What size hose adapter for water temp gauge?

[tkuss]

What size hose adapter do I need to splice into a coolant line for a water temp gauge. Is it a 1"/26mm hose? Is it larger?

 

[Grimm]

It is a 1 inch diameter, however I was able to use a 28 mm diameter adapter, pictured here:

 

[bklynantnee]

I'm trying to use an auto meter gauge and sender. they don't have a 1" sending unit only 3/4 and 1 1/4. Is the 1" in id? will the 1.25 work? any ideas???