Re: Missing Washer on Force 120hp Trim Cylinder
That "washer" holds in a "wiper" that cleans the ram as it retracts and keeps the internal O ring ram seal from leaking. It MAY also be zinc to help protect from corrosion.
If you can find an aluminum washer that can be modified to the correct size and staked in place, you can replace it. Otherwise run it until it leaks (it may never) then replace the cap as RRitt said.