Fig. 375.
In addition to these general names for bolts, there are others applied to special cases. Thus [Fig. 375] represents a patch bolt or a bolt for fastening patches (as plate c to plate d), its peculiarity being that it has a square stem a for the wrench to screw it in by. When the piece the patch bolt screws into is thin, as in the case of patches on steam boilers, the pitch of the thread may, to avoid leakage, be finer than the usual standard.
In countersink head bolts, such as the patch bolt in [Fig. 375], the head is very liable to come off unless the countersink in the work (as in c) is quite fair with the tapped hole (as in d) because the thread of the bolt is made a tight fit to the hole, and all the bending that may take place is in the neck beneath the head, where fracture usually occurs. These bolts are provided with a square head a to screw them in by, and are turned in as at b to a diameter less than that at the bottom of the thread, so that if screwed up until they twist off, they will break in the neck at b.
Fig. 376.
Instead of the hole being countersunk, however, it may be cupped or counterbored, as in [Fig. 376], in which the names of the various forms of the enlargement of holes are given. The difference between a faced and a counterbored hole is that in a counterbored hole the head or collar of the pin passes within the counterbore, the use of the counterbore being in this case to cause the pin to stand firmly and straight. The difference between a dished and a cupped is merely that cupped is deeper than dished, and that between grooved and recessed is that a recess is a wide groove.