A crank-pin is turned in about the same manner as that described for a piston-rod, the entire pin being first rough-turned, and the cone next fitted, previous to finishing the other portions. A handle which exists solid with the cone is also necessary, similar to that mentioned for a piston-rod; although no such stem is required if the crank-pin in hand is one which has the smaller end of the cone outwards. A pin of this class is to have a screw on the end beyond the cone ; therefore on this portion a carrier can be fixed.

The cone of a crank-pin which is to be fastened with a key only, should subtend but a small angle - six or seven degrees is sufficient - in order to prevent it getting loose while at work. But a nearly parallel cone renders the pin difficult to get out, in the event of breakage or other emergency. Consequently, it is advisable to provide a crank-pin with a screwed end and nut, if room for it exists, and to furnish a cone having an angle of about twelve or fifteen degrees, which is the angle for a piston-rod's cone. Such a cone will allow the pin to be easily detached when necessary, but must have a nut to prevent it getting loose.

The threads for piston-rods and crank-pins should never be larger, for the respective sizes, than those mentioned in Table 7, page 180 ; but threads having smaller numbers of steps for the respective diameters are preferable for the nuts here treated, to prevent them getting loose after being fixed. The office of these nuts principally consists in drawing the rods or pins to their proper places in the holes, and in afterwards keeping them in their relative positions during use, which will be the result if the cones exactly fit their holes. The strain upon a crank-pin nut when in ordinary work is nothing, unless the pin shifts in its boss through not fitting properly; consequently the thread may be very small, about five or six steps per inch being quite coarse enough for a screw seven or eight inches in diameter. Smaller than this would suffice, if it were not necessary to remove the nut and pin at some future time.

But the thread of a piston-nut differs from a crank-pin nut in that it really does sustain a great strain, which is nearly equal to the entire strain exerted by the engine to which it belongs; and supposing that the piston-rod is kept in the piston solely by virtue of the nut, the strain is exactly equal to that exerted by the engine; consequently, a too small thread would be dangerous. But it is to be remembered that the strength of a thread does not depend on its thickness or step, but on its length ; therefore any desired strength of nut-screw may be obtained, however fine the thread may be, by making the nut of sufficient length. The length or distance through the hole of a piston-nut is, however, always limited, to avoid having a great portion of the nut outside of the piston. The length is seldom more than half the diameter of the hole ; but although finer threads than those in Table 7 are used, and although so short, such threads never break, neither from the rods nor from the nuts, unless the material is bad, or the threads are not fitted to each other. It may therefore be seen that thin threads are highly efficient for the purposes referred to, both because they occupy less time in making, and because they are less liable to become loose after having been fixed.

The exact turning of a crank-pin cone to fit its lever-boss is not so important as that of a piston-rod cone, because, when the pin is to be put into its final place in the boss, the boss can be easily heated; and after the pin is put in and tightened with its nut, or with the key, the subsequent cooling of the boss will cause it to exactly fit every part of the cone. But a piston can be only very slightly heated when the rod is to be fixed, so that the cone must be carefully fitted with the lathe. The final fitting of such a cone may be done with a file, after the piston -nut is fitted, and has been screwed tight in the recess while the rod was in the piston. By this time, in consequence of the hammering which has been given, and through the nut having been screwed tight, the cone is plainly marked, and a slight filing can be given accordingly.

To facilitate the screw-cutting of bolts, piston-rods, and nuts, the nut of the lathe screw is disconnected from the screw at the conclusion of every advancement of the screw-tool to the end of the thread; and while disconnected the carriage and rest is moved back by the operator working the carriage-handle. By this means the comparative slow backward motion of the lathe is avoided; but by the use of the authors double-screwing apparatus all this laborious moving back of a heavy carriage and slide-rest is abolished. This affair, although one of the oldest inventions by the author, has never been superseded by any later method (for screw-cutting).