AN16 is one of the bigger sizes in the AN range and turns up on high-volume fuel returns, dry-sump tank connections, oil cooler feeds and catch-can breathers. This cap threads onto a male AN16 fitting and seals against its 37-degree flare, closing the port completely without any sealant. On large-bore ports the open thread is an obvious dirt trap, so capping is worth doing whenever a hose comes off for servicing or a tank is stored between race meetings. Tighten it firmly by hand and nip it up with a spanner — the flare does the sealing, so excessive torque is unnecessary and only marks the cone.