Stay-bolts should never be subjected to a strain of more than one-eighth or one-tenth of their breaking strength. QUESTION 92. _How do stay-bolts often fail without breaking?_ _Answer._ By tearing or _stripping_ the thread of the bolt, or that in the plate, but oftener perhaps by the stretching of the plates between the holes. With a heavy pressure, the tendency of the plates between the holes, especially if they are heated very hot, is to “bulge” outward and thus stretch the hole in every direction until it is so large that the bolt is drawn out without much injury to the screw-thread. QUESTION 93. _How is the flat-top or crown-sheet strengthened?_ _Answer._ It is sometimes strengthened with stay-bolts similar to those used for the sides, which pass through the inner and outer shells;[25] but usually the crown-sheet is strengthened by a series of iron bars, (_f_, _f_, fig. 41 and 42) called _crown-bars_, placed on edge, and of considerable depth, which are firmly fastened to it by T-head rivets or bolts. The crown-sheet can therefore only be crushed downwards by bending these bars, which are of great strength. They usually extend crosswise of the length of the fire-box, but are sometimes placed lengthwise. These bars bear on the fire-box only at each end, as shown in fig. 42, and are usually made with a projection, _k_, _k_, which rests on the edge of the side plates. Iron rings or washers from ³⁄₄ to 1¹⁄₂ inches thick are interposed between the plate and the bars at the points where the bolts or rivets which secure the rivets pass through. This permits the water to circulate under the bars, and prevents the crown-sheet from being burnt or overheated, as it would be if the water were excluded from the whole under surface of the crown-bars. [26]The crown-bars are also attached to the outer shell and the dome by _braces_, _e_, _e_, _l_, _l_. [25] This method of staying crown-sheets has been extensively used on the Baltimore & Ohio and Reading railroads, and is now very generally used in Europe. [26] Colburn’s Locomotive Engineering. The opening _c_, fig. 41, at the back end is for the door through which fuel is supplied to the grate. QUESTION 94. _How are the grates constructed?_ _Answer._ They are generally made of cast-iron bars, and for burning coal are usually arranged so that the fire can be shaken by moving the bars. For burning anthracite coal, the grates are sometimes made of wrought-iron tubes, through which a current of water circulates to prevent them from being overheated. QUESTION 95. _How are cinders and burning coals prevented from falling through the grate upon the road?_