Re: Old Mercury 1500 trim
Match the orings up at the HW store. When you open the trim cylinders, be sure not to pull the piston out of the inner cylinder. Remove the inner cylinder piston and trim rod as a unit. Now unbolt the cylinder from the rod, pull the rod out of the cap and replace the oring.
If you remove the piston from the inner cylinder, it is hard to get it back in, and easy to ruin the teflon piston seal.