Technical Drain Pan issue with a 340 T/H Trans

Drain pan is leaking .This is the 3rd time I have replaced it Pan perfectly level at bolt holes ,checked on granite flat surface / perfect, not sprung, not leaking at the transmission & kick down connections ,always use a quality gasket ,torque the bolts and stager as I tighten. For sure- leaking at the gasket. Any suggestions ? What is the best ( gasket) for this application ? Thx Gene

 

Are the bolt landings perfectly flat? If not peen them flat.

 

There are about a dozen places above the pan that can leak on a TH350. Some of them are pretty hard to see. Here's a partial list of things to check. Yeah, it's long.

shifter shaft seal
shifter shaft worn
accumulator cover O ring
governor cover O ring
speedo gear worn
speedo gear sleeve inner seal
speedo gear sleeve O ring
front pump seal
front pump O ring
front pump bolt washers
kickdown cable housing
kickdown cable O ring
dipstick tube O ring
dipstick tube cracked
cooler line nut loose
cooler line fitting washer
rear extension housing O ring
line pressure plug

 

You can't use a TH 350 gasket on 340 th. The 340 th doesn't use one . No tranny, no gasket.

 

Of course Squirrel ruin my comedy by correcting the trans description but I still had to say it.

 

Most of the new metal pans are pretty thin and weak hence the bolts pull through the gasket easily. I've gone to using only a quality aluminum pan like TCI.

 

Oop s My fingers are old and stiff and I hit the wrong key all the time... 350 T/H

 

Yikes squirrel. I guess I have a lot of things to look at . Nothings easy when building these old cars. One step forward and 5 steps backward.