Backset, Bore, and Cross Bore: The Measurements That Decide Fit

The most common reason a new lock goes back to the store is not quality. It is that the holes in the door and the holes in the lock do not agree. Door hardware is interchangeable only because a handful of dimensions became conventional, and the moment a door falls outside those conventions, nothing bolts up.

The Three Dimensions, and How to Measure Them

Backset is the horizontal distance from the edge of the door to the center of the large bore hole. Measure it by hooking your tape on the door edge and reading to the center of the hole, not to its near side. If the lock is still installed, measure from the door edge to the center of the spindle or cylinder.

Bore, sometimes called the cross bore in older usage, is the large hole through the face of the door that the lock body or cylinder passes through. Measure its diameter across the widest point on the face of the door.

Cross bore, as most manufacturers use the term today, is the smaller hole drilled into the edge of the door, perpendicular to the face bore, that the latch or bolt assembly slides into. Measure both its diameter and its depth, because a bolt that is too long for a shallow cross bore will not seat.

Also record door thickness and the faceplate shape on the edge, either a radius corner or a square corner drive-in style. Those two details cause as many returns as backset does.

Standard Residential Conventions

In North American residential construction, two backsets dominate: 2 3/8 inches and 2 3/4 inches. The shorter one is typical on interior doors and the longer one is more common on exterior doors, though both appear in both places. Because a manufacturer cannot know which one a given door has, most residential latches ship as adjustable units that expand or collapse between the two positions. If your latch will not adjust, you either have a fixed-backset unit or a door outside the standard.

Face bore diameter is conventionally 2 1/8 inches for both cylindrical locks and deadbolts. Cross bores are typically 1 inch in diameter. Standard door thickness for residential slabs is 1 3/8 inches for interior and 1 3/4 inches for exterior, and most locks are supplied to cover that range with an adjustable or shimmed mounting.

Why Standardized Prep Exists

Interchangeability is not accidental. There are published preparation standards that fix these dimensions so that hardware from one manufacturer mounts to a door from another. ANSI/BHMA A156.115, Hardware Preparation in Steel Doors and Frames, is written to cover all significant dimensional attributes for mounting common hardware products in steel doors and frames, providing dimensional drawings and acknowledging that multiple configurations exist in common usage.

That last point is worth absorbing. The standard itself notes it may be used in part, or with exceptions, while still providing basic guidance and standardization. Preparation conventions are a strong norm, not a guarantee. BHMA maintains a broader catalog of A156 series standards covering hinges, closers, cylinders, auxiliary locks, and door preps, which is the right reference when you need to confirm what a spec sheet is claiming.

Non-Standard and Imported Doors

Several situations put you outside the conventions:

• European and other metric doors commonly use euro profile cylinders in mortise cases, with entirely different prep and no 2 1/8 inch face bore at all.
• Narrow stile aluminum and glass doors use much shorter backsets to fit within a slim stile, and require hardware specifically made for them.
• Older American doors may be prepped for mortise locks, a rectangular pocket routed into the door edge rather than a pair of round bores.
• Fire rated and steel doors may have factory prep that cannot legally be modified in the field without affecting the assembly rating. Check with the authority having jurisdiction before drilling one.

The Pre-Order Checklist

Five minutes with a tape measure before you buy:

• Backset, edge of door to center of face bore
• Face bore diameter
• Cross bore diameter and depth
• Door thickness
• Faceplate style on the edge, radius or square or drive-in
• Center-to-center distance between deadbolt bore and latch bore, if you are buying a one piece handleset

Write those six numbers down and compare them to the product specification rather than to the product photo. If any of them fall outside the standard values, a licensed locksmith can tell you whether the door can be reworked or whether you need hardware made for the prep you already have.