Why is piston knurling all the rage on YouTube?

fishmens67 said

That was standard practice on the first valve job on cast iron heads.

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Yup and now If I see it I know they need gone over ... I usually disassemble and feel the play though as soon as I find 1 loose one I call it done "they need sent out" etc. especially now that the X and Js are 55 or 58 years old etc.