Make sure the gas piston rings are present, and that they are correctly staggered.
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Unless there is something visibly wrong with them, leave them alone. Changing them is unlikely to help, and changing them unnecessarily merely subjects the rings or the piston to the possibility of damage or loss. I mentioned them only because some users don't even know they are there, and a replacement piston might not have any installed.I've inspected the gas rings. They were fine but it wouldn't hurt to change them.