Sure, there's defective manufacturing...everyone who makes anything gets
a certain percentage of bad ones.
Doesn't mean a "bad" manufacturer necessarily...just that this person
got unlucky.
I got unlucky with a head on my '82 Westy. Valve seat separated from the
head (I got lucky because it stayed whole and didn't go inside), and the
head was not salvageable. Mechanic couldn't find any evidence of unusual
stress, heating...nothing...just something in the metal that may have
never happened before or since.
Things DO just happen.

Horace K. Sawyer wrote:
>
> >Good luck, and that is definitely a weird one! That area is not subject
> >to any particular stress, so it is hard to guess why it cracked.
>
>         Hmmmm . . . sounds like defective manufacture.  Defective metals,
> defective casting, defective moulding, defective pouring process, or
> something.