Yup, I've had few go 'rounds with machine shops and their "testing" methods. For whatever reason some of them have decided that if you apply vacuum to the valve guides and the valves stay closed then the grind must be good.

I'm just too old school I guess. I don't accept assembled heads from them any more unless I see evidence of lapping compound and I usually request the heads to be disassembled so I can do a final lap just to ensure a good "ring".

Twice now while doing a final lap I've run across loose seats and this is from a decent machine shop I've dealt with for years.

My maxim has always been, "Trust no one else's work and double-check your own".

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“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now