The 6-60 was continued in production and development until it became the 6-70, and indications are that it eventually approached an output of 80 horsepower. [Illustration: _Figure 14._--6-Cylinder 6-60 and 6-70 engine, right rear intake side. (Pratt & Whitney photo.)] [Illustration: _Figure 15._--6-Cylinder 6-70 engine, incorporating flexible flywheel drive, exhaust side. (Smithsonian photo A-54381.)] Minor Design Details and Performance of the Wright Engines In the Wright brothers' various models were many minor design items which altogether required a great deal of consideration, but which did not materially affect overall engine performance. The results generally could all be classed as good practice; however, one of these utilized in the 4-cylinder vertical engine was rather unorthodox and consisted of offsetting the cylinders with relation to the crankshaft. This arrangement, which can be seen in the drawing (Figure 11) was apparently an attempt to reduce the maximum side load on the piston during the power stroke, but since the peak gas loading usually occurs at about 10 to 15 percent of the power stroke, this probably did not have much effect, and it was not carried over to the 6-cylinder design. All engine bearings were of the plain sleeve type and, except for the bronze and steel bearings in the connecting rod, were of babbit. The advantages of babbit for bearings were discovered very early in the development of the mechanical arts, and apparently the Wrights never encountered a bearing loading sufficiently high to cause a structural breakdown in this relatively weak material. Valve openings show no variation through the successive production engines, although the Wrights most probably experimented with different amounts. The 1903 engine and the vertical 4-and 6-cylinder all had lifts of 5/16 in., but the valve-seat angles varied somewhat; the records show included angles of 110° to 90°--not a large difference.