Stainless steel is normally not as strong as regular carbon steel.  Even
the strongest stainless is still weaker then high yield steel.  Also many
SS fasteners are actually quit brittle.

I'm curious to find out how the studs are breaking.  When looking at the
surface that broke are they shiny with little curved lines that seem to be
diverging from a point or is the surface mostly dull?  If the surface is
dull right after the piece breaks the fracture is usually a brittle one and
could have been caused by a sudden shock.  A mostly shinny surface with
concentric lines on it is usually indicative of a fatigue failure which is
caused by repeated deflection, such as vibration or loading unloading
cycles.  If the part has been in there awhile rubbing against something
then the observations above can't be done.

Since you have has several sets of these studs break in the last few months
I'm thinking something is amiss.  Your engine could have a bad vibration,
this should be easily checked.  Your alternator may be causing a vibration.
 Your engine mounts maybe wornout allowing jolts to be transmitted to the
engine case.  The enginemounts happen to be in the same plane as the
alternator mount, which makes me think this maybe it.

Dave K.

>Are other 1.9l folk having the same problem? I'm even using extra-strong
>stainless studs, they just snap off. Do I have some kind of awful engine
>vibration? How could I fix that?