Durability of the VES (power valves) found in the big block Liberty 800 engines has been an issue for a number of Polaris riders. The 800 mill does not have the same water jackets as the smaller bore 700, and thus the valves are subjected to higher operating temperatures due to the greater amount of heat being generated with the bigger displacement engine.
Polaris has discontinued the sale of the (accessory) titanium valves, and now offers a relieved stainless steel valve as a premium accessory part. This is the valve of choice for replacement of the original cast valves that seem to be more prone to deformation. Order Polaris part #2202838 for the Stainless Steel VES Kit that includes two SS valves and a set of springs with instructions.