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82 by slightly moving the parts. It is essential that the engine should be built perfectly “square,” otherwise it will not run freely, and endless trouble and extra repairs will result.

Fixing the Cylinders. The cylinders are then lowered into place by a crane, and fastened temporarily by ordinary bolts. They rest upon two screw jacks which are packed up on strong planks placed across the pit. In place of the jacks some shops use angle irons fixed temporarily to the frames as supports for the cylinders. It is absolutely necessary that the centre lines of the cylinders, when the latter are fixed permanently, should be exactly parallel to the frames, and the faces of the hornblocks, which determine the alignment of the axles, must be exactly at right angles to the cylinder centre lines.

A long steel straight edge a (Fig. 42) is fastened across the front of the frames at a suitable height, which may be measured from the drawings. Another straight edge b is fixed across the driving hornblock faces. Thin wires representing the cylinder centre lines are stretched tightly between the two straight edges, their distances apart and from the two frames being measured from the drawings. These wires it will be noticed pass through the bores of the cylinders. In place of the straight edge a, it is very usual to fasten the front ends of the two wires to steel strips fitted across the front end of each cylinder, each strip having a small hole exactly in the centre of the