_Answer._ They are made of wrought iron or steel and accurately turned to the size required for the journals for the connecting-rods. Fig. 164 represents one of the main crank-pins, and fig. 163 a back pin for an American engine. The main pin has two journals, one, _B_, to which the main connecting-rod is attached, and the other, _A_, receiving the coupling-rod. The back pin has only one journal, _A_, for the coupling-rod. QUESTION 269. _How are the crank-pins fastened to the wheels?_ _Answer._ They are turned so as to fit accurately holes which are bored in the wheels, and are usually “straight” or cylindrical. The pins are then either driven in with blows from a heavy weight swung from the end of a rope, or else pressed in with a screw or hydraulic press. Sometimes the holes are bored tapered or conical and the pins turned to the same form. They are then ground in with emery and oil, so as to fit perfectly, and are secured by a large nut and key on the inside of the wheel. [Illustration: _Fig. 163._ _Fig. 164._ Scale 1¹⁄₂ in. = 1 foot.] QUESTION 270. _What are the pieces A, A, fig. 161, between the spokes of the wheel for?_ _Answer._ They are called _counterbalance weights_, and are put in the wheels to balance the weight of the crank-pins, connecting-rods and pistons. The principle of their action will be explained hereafter. QUESTION 271. _On what part of the axle does the weight of the engine rest?_ _Answer._ It rests on the driving-axle boxes _L_, fig. 161, which are placed just inside and close to the wheel. [Illustration: Fig. 165.] QUESTION 272. _What are the driving-axle boxes for and how are they made?_ _Answer._ They are cast iron blocks, _L_, fig. 161, which embrace and rest on the axle. The part of the axle on which the box bears is called the _journal_. Each box has a brass bearing, _c_, fig. 165, which bears on top of the journal and which is consequently exposed to the friction and wear. Fig. 165 is a perspective view of a driving-box, which shows what is called the _oil-cellar_, _d_. This is a receptacle underneath the axle which is filled with wool or cotton waste and saturated with oil for the purpose of lubricating the journal. The oil-cellar is held in its position by two bolts, _f_, _f_, which pass through it and the driving-box casting. By removing the bolts the oil-cellar can easily be removed and the box can then be taken off the axle. [Illustration: Fig. 166. Fig. 167. Scale ¹⁄₂ in. = 1 foot.] QUESTION 273. _How are the truck wheels made?_