A Reducing Adaptor is a fitting used to modify a port size from one thread dimension to another, typically from a larger port to a smaller port. This allows compatibility between components with different thread specifications.
To determine the correct adaptor, two key details are required:

1. Thread type and size (larger side) – Identify the specification of the existing port.
2. Thread type and size (smaller side) – Identify the specification of the component to be connected.