Looks like a stress crack to me. Maybe your machinist doesn’t want to admit he missed it? I’m not sure you can drill a small hole at the end of the crack to create a termination point, kinda like glass. Not sure. Hope others will chime in.
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1964 Tempest Custom
421 SD clone
KRE 74cc heads
KRE 230°/236° hydraulic roller cam
Rebuilt 3.23 rear end
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