This engine with a cylinder size of 20 ½” x 26” and a firebox of radial-stay was built when?
CREATE TABLE "engine_num7002" (
  "class" text,
  "firebox" text,
  "cylinder_size" text,
  "valves" text,
  "valve_gear" text,
  "number_built" text,
  "years_built" text
);
SELECT "years_built" FROM "engine_num7002" WHERE "cylinder_size"='20 ½” x 26”' AND "firebox"='Radial-stay';