Honda Outboards BF40A/BF50A Service Manual, Page 421Get this manual
BF40ABF50A 18) Apply ATF to new O-ring, and install it on the bottom piece19) Hold the valve body in vise with shop towel or soft jaws as shown
CYLINDERIPISTON ASSY
Take care not to tighten the vise too tight as It damages the valve body20) Loosen the manual valve turns or more and pour the fresh ATF into the cylinder chamber up to the top edge of the valve body21) Install the cylinder piston assembly to the valve body[2]