Shimano GRX RX 815 Series Dealer's Manual page 122

ADJUSTMENT
Adjusting the rear derailleur
1.
Adjust the low-side stopper bolt.
(1) Shift the rear derailleur to the largest sprocket.
(2) Then, tighten the low-side stopper bolt until it just touches the low-side stopper.
2.
Adjust the top-side stopper bolt.
(1) Shift the rear derailleur to the smallest sprocket.
(2) Then, tighten the top-side stopper bolt until it just touches the top-side stopper.
(3) Turn the top-side stopper bolt counterclockwise one turn from position (2) so that an
over-stroke allowance can be maintained.
Top-side stopper bolt
Low-side stopper
Low-side stopper bolt
122
Top-side stopper