After a couple of brasses are properly fitted into their strap, they should appear as in Fig. 982, the faces being quite close together, and both brasses tightly fixed in their places by means of packing-pieces in contact with the key. They can now be properly prepared for lathe-boring by a suitable lining. The lining commences by fixing a wood ender at one mouth of the rough hole; and a centre-dot is put into the ender at a point exactly coinciding with the length-marks on the strap, when it is known that these marks are carefully put at the proper places; if not, it is now needful to accurately mark the centre of the required hole by an additional measurement, or by using the radius-gauge as before.

It is now requisite to ascertain the centre of the gap in the strap, and indicate it on the brasses. This is necessary in order to cause them to be so bored that an equal thickness of metal shall exist at either side when bored. The simplest mode of showing the gap's centre is performed with a straight-edge and scriber. A thin flexible straight-edge is used, and while in position it appears as in the Figure (982), being denoted by S. One edge of the tool is first put into close contact with the strap's inner plane surface, and it is there held by the operator or an assistant, while the opposite end of it is bent sufficient to make it touch both brasses. A line is now marked upon the brasses by moving a scriber along the straight-edge, which line will indicate one of the gap's sides. In the Figure, on the left of the brasses, a dotted line is seen, which has been marked by placing the straight-edge as directed; and the tool is now shown situate at the opposite side, in position for scribing another line. When the two lines are scribed both sides of the gap are indicated ; and the centre of the gap can now be easily shown by placing a compass-point to each line.

When a thin flexible straight-edge is not available, and the strap is not too heavy, the sides of the gap can be easily shown on the brasses by means of a parallel block or blocks on a lining-table. By this mode, the strap is held with its inner surface on the top of a block having a ledge extending into the gap of the strap. While resting on the block, the point of a scriber-block on the table is adjusted to the exact height of the strap's inner surfaces, and the two required lines showing the gap-sides, are scribed upon the brasses. These lines resemble those scribed by means of a straight-edge as before described. If a parallel block having a ledge is not accessible, two blocks may be selected, a thick one and a thin one, the thin one being put upon the top of the thick one, and the strap put above both.

As soon as the centre of the hole required in the brasses is shown, with regard to both the centre of the gap, and the length of the rod, a circle can be scribed showing the diameter to which the brasses are to be bored. Another circle also is usually marked, of larger diameter, and this is that which is used for adjusting the strap to be bored.