Measurement on fragment spin as a function of its mass has been carried out over a wide range of bombarding energy in 12 C, 16 O + 209 Bi and 12 C, 16 O + 232 Th reactions. The mass dependence of fragment spin for 12 C, 16 O + 232 Th reactions is observed to be in marked contrast to that for the 12 C, 16 O + 209 Bi reactions. The present results have been analysed within the statistical model by taking into account the finite relaxation time for the equilibration of the collective spin modes. It is found that the observed feature in 12 C, 16 O + 232 Th reactions are characteristic of incomplete equilibration of the collective spin modes where the mass relaxation process at extreme mass asymmetry terminates before full statistical equilibration of the collective spin modes are reached.