Teflon Tape...? Nope. The 1st INNER Silicone "O" Ring hidden down inside of the Aluminum Radiator Female Fitting mates up at the BASE of the Cadmium Plated Fitting leading into the Cooling Tubes inside of the Radiator. With Modest Torque applied ... It Seals the Breach between the OD Threaded (Flat End) of that fitting and the Larger Aluminum Insertion point .
If you Lather on any Thread Sealant or Thread Locker or Wrap any Teflon Tape around External Fitting Threads, that stuff might get stuck in between the Base of the Fitting and the In-Dwelling 1st Silicone "O" Ring and potentially defeat the seal in the presence of Very HOT Transmission Fluid being contained by the Piping Lines and Fitting(s) ... and result in a Leak.
For any Curious Others intent on Performing this Repair:
As described in the Video from
@CajunWon ... Look at this attached image. AFTER the Transmission Cooler Line Fitting is installed and Tightened Down inside of the Aluminum Fitting inside of the Radiator (Post Installation in the Vehicle)... Take that Short Tube segment with the Beveled (ramped) End highlighted inside of the
Red Circle... and Stick it down inside of the Open Tri-Lobe Spring Decorated End of that Cadmium Plated Steel Radiator-Trans-Cooler Fitting and Push on it inwards until it solidly "Clicks".
Those Three Wire-Wound Spring Lobe segments will spread open as the Tube glides inside and up the smooth beveled portion of that Tube and then suddenly spring back and LOCK over the Broader Flat Section. Meanwhile, the smooth surface of the Inlet Tube will be held inside that Fitting tighter than a Prophylactic on a "Tumescent Teenager" on Prom Night.
After that... The Tube and Pressurized Fluid Filled Line WILL be able to gyrate and move around just a little bit inside as it is expressly designed for this purpose. BUT... it will NOT LEAK any Transmission Fluid because of the constant Compression Tension created from the 2nd Silicone "O"Ring hidden inside of the Cadmium Plated Fitting... squeezing uniformly around the OD of the In-Dwelling Tube.