Helps to know exactly what year/model your motor is. I did mine without too much problem. I have a 1988 125 on a 19' Bayliner Cobra. Trim and tilt cylinders were easy. There are some "O" ring seals on the piston inside. Pretty straight forward. Had no problems replacing the "O" ring seals. Hardest part was the valve assy. attached to the bottom of the trim/tilt motor. There are some "O" ring seals in it too, but also, small steel balls to control fluid. Couldn't find any diagrams to tell me exactly where these balls go. They fall out of place when you take if off of the trim/tilt motor. 1st try after re-assembly, motor would trim/tilt up OK, but wouldn't go down. Took me a coulpe of days to figure out exacly where the balls went. Once I figured it out, it works like a champ. Cost about 20 dollars in parts. A lot cheaper that a replacement unit. Check out this site.
http://www.crowleymarine.com/index.cfm. They sell the "O" rings. I just took my old "O" rings and piston assy. down to the local hydraulic cylinder repair shop and they matched up the "O" rings with better "viton" rubber replacements. There are some posts on this site that were very helpful in giving me the information needed to complete the task.
