Strap-brasses are bored while they remain tightly keyed in their respective straps as when they were being lined. They may be bored either on an ordinary disc-chuck in a lathe, or on a table of a drilling-machine or boring-machine.

In order to cause the hole of a couple of brasses to be bored square to the length of their bar or connecting-rod as required, the smooth narrow sides or edges of the strap's arms are considered as standard planes which are to be put parallel with the face of the lathe-chuck, because these surfaces are parallel with the broad sides of the connecting-rod's end, and the lathechuck's face is square to the motion of the lathe-spindle which performs the boring. These surfaces cannot be put into direct contact with the chuck, because the flanges of the brasses project beyond, and because a space must exist between the chuck and the brasses, in which the boring-tool may disengage from the metal. To provide this space, parallel strips of proper thickness are placed between the chuck and the strap ; or, instead of two separate strips, a single bent packing-piece similar to a letter U, may be used; but two separate strips are more useful for a great variety of straps of all sizes, because, when the two are to be used, they can be fixed on the chuck at the exact required distance from each other to suit the particular size of the strap in hand. Each parallel strip should be furnished with slots and small screws, with which it can be fastened at any part of the chuck.

As soon as a strap is put into contact with the parallel packing on the chuck, by the aid of an assistant, or with pulley-blocks above, it is partly fastened to the chuck with plates and poppets, and becomes ready for adjustment by the gauge-circle on the brasses. This is effected by gripping a tool-scriber or pointer in the tool-holder, and adjusting until the point is near the gauge-circle. The lathe-chuck and strap are next slowly rotated while the operator observes the point and the gauge-line; and the strap is gradually shifted with the poppet-screws until the circular line is seen to rotate exactly concentric with the path of the lathe-spindle, which condition is known by the pointer exactly coinciding with the path of the circle as it is moved slowly around.

After the strap is adjusted, it is finally fastened with additional holdfast plates, if its size requires them; and weights also are bolted to the chuck on the portion opposite the strap, in order to balance it properly during rotation. The boring next commences with a drill, if the hole is of comparative small diameter, and a great amount of metal is to be taken out; or with a borer similar to Fig. 431 if the hole is five or six inches in diameter. A borer similar to Fig. 435 is only employed for small holes which will not admit a thick strong tool like Fig. 431. The drill, or whatever tool is used, is held tight in the rest, and advanced through the hole with the usual long traverse belonging to the lathe, the traverses being repeated a proper number of times to obtain the desired diameter for the hole. It is next necessary to smoothly turn the flanges of the brasses, that they may be reduced to the proper thickness, and be parallel with the strap, and also square to the bored hole.

Some strap-brasses require the mouths of the holes to be curved instead of presenting sharp corners. The curved surface is necessary to fit a pin, spindle, or gudgeon that is curved at a corner, for the purpose of obtaining great strength with a comparative small amount of extra metal. The edges of the brasses are curved after the hole is bored to the finished diameter, and also after the flanges are turned to the proper thickness. For this curving a pointed corner tool should be first used, and the curve finally smoothed with a springy tool having a concave edge of proper curve.

Those strap-brasses that require but one mouth of each pair to be curved are, for boring, lined on that same side which is to be curved; this causes the lined side to be outwards while the strap is fixed on the chuck ; so that the curving can be done at the first fixing, which avoids the necessity of again accurately adjusting the strap to the truly bored hole, after one side is turned and the strap is reversed. But when both mouths of the hole are to be curved, the strap must be adjusted to the hole after being reversed, in order to cause the surface of the curved corner to be concentric with the hole.