Hi, it's common issue due to flaws in original design. Here's discussion I made on them:
https://www.ninjette.org/forums/showthread.php?t=344615
Discovered seal issue under max-braking going into T-10 @ Thunderhill and about 2-gallons of gas flowed out from gap between cap & tank!
Original SSR O-ring is low-quality material and degrades quickly. I rummaged through back of auto-parts store and found genuine butyl-rubber flat O-ring that matches. I've had to use 2 O-ring seals. Glued them together with Yamabond-4 and to cap & tank on top & bottom as well.
BTW - OP mis-installed his cap. Should've been clocked 60-degrees to left and use short "dummy" holes for mounting so nozzle fill-holes line up. Longer mounts hang over nothing and their longer-length is accomodated by larger non-threaded holes. No grinding necessary.