The neck finish is the shaped opening at the top of the bottle — the diameter, thread form, and shoulder geometry that the lid mates with. Threads here are typically rolled or machined into the steel rather than cut separately.

The thread profile does the sealing work. A clean, consistent thread lets the gasket compress evenly all the way around the rim. If the thread is rough, too shallow, or misaligned, the lid can bottom out before the seal is fully compressed, and the bottle leaks slowly.

A poor thread finish is one of the most common causes of the slow-leak complaints buyers report after months in the field. The bottle may pass an initial pressure test but seep liquid once the gasket takes a set or the user cross-threads the lid.

Practical takeaway: Request sample lids from the same tooling as production and inspect thread engagement and gasket seating, not just the printed thread specification.