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64 break across the holes. In the alternative construction, Fig. 29, the tyre is held by a retaining ring R, which is heated and driven into the recessed groove shown, and the lip B of the tyre is then hammered down all round the wheel on to the ring.

The outside crank pins, upon which work the coupling rods, and also the connecting rods of outside cylinder engines, are turned and then ground on a grinding machine, and pressed into the holes bored into the wheel boss to receive them. As the pin on one side of the engine has to be fixed at exactly 90° with the pin on the opposite side, the boring of the holes in the wheels is done on a special “quartering machine,” in which both holes are bored simultaneously in the opposite wheels. The arrangement of the machine is such that the 90° angle is automatically maintained.

Finally the complete set of wheels and axle are lifted into a heavy wheel lathe, of the type shown in Fig. 30, to have the tyres turned on the outside. These lathes are very rigid and powerful machines, since the extremely hard steel of the tyres places a heavy stress on both the cutting tools and the machine. Figs. 31 to 34 show a set of sheet iron gauges used for turning the tyres. Figs. 31 to 33 are used to see that the cone of the head of the tyre (usually 1 in 20) and the height of the flange are correct. Fig. 34 tests the proper distance apart or the “gauge” of the two tyres on a pair of wheels.