Is my Tiagra rear derailleur missing a screw? Just flipped my bike over, spotted this hole in the underside of my Tiagra derailleur. Looks like a screw should be in there, but there it is not. Should there be? Should I worry? What is this screw for/called? If no, what purpose does this bike serve? Note these are pics of the **underside** of the derailleur. ![Tiagra rear derailleur 1]( ![Tiagra rear derailleur 2](

That hole is for the outer plate stopper pin, one component of part number 8 in the EV diagram linked in the other answer. It prevents the pulley cage from pulling out of the derailleur body.