I on eBay and I came across same good priced radiators. In some of them description, is spec is for Without Secondary Air Injection. What does the SAi have to do with the radiator?

 

It's the wired plug at the bottom by the outlet hose. If the radiator is w/o sai, it won't have the threaded port there.

 

Will anything (hose, sensor, etc) be inserted in the plug?

 

The SAI trucks have a sensor threaded into a bung in the passenger side tank that tells the computer if the engine is cold enough to need the air injection for the catalysts. Most radiators now come with a threaded plug so that they work for both SAI and non-SAI trucks. If you have the sensor you remove the plug and install it otherwise you just leave the plug in place.