Shimano GRX RX 815 Series Dealer's Manual page 116

ADJUSTMENT
Checking the chain length
NOTICE
• The rear derailleur plate assembly is equipped with a pin or plate that prevents
the chain from derailing. When passing the chain through the rear derailleur,
pass it through the main body of the rear derailleur from the side of the chain
derailment prevention plate as shown in the figure.
• If the chain is not passed through the correct position, damage may be caused to
the chain or rear derailleur.
RD-RX815
Chain derailment prevention plate
RD-RX817
Chain derailment prevention plate
Chain derailment prevention plate
Chain derailment prevention plate
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