Abstract: An automatic device (1) for index guiding a part (2) pivotably mounted about an axis (3) on a part (4) that is stationary relative to the axis (3). The device uses a mechanism (5) provided with a ratchet (6) that is pivotable between a number of positions including a fully engaged position, an adjustment position, and a fully spaced position. A toothed surface (6B) of the ratchet (6) is formed in an angular sector (6S) which, in the fully engaged position, is centered on the rotational axis (3) of the movable part and does not include a pivot axis (6A) of the ratchet (6).