Boss-brasses are those that fit the holes in the bosses of levers, side-rods, joint-rods, and a few classes of connecting-rods. Such bosses are represented by Figs. 890, 891, and a few others adjacent. The boss having an octangular hole, shown by Fig. 893, requires a couple of brasses each like Fig. 803.

Boss-brasses have no flanges, and are cast separately, in the manner of strap-brasses, each having a half-round gap formed at the time of casting, unless the hole is to be only about three-quarters, or an inch, in diameter; in which case no gap is formed at casting, but the entire hole is formed by boring. In order to cause the gaps in a couple of boss-brasses to be of equal depth, it is advisable to place length-marks upon both faces of the boss, similar to the marks described for straps.

Boss-brasses should be fitted to their respective holes in about the same manner as brasses for straps, one of a pair being first made to properly bear on one end of the hole, and the superfluous metal next removed which extends beyond the length-marks on the boss-faces. The fellow-brass can now be fitted to the opposite part of the hole, while the first one remains out; after which both brasses should be fitted together, both being driven together several times into the boss by hammering them with a tin hammer or wood blocks, which will mark the places of contact, and thus indicate the prominent portions to be removed.

A pair of boss-brasses cannot be properly fitted to their hole unless it is taper, as directed in page 294. It should also have straight sides, or sides which are rather concave, but not to any extent convex; a convex form prevents the brasses bearing at the small end of the hole, however tight they may fit at the large end. After each pair of brasses are made to tightly fit their boss, the superfluous metal which extends beyond both faces of the boss is taken off, and the two ends of the brasses are thus made plane, and made to exactly coincide or be level with the boss-faces; therefore when a pair of brasses are fitted, their length is the same as the distance through the hole in the boss.

When a pair of brasses are fitted tight in their boss, and a wood-ender fitted in, they are ready for lining, to cause the hole to be properly bored; and the two surfaces or faces of the brasses which touch each other will indicate the place for the centre of the required hole with regard to the length of the rod or bar, supposing that the length-marks are properly situated on

2 t2 the boss, and have been attended to during fitting. It is next necessary to indicate the centre with regard to the sides of the hole in the boss, that the lining may cause the hole to be bored so that the metal shall be of equal thickness at either side. To show this centre-point between the hole's sides, a straight-edge is used in a mode somewhat resembling that for showing the centre of a strap-gap. But on a boss the straight-edge is laid flat, no bending being necessary. One edge is put to exactly coincide with one side of the brasses, which is the same as one side of the boss-hole, if the brasses fit properly and no spaces are seen. A line is now scribed along the opposite edge of the straight-edge, which will be near the centre ; when this is scribed, the straight-edge is put to the opposite side and another line marked; a middle line between these two is the centre bine required, and the point in this line which is intersected by the line connecting the length-marks on the face, is the required centre for boring the hole. A circle is therefore scribed from this point to show the desired diameter of the hole, and another larger circle is marked to be used while adjusting the boss to be bored, in a mode similar to that described for strap-brasses.

The middle of the boss-hole can be shown also with compasses, if the brasses fit accurately to the edges of the hole, in which case the points of the compasses are extended until the distance between is a trifle greater than half way across the width of the hole; one point is then put into the place where the brass touches the boss, while the other point is put to about the centre and an arc scribed. The compasses is next shifted to the opposite side, to scribe another arc, the midpoint between the two being the centre required.