I agree 100% with your notion about assemblers vs builders, but this problem is somewhere else entirely. The original builder probably had nothing to do with this flywheel and clutch. Like others in the thread, I'd suspect a good ol' home mechanic in a hurry to get his shiny new engine in his shiny car, and these-here old porch bolts are good enough. That's neither assembler nor builder (nor an engine issue)... it's just a crappy and not very bright mechanic.
