I use the black o-rings from Ace Hardware and they do fine. No idea what they are. I have always removed contrapistons from the bottom, that is I beat on them from the bottom and they come out the top. Get a piece of hard motor mount and carve it to fit into the cylinder. Cut an appropriate size hole in a piece of 1/4 ply. Set the cylinder on it upside down and beat on the contra piston until it comes out.