705. Q.—­How are the parts of a piston fitted together so as to be perfectly steam tight?

A.—­The old practice was to depend chiefly upon grinding as the means of making the rings tight upon the piston or upon one another; but scraping is now chiefly relied on.  Some makers, however, finish their steam surfaces by grinding them with powdered Turkey stone and oil.  A slight grinding, or polishing, with powdered Turkey stone and oil, appears to be expedient in ordinary cases, and may be conveniently accomplished by setting the piston on a revolving table, and holding the ring stationary by a cross piece of wood while the table turns round.  Pieces of wood may be interposed between the ring and the body of the piston, to keep the ring nearly in its right position; but these pieces of wood should be fitted so loosely as to give some side play, else the disposition would arise to wear the flange of the piston into a groove.

706. Q.—­What kind of tool is used for finishing surfaces by scraping?

A.—­A flat file bent, and sharpened at the end, makes an eligible scraper for the first stages; or a flat file sharpened at the end and used like a chisel for wood.  A three-cornered file, sharpened at all the corners, is the best instrument for finishing the operation.  The scraping tool should be of the best steel, and should be carefully sharpened at short intervals on a Turkey stone, so as to maintain a fine edge.

707. Q.—­Will you explain the method of fitting together the valve and cylinder faces?

A.—­Both faces must first be planed, then filed according to the indications of a metallic straight edge, and subsequently of a thick metallic face plate, and finally scraped very carefully until the face plate bears equally all over the surface.  In planing any surface, the catches which retain the surface on the planing machine should be relaxed previously to the last cut, to obviate distortion from springing.  To ascertain, whether the face plate bears equally, smear it over with a little red ochre and oil, and move the face plate slightly, which will fix the color upon the prominent points.  This operation is to be repeated frequently; and as the work advances, the quantity of coloring matter is to be diminished, until finally it is spread over the face plate in a thin film, which only dims the brightness of the plate.  The surfaces at this stage must be rubbed firmly together to make the points of contact visible, and the higher points will become slightly clouded, while the other parts are left more or less in shade.  If too small a quantity of coloring matter be used at first, it will be difficult to form a just conception of the general state of the surface, as the prominent points will alone be indicated, whereas the use of a large quantity of coloring matter in the latter stages would destroy the delicacy of the test the face plate affords.  The number of bearing points which it is desirable