Please note: This model has adjustable external voltage regulator. New units come with non adjustable regulator only. Conversion for External Voltage Regulator available. This alternator is equipped with directional fan for better cooling. It therefore needs to rotate clockwise (viewed from the front). Less efficient bi-directional fan is available, if needed. 621 & ATSF(165) (1"’ Single Foot): Replaces most domestic styles using a single1" mounting foot (eg., Motorola, Prestolite). 621 is used without spacer.