_Answer_. They are first planed off smooth, and then filed and scraped until the two touch each other over the whole of their surfaces in contact. The valve-stem _v_, fig. 92, works steam-tight through a stuffing-box on the steam-chest. QUESTION 163. _How are the valves and pistons oiled?_ _Answer_. The oil is usually introduced into the steam-chest through a cock, _c_, fig. 92, called an _oil-cock_. From this cock it flows down upon the valve and is conducted by suitable holes and channels to the valve-face and from there through the steam-ports to the cylinder and piston. Sometimes, for greater convenience, oil-cocks _c_, _c_, fig. 71, are placed inside of the cab and communicate by pipes with the steam-chests. The valves are oiled by pouring oil or melted tallow into the oil-cocks when the steam is shut off from the steam-chests and cylinders. When the pistons are working in the cylinders without steam, they create a partial vacuum, so that if oil is then poured into the oil-cocks it will be sucked into the steam-chests, or, in other words, it will be forced in by the pressure of the air above it. _Q_, fig. 71, is a shelf attached to the boiler to receive an oil-can filled with oil or tallow, which is thus melted or kept in a fluid condition by the heat of the boiler. QUESTION 164. _How are the cylinders and steam-chests protected so as to prevent, as far as possible, the heat in the steam from being lost?_ _Answer._ The sides of the cylinders are covered with wood, _w_, _w_, _w_, fig. 95, called the _cylinder lagging_, and the wood is covered outside with Russia iron or brass, which is called the _cylinder-casing_. The ends of the cylinders have light metal covers, called _cylinder-head covers_, made of cast iron, brass or sheet metal. The steam-chest is covered in a similar way so as to be surrounded either with a covering of wood or of confined air. Sometimes coarse felt is used for the purpose. The covering, excepting the cylinder lagging, is not shown in the engravings. QUESTION 165. _For what purpose are the cocks C, C, figs. 92 and 95, at each end of the cylinder, used?_