The O-ring is a rubber sealing ring on the valve stem that blocks the flow of soda water or whipped cream in the valve bore.
It is located on the valve stem in a soda siphon and in the middle of the dispensing piston in a cream whipper.
The material of the O-ring will age over time and become rigid. Soda water then can flow into the spring housing and can leak near the push lever or at the top of the head at the clamping screw. Replacement is recommended at every maintenance.
The old O-rings are made of a more flexible material, while the ones currently on the market are more rigid. I did not notice any difference in terms of performance. As they move in a bore, it is worth applying a thin coat of silicone grease after replacement for easier operation.
The size of the O-ring is 6×3 mm, i.e. the diameter of the inner hole is 6 mm and the thickness of the rubber is 3 mm.