Coolant Gauge Adapter Large
Large Coolant Gauge Adapter. Specifically designed for Spec Miata's, this radiator hose adapter will work for any car with a 1-1/8" hose. This adapter features a 1/8" NPT port for electric senders on one side and a 1/2" NPT port for mechanical gauges on the other side. Plugs for both ports are included.
(page 72 in Spec Miata Construction Manual)