Shimano GRX RX 815 Series Dealer's Manual page 188

MAINTENANCE
Replacement of the plate and the plate tension spring
3.
Check that the switch lever is in the OFF position.
If the switch lever is in the ON position, be sure to set it to the OFF position.
4.
Set the chain stabilizer into the cam unit as shown in the figure.
Check that the convex section of the cam unit is in the position shown in the figure.
Cam unit
Chain stabilizer
Switch lever
ON
OFF
Convex section
Convex section
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