience.