Shimano GRX RX 815 Series Dealer's Manual page 68

INSTALLATION/REMOVAL
Temporarily installing the front derailleur
4.
Temporarily install the front derailleur to the seat tube.
(1) Remove the clamp bolt, and expand the clamp band of the band adapter.
(2) Set the clamp band to the frame, and reinstall the clamp bolt.
* Use a seat tube adapter (for Ø28.6) according to the size of the seat tube.
Seat tube adapter (for Ø 28.6)
Clamp bolt
(temporary)
Seat tube
68