The fish-bolts, Figs. 320 and 321, are of a form which is in very 220 general use both for steel bull-head rails and steel flange rails. By making the neck square or pear-shaped, to fit into corresponding hole in the fish-plate, the bolt is prevented from turning round when the wrench or spanner is applied to tighten the nut. A channel or groove is sometimes rolled on the outside of fish-plate to grip bolts made with square heads. Some engineers adopt two nuts, others prefer one nut of extra depth. Washers are used in some cases, but are not universal. With a deep rail it is preferable to place the nuts inside, so that the platelayer inspecting his length can see both rows of nuts as he walks along between the rails. With shallow rails the nuts must be placed outside and the cup-heads inside, to give ample clearance to the wheel-flanges. Fish-bolts are subject to very severe work. Heavy rolling loads passing over the rail-joints--frequently at very high speeds--bring into play all the gripping power of the fish-bolts to maintain a firm support of the fish-plates to ends of rails, and the constant action of pressure and release produces a loosening or unscrewing motion in the bolts which is very difficult to counteract. Loose fish-bolts cause clattering joints and uneven road, and unless promptly remedied, the screw threads are soon destroyed and bolts rendered useless. Many devices have been invented to prevent or check this loosening of the bolts; one of the methods, and a very simple one, consists of a plain steel bolt with a steel lock-nut, made as shown in Fig. 322. As will be seen from the section, one-half of the nut is tapped of the same size as the bolt, and the remainder with deep-locking threads. The first half of the nut is readily screwed on to the bolt, but considerable force must be exerted to screw on the portion having the deep-locking threads; practically the second half of the nut has to cut a new or deeper thread for itself when screwing round the bolt. [Illustration: Fig. 320, 321, 322, 323, 324, 325, 326, 327, 328, 329, 330, 331, 332, 333, 334, 335] The slits or grooves at the angles of the nuts form four distinct cutting edges for shaping the deep threads. As the upper part of the lock-nut is divided by the grooves into four separate or detached segments, these segments will be forced slightly open or outwards during the action of cutting the deep thread on the bolt, and from their natural tendency to return to their original position they must exercise a strong gripping power on the bolt. This combined operation of cutting the deep threads and of forcing open the upper or detached 222 segments, give an enormous holding and retaining power to the lock-nut, and enables it to withstand the train vibrations for a very long time without any perceptible slackening. In case of line repairs the nut can be readily unscrewed, and taken off the bolt.