3 years is what I figured.  Now I know.  Only had two go bad.  Got all of
the flanges as flat and matched and non-leaking the last time.  Guess it is
just the fiber quality or something that allowed the #1 and #3 pipe gaskets
to the head to start leaking air.  My shim to the Y-pipe worked perfectly.
No leaks.  Have to see how I installed it last time.  Have to see which
gaskets I used sealant on.  If all of the sealant gaskets held, then I will
use the sealant throughout.

Any ideas on what to do for the leak coming from the threads on the O2
sensor?  I will more know more about how it is leaking when I get things
apart.  Right now I am guessing a crack.

William