The smallest plug in this family, an AN3 flare plug sealing on an O-ring into a female AN3 port. AN3 is the size used for brake and clutch hydraulic lines and for small gauge take-offs, where blocks and tees regularly carry an outlet that a particular installation does not use. This plug closes that outlet neatly instead of leaving it improvised. The O-ring does the sealing work, so it needs only light tightening and can be removed later without wearing the flare seat. Given the small size, take care not to overtorque. Always confirm the O-ring material is compatible with the fluid in the circuit, especially brake fluid.