When doors are hung in pairs it is necessary to secure one of the doors to the frame by means of bolts, and the other door is locked to it. The common method of securing the "standing leaf" is by means of bolts placed at the top and bottom of the door. Wherever a neat finish is desired, "flush" bolts are generally used which are either let into the edge of the door or into the inside face of the meeting stile. When the bolt is placed on the edge of the door it is, of course, inaccessible when the other door is closed and locked, hence the doors cannot be opened without the key. When placed on the face of the door, the doors may be opened from the inside by drawing the bolts and pulling both doors open: usually, however, it is not necessary to secure the door from being opened from within.
When the bolt is placed on the edge of the door, the bolt is usually operated by a flush thumb piece, sliding in a sunk slot in the face of the plate, and much trouble and vexation has been experienced with such bolts from the fact that the thumb piece is usually too small to afford adequate means of pushing or pulling them. This has been overcome by the lever device, shown in Fig. 405, which, while still flush with the plate, affords a good hold for operating the bolt, and by its long lever arm enables the bolt to be moved easily under any circumstances. This device is used on the Yale & Towne, and Russell & Erwin bolts, both flush and extension.
When a flush bolt is used on the face of the door it should have either a good sized knob, as in Figs. 406 and 407, or the lever device, shown in Fig. 405. The bolt shown in Fig. 406 is made by the Stanley Works, and differs from the ordinary flush bolt in having a hooked plate, which adds both to the strength of the bolt and of the door.
Flush bolts are not usually made over 16 inches long, and when the door is over 7 feet high, it is much better to use an extension flush bolt, of which one form is shown in Fig. 407. The extension bolt differs from the flush mortise bolt, in that the bolt with its connecting rod is set in a hole bored in the thickness of the door, and the plate in which the knob or thumb piece slides is only about 6 inches long. The short plate looks ranch better than a very long plate and does not weaken the door as much by cutting away- Extension bolts are made with rods varying from 12 inches to 6 feet in length, and may be used either on the face or edge of the door. When used on the face of the door they are often provided with T-handles. The lengths of both flush bolts and extension bolts are measured from the centre of the slide or lever to the end of the bolt. As a rule 10 or 12-inch bolts are placed in the bottom of the door, and the thumb piece or knob for the upper bolt should be about 5½ feet from the floor, no matter what the height of the door may be; the difference in the length of the rod adds but little to the cost.
Nearly all forms of flush bolts are provided with a spring which prevents the bolt from dropping when shot.
The face plate of both flush bolts and extension bolts may usually be obtained to match the finish of the knobs and escutcheons, and occasionally the ornamentation. The Stanley bolts are all made of wrought steel, planished or polished and plated; most of the other leading manufacturers make their best bolts with bronze metal plates and knobs.
For store doors it is customary to use large square bolts screwed to the inside face of the door, the upper bolt ("chain bolt ") being a spring bolt provided with a chain to pull it down. The lower bolt ("foot bolt") is made so that the bolt can be pushed down with the foot, while a spring keeps it from dropping when raised. Store door bolts should be either of wrought steel or bronze metal. Cast iron bolts are not desirable, for the reason previously stated.
A special form of flush bolt, known as a " Flush Dutch Door Bolt," is made for connecting the two portions of a "Dutch door" (see Section 164.)