Mazda RX-8 Workshop Manual page 4151

Cap Installation Note
1. Align the convex of the set plate with the notch of the retainer as shown in the figure.
NOTE:
Remove the residue of gasoline on gasket or cap completely. Any residue of
gasoline can make the set plate turn round and prevent it from fixing.
2. Adjust the start mark of the fuel tank with the center one out of three rib lines of the cap,
then turn it around 360° degrees by hand as shown in the figure.