Rubber dampener in the outdrive cap

[findinghomer]

This is an alpha 1 Gen 2. Long story short, I pulled the cap this summer Because it appeared to be heat damaged and I just wanted to make sure I had fluid up in the top gears. Everything was fine but now I don't know how to reseal it, and the dampener thing broke it was dry rotted. Do I need it?

 

[findinghomer]

Never mind I found the part, I went ahead and ordered it, however can I seal it up with a gasket maker just to run it on muffs In the driveway so I can get my advanced timing set? I don't know what this thing does