This is a replacement piston O ring for Red Paddle HP Pumps and many other high pressure sup pumps. This is a large O ring that sits on the piston of the pump inside. Can wear when lubrication wears out so needs replacing or a new pump! Check your pump has the O ring and rough size before ordering if you don't have the red pump.
Please see picture below for rough position of the Piston O Ring. We also do handle shaft O rings on this product.