Standard A-series pistons are not cut for valve reliefs. The only ones that were notched are the flat-top A12/A10 pistons. And of course competition pistons.

I can't see how the valves would hit the bore just due to the flywheel letting go. I suggest they were scraping the bore before that happened. Stock engines havea relief cut in the bore for that purpose. If you fit larger valves be careful of this.