M12 extension housing oil leak

ATF transmission oil (red color) accumulating between the transmission body and the extension housing (This is where the 5th, 6th and reverse gears are located). This leak is caused by a failed RTV seal between the two housings, and the only way to repair this is by removing the extension housing and resealing the flanges.

The failed factory RTV sealant. The earlier effort to seal the leak from the bottom/outside was only a partial success, but who wants to remove the transmission only to stop this leak?

The magnets trap metallic particles produced as the transmission wears, which would otherwise be circulating in the oil and cause further wear. I wanted to remove the magnets to clean out all the particles, but I had to use more than sensible force, so I cleaned them in place. I found no large particles or shavings, only fine metallic dust that disappeared into the cleaning cloth.  A good sign.

Another source of oil leaks are the inner and outer rear seals of the extension housing, so I changed these too while they were accessible. If you have red ATF oil leaking between the transmission and the differential, these seals have failed.

PS! Notice the bearing cap to the right. The cap is floating, with a stack of shims underneath to set the preload of the bearing/transmission shaft. As I inverted the housing to insert new seals, the cap somehow moved and seized in the housing! I could not release it by hand force, but a very light tap with a plastic rod fortunately solved it.

Flanges cleaned. I'm using RTV to seal the flanges. My father, an aircraft maintenance technician, advised me to put down a thin linen thread in the sealant (A Google search revealed that this is standard procedure on Lycoming aircraft engines).

Also notice the two upper bolt holes (red arrows), they extend into the transmission! I applied Permatex Form-A-Gasket on these bolts to stop transmission oil from seeping through the bolt threads.

Extension housing reinstalled. The T56 manual stated that the transmission should be placed horizontally when installing the housing, and it slid on with ease. I torqued the bolts by hand, allowed 1 hour for the RTV to cure, then did the final torquing in a crisscross pattern.

There was also an oil leak from the reverse lockout solenoid cover. The RTV seal had failed in the same manner, so I applied new RTV gasket here as well. As the bolts extend into the transmission, I also applied Form-A-Gasket to the bolts to stop oil seeping through the threads. I replaced the reverse lockout solenoid O-ring, as it had flattened.

Flange sealant: Permatex RTV, Ultra Black

Flange bolt torque: 26 ft lbs. (35,3 Nm)

Bolt sealant: Permatex Form-A-Gasket