At this point the question arises: Why was the engine layout such that the exhaust took place close to the operator's ears? It would have been possible, starting with the original design, to turn the engine around so that the exhaust was on the other side. This would have little effect on the location of the center of gravity, and the two main drive chains would then have been of more equal length. However, of the many factors involved, probably one of the principal considerations in arriving at their final decision was the location of the spark-advance control, which was in effect the only control they had of engine output, except for complete shutoff. In their design this was immediately adjacent to the operator; with a turned-around engine, an extension control mechanism of some sort would have been required. The noise of the exhaust apparently became of some concern to them, as Orville's diary in early 1904 contains an entry with a sketch labeled "Design for Muffler for Engine," but there is no further comment.

The problem of keeping joints tight, and for that matter the entire construction itself, were both greatly simplified by their decision to water-jacket only a part of the cylinder head proper, and the valve box not at all. This was undoubtedly the correct decision for their immediate purpose, as again they were effecting savings in time, cost, complexity, and weight. There is nothing in the record, however, to show why they continued this practice long after they had advanced to much greater power outputs and longer flight times. Their own statements show that they were well aware of the effect of the very hot cylinder head on power output and they must also have realized its influence on exhaust-valve temperature.

The cylinder assembly was made somewhat more complicated by their desire to oil the piston and cylinder by means of holes near the crankshaft end in what was, with the engine in the horizontal position, the upper side of the cylinder barrel. This complication was no doubt taken care of by not drilling the holes until a tight assembly had been made by screwing the barrel into place, and by marking the desired location on the barrel. Since this position was determined by a metal-to-metal jam fit of the crankcase and cylinder barrel flange, the barrel would reassemble with the holes in very nearly the same relative position after disassembly.

With the valve box, or housing, cylindrical, the task of locking and fastening the intake and exhaust valve guides and seats in place was easy. The guide was made integral with and in the center of one end of a circular cage, the other end of which contained the valve seat (see Figure [5]). Four sections were cut out of the circular wall of the cage so that in effect the seat and guide were joined by four narrow legs, the spaces between which provided passages for the flow of the cylinder gases. These cages were then dropped into the ends of the valve boxes until they came up against machined shoulders and were held in place by internal ring nuts screwed into the valve box. The intake manifold or passage was placed over the intake valves so that the intake charge flowed directly into and through the valve cage around the open valve and into the cylinder. The exhaust gas, after flowing through the passages in the valve cage, was discharged directly to the atmosphere through a series of holes machined in one side of the valve box.

Figure 5.—First flight engine, 1903, assembly. (Phantom cutaway by J. H. Clark, with key, courtesy Aeroplane.)

KEY

The intake and exhaust valves were identical and of two-piece construction, with the stems screwed tightly into and through the heads and the protruding ends then peened over. This construction was not novel, having had much usage behind it, and it continued for a long time in both automobile and aircraft practice. One-piece cast and forged valves were available but here again it was a choice of the quick, cheap, and proven answer.