2005 Yamaha F90TR 90HP Outboards Factory Service Manual, Page 115Get this manual
Power unit NOTE: Install the bearings in their original positions4Install the piston with the "UP" mark on the piston crown facing towards the flywheel magnet
Assembling the power unit
1Install the oil ring 1, second ring 2, and top ring onto the pistons with the "T" marks on the piston rings facing upward2Offset the piston ring end gaps as shown