Cooler line and fitting sizes vary by manufacturer. Pick the wrong adapter and you spend the whole day chasing the right fitting. Here is the reference for the most common vehicles.
Cooler line size: 3/8 inch.
Fitting: 1/4 NPSM (National Pipe Straight Mechanical).
Note: Quick-disconnect on newer factory lines; barb on aftermarket replacements.
Cooler line size: 3/8 inch.
Fitting: Quick-disconnect (factory) or 5/16 inverted flare (aftermarket).
Note: Factory plastic quick-disconnects are a known failure point — upgrade to braided steel for tow.
Cooler line size: 1/2 inch.
Fitting: 3/8 NPT or 5/8 inverted flare.
Note: HD applications use larger lines for higher fluid volume.
Cooler line size: 3/8 inch (6R80), 1/2 inch (10R80).
Fitting: Quick-disconnect (factory).
Note: 10R80 quick-disconnect is a different size than 6R80 — verify before ordering.
Cooler line size: 1/2 inch.
Fitting: Quick-disconnect or 3/8 NPT (aftermarket).
Note: Powerstroke trucks need 1/2 inch minimum for adequate flow.
Cooler line size: 1/2 inch.
Fitting: 5/16 inverted flare or 3/8 NPT.
Note: Cummins trucks should run 1/2 inch minimum — stock lines can restrict flow under heavy tow.
Cooler line size: 3/8 inch.
Fitting: 14mm or 5/8 inverted flare.
Note: Metric — verify thread pitch before ordering aftermarket adapters.
Up to 5,000 lbs: 18,000-25,000 BTU cooler.
5,000-10,000 lbs: 25,000-40,000 BTU cooler.
10,000+ lbs: 40,000+ BTU stacked-plate cooler.
Diesel pickups: always go one size larger than calculated minimum.
For shop use: use AN-grade braided steel hose with reusable AN fittings. Cheaper rubber-and-clamp setups fail at the worst time. Tru-Cool and Hayden sell pre-made -6AN and -8AN sets sized for common applications.
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