This paper is aimed at elucidating the cause of a series of malfunctions involving the bending or breaking of main steam turbine throttle valve spindles which occurred at service times ranging from hundreds to several thousand hours in a number of 169 MW steam raising units[ It was clearly established\ by two distinct approaches "one engineering\ one micromechanistic# that the stresses which produced these malfunctions were bending in nature and were the result of out!of!alignment de~ections[ In the case of the bent spindles the stresses were very high and approached ~ow strength levels of around 7999 MPa while the broken spindles were the results of fatigue initiation and subsequent growth from a thread root "stress concentration# location on the spindle[ Using relevant fatigue crack propagation data for the valve spindle material at 299>C it was demonstrated that fatigue failures occurred at spindle de~ections of between 9[8 and 0[5 mm[ Finally\ it was demonstrated that the fatigue breakage problem could be signi_cantly reduced\ especially at the lower end of the valve spindle de~ection range\ by a combination of re!pro_ling the thread root and shot peening[