This nut is at the outer end of the generator shaft and tightens the pulley/shim stack. When installing, be sure you have the correct number of shims (total of ten). If you are using the correct size fan belt, you will need 4-5 shims between the pulley halves and the remaining shims go under the nut. Failure to put shims under the nut will result in insufficient tension against the pulley halves.