CREATE TABLE table_25695027_1 (years_built VARCHAR(255), cylinder_size VARCHAR(255), firebox VARCHAR(255))
This engine with a cylinder size of 20 ½” x 26” and a firebox of radial-stay was built when?
    
SQL: SELECT years_built FROM table_25695027_1 WHERE cylinder_size = "20 ½” x 26”" AND firebox = "Radial-stay"