The mechanical strength and subcritical crack growth (SCG) behaviour of four types of MnZn ferrite E cores were determined using constant stress rate tests. Decreasing the stress rate or introducing water decreases the mechanical strength. Using fractography it was shown that the mechanical strength of the standard type of cores depends on the behaviour of two kinds of defect: large grains and pores surrounding agglomerates, having a different SCG susceptibility. Therefore a bimodal strength distribution was used to give a statistical description of these data. Eliminating the large grains by adapting the processing results in cores with only the pores surrounding agglomerates as critical defect. The strength distributions of these cores are comparable to those of the original that have failed on this type of defect. Removing both defect types, besides improving strength, also reduces the susceptibility to subcritical crack growth.