  We study the pionic decay of a possible dibaryon d' --> N N \pi in the
microscopic quark shell model. The initial d' dibaryon wave function (J^P=0^-,
T=0) consists of one 1\hbar\omega six-quark shell-model s^5p[51]_X
configuration. The most important final six-quark configurations s^6[6]_X,
s^4p^2[42]_X and (s^4p^2-s^52s)[6]_X are properly projected onto the NN
channel. The final state NN interaction is investigated by means of two
phase-equivalent - but off-shell different - potential models. We demonstrate
that the decay width \Gamma_d' depends strongly on the short-range behavior of
the NN wave function. In addition, the width \Gamma_d' is very sensitive to the
mass and size of the d' dibaryon.
  For dibaryon masses slightly above the experimentally suggested value
M_d'=2.065 GeV, we obtain a pionic decay width of \Gamma_d' = 0.18 - 0.32 MeV
close to the experimental value \Gamma_d' = 0.5 MeV.
