to be done the professional way yes.....but you can also buy a tool that goes in a bare head like a valve..and then you set the head on the block and use a drill to spin the tool and notch the pistons with the block still in the car..very unprofessional but it works
It can be done in the car, but extra care needs to be taken as to get all metal shavings out. When I helped do this, we used greese and covered the pistons and sealed them up, when done, just clean the greese up. Not the best way, but didn't have any problems with the motor. When on a budget, doing a cam swap & get piston to valve interference you do what you got to do. Just know consequences.
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