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Rh which the piston reaches the two ends of its stroke, have to be found. It may be noted here that there are four dead centres, two for each side of the engine. As the valve-setting for one side is the same as for the other, it will be sufficient to describe one side only.

The driving wheel (Fig. 44) is turned in the direction of the arrow for forward motion until the slide block D is nearly, but not quite, at the front end of its stroke as shown in the illustration. The crank will then occupy the position shown at C₁. At the extreme end of the stroke the edge of the slide-block would reach the dotted line A. A centre punch mark P is made on the side of the slide block, and another similar mark S on the side of the slide bar. A pair of compasses is then set to the distance PS. A centre punch mark is made on the engine frame at Q near the driving wheel, and a mark m₂ is made by a pair of compasses or a trammel on the tyre. The wheel is then slowly revolved so that the slide block D reaches the end of its stroke A, and then begins to move in the opposite direction towards the crank. A man in the pit keeps the compasses set to PS with one point in S and as soon as the other point enters the pop mark P the rotation is stopped. With the trammel having one point in Q, a second mark m₂ is made on the tyre, the first mark having moved on to the position m₁, as the crank has moved round from C₁ to C₂. The distance m₁m₂ on the tyre is divided equally, and a “pop” is made in the mark M, the centre