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doesnt the blasting then require redoing the bores?



Shot blasting changes all the machined surfaces , you would do this before all machining operations . You could make covers for certian areas if you don't want the machined surfaces touched , I made alum plates with holes when I had a block done that protected the deck surface , I didn't worry about the mains and the bores since those were going to be remachined . It also stress relieves the casting .