What size Water Temp Sensor Adaptor should I buy

[stephenpen]

I'm putting an Autometer water temp gauge on my nytro so I need a sensor adaptor.
They list 3 sizes 18mm, 22, and 26mm inside diameter
Which one do I get

 

[Grimm]

The coolant hose inside diameter is 1 inch which equals 25.4 mm, so go for the 26 mm one.