AN4 is the workhorse small size for oil pressure feeds, turbo oil supply, vacuum and boost reference lines and nitrous plumbing, so spare AN4 ports are common on manifolds, blocks and adapter fittings. This cap seals one off cleanly by clamping onto the male fitting’s 37-degree cone — metal to metal, no washer, no thread sealant. It is a much better answer than a rag or a bit of tape when you have stripped a line for a repair. Remember it is the female half of the pair: it caps male AN4 threads and will not screw into a female AN4 port.