Table schema:
CREATE TABLE table_25695027_1 (years_built VARCHAR, cylinder_size VARCHAR, firebox VARCHAR)

Question: 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"