Changing out a masthead sheave isn't a super complicated task, but doing it while working aloft becomes complicated quickly. If you can't locate a drawing of your masthead arrangement just go aloft with a camera (camera/phone) and take a few pictures. Also inspect to see if there is any galv corrosion between the stainless pin and the aluminum.
Once you know what you are dealing with you can buy the correct sheaves (I'd go ahead and replace them all). Take aloft only the tools you will need, attach lanyards from the tools to your tool belt for safety. Plan ahead every step carefully! The most important thing is that you don't want to drop any of the parts. Remember to apply some tef-gel to the stainless pins, I would do that on deck and then put the pins in a plastic bag before going aloft.
