Below is the list of tables from the database along with their columns and datatype. Need to generate an accurate SQL query for the question.
Only use the columns present in the respective table. Only use the columns which are required to generate the SQL query. Only use table joins if required and not otherwise.

### Tables ###
TABLE: CREATE TABLE table_28017 ( "Class" text, "Firebox" text, "Cylinder size" text, "Valves" text, "Valve gear" text, "Number built" text, "Years built" text )

### Question ###
This engine with a cylinder size of 20 x 26 and a firebox of radial-stay was built when?

### Accurate SQL ###

SELECT "Years built" FROM table_28017 WHERE "Cylinder size" = '20 ½” x 26”' AND "Firebox" = 'Radial-stay'