O-Ring, NBR, ID.18mm.THK : 2.5mm.
Replacement O-ring for TYLY-0072 Front Fork Preload Adjusters, Pair, CBR250RR MC19/MC22.
This O-ring is used between the preload adjuster inner piece and the outer fork cap.
To replace the O-ring, remove the fork cap assembly from the fork, and wind the preload adjuster counter clockwise to remove this piece from the outer. The EXOR-0008 O-ring is located in a groove on the outer piece. Apply a small amount of rubber grease to the O-ring before fitting and reassembly.
Reassembly is just a reversal of disassembly.