Honda Outboards BF40A/BF50A Service Manual, Page 418Get this manual
BF40A-BF50A 2) Apply ATF to new O-ring, and install it on the free piston[1] FREE PISTON
3) Apply ATF to new O-ring and install it on the piston rod4) Apply ATF to the inner wall of the cylinder5) Install the free piston in the cylinder taking care not to damage the O-ring, and pour the fresh ATF on the free piston6) Install the piston rod in the cylinder taking care not to damage the O-ring7) Push in the piston rod until the free piston end aligns with the bottom end as shown