Gouging from gravel is normal. There is considerable surface on the liner to provide pressure sealing. If it is worn such that all the shim washers are on top of the impeller, and blade clearance is still more that 1/32 inch, the liner and/or impeller need to be replaced. If you do replace the liner because of gouging, check performance before and after replacement to see how much, if any, loss the gouging caused.
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