the boiler for the s1 was the largest built by the pennsylvania railroad; with of direct heating surface and 500 one-inch diameter tubes and flues, the total heating surface area of s1 was 7,746 sq ft (719.6 m2), it was 99.3% as massive as the boiler for union pacific's 4000-class 4-8-8-4 locomotive, the "big boy". in terms of drawbar horsepower, the s1 was 13% more powerful than the "big boy," 7200 hp and 6345 hp respectively. the large belpaire firebox met the pennsy's standards; its heating surface area included that supplied by seven american arch circulators. water passed through 5 1/2" horizontal tube met at the centerline with the other cross tube forming the bottom of the 7" vertical tube that sprayed the water up into the steam space above the crown sheet. the lowest set pair of tubes was forward with the side openings of the other six steadily rising toward the back.