Honda Outboards BF40A/BF50A Service Manual, Page 229Get this manual
HONDABF36A45A Thermal valve
The outboard motor is equipped with the thermal valve to protect it and allow it to cruise at designated trim angleBecause the piston lowers and oil in the upper chamber of the cylinder is pressured when force is applied to the outboard motor from rear while cruising at designated trimtilt anglethe oil flows through the right side check valve and activates the thermal valve to release the oil pressure and thereby to protect the outboard motor