Is there only a crack but the shape hasn't changed? Because when you really have excessive crankcase pressure, the bathtub will balloon "upside down" before cracking. If not, then I believe you've got somekind of harmonics that crack the pan. I believe that's the reason why factory also used a "sandwich" between the intake and valley pan, to avoid cracking caused by harmonics.


Plynouth VIP '67 TT IC EFI