Adding a deadbolt bore to a door that has none

Everything else about fitting a smart lock is reversible. Wrong finish, wrong app, wrong keypad: unscrew it, box it up, send it back. Cutting a 2-1/8 inch hole through the face of a front door is the one step that is not. Put it at the wrong height or the wrong backset and you own a door with a hole in the wrong place, which is a repair rather than an adjustment.

None of which is an argument against doing it. On the right door, with a jig and a sharp hole saw, a deadbolt bore is an hour or two of careful work, and a great many doors have only a knob bore because whoever hung them was saving a few dollars a door. It is an argument for spending the first part of the job deciding whether this particular door is a candidate. If what you actually want is keyless entry rather than a deadbolt specifically, there is a lower-effort route worth ruling out first, since a keypad knob or lever drops straight into the hole that is already there.

Doors that take a bore, and doors that do not

A deadbolt needs solid material 2-1/8 inches across the face of the door and a full inch into its edge, and it needs the door to hold that material together when somebody puts a shoulder into it. Several common constructions cannot do that, and none of them advertise the fact.

Hollow core

An interior hollow core door is two thin skins over a cardboard honeycomb, with a solid block of timber only where the knob goes. Cut above that block and the saw comes through into air, leaving a bolt held by two skins roughly the thickness of a cereal box. Knock along the door at chest height and the lock area sounds solid while the rest sounds like a drum. These were never intended to carry a deadbolt.

Insulated steel and fiberglass

Most residential steel and fiberglass entry doors are a thin skin over a foam core with a wood or composite lock block behind the knob bore. Some manufacturers run that block tall enough to take a deadbolt above the knob and some do not, and you cannot tell by looking at the door. The manufacturer’s literature will say, and so will the supplier if the door is recent. Worth the phone call, because a bolt anchored in foam is decoration.

Fire-rated doors

The label on the hinge edge of a fire-rated door ties its rating to the condition it was tested in, and cutting fresh holes is generally treated as ending that. Garage-to-house doors, doors onto shared corridors and anything in a commercial building are the usual cases. What is actually permitted varies by jurisdiction, so ask your local building department or the authority having jurisdiction before drilling rather than after.

Narrow stiles and glass

A glazed or heavily paneled door may not have enough flat vertical stile at deadbolt height. Measure the stile on the lock side. The bore is 2-1/8 inches wide and needs wood on both sides of it, so anything under about four inches is already tight and worth checking against the lock’s own template first.

The numbers that decide the job

Bore sizes

The face bore, the hole through the door face, is 2-1/8 inches on essentially every residential deadbolt sold in the US. The edge bore, drilled into the door edge to receive the bolt, is 1 inch. Some older doors carry a 1-1/2 inch face bore from an earlier generation of hardware, which will not take a modern deadbolt until it has been opened out.

Backset

Backset is the distance from the door edge to the center of the face bore, and residential doors use either 2-3/8 or 2-3/4 inches. Most deadbolts ship with an adjustable bolt that covers both. What matters is that the new bore takes the same backset as the existing knob bore, or the two escutcheons will sit out of line with each other for the life of the door.

Height

Templates commonly place the deadbolt five to six inches above the center of the knob bore, but the template in the box is the authority for the lock in your hands, not a figure in an article. Measure up from the knob bore rather than from the floor, since floors, thresholds and the bottoms of old doors are rarely square to one another.

Door thickness

Most exterior doors are 1-3/4 inches thick, and most deadbolts cover 1-3/8 to 1-3/4 as standard. Anything thicker needs the manufacturer’s long spindle and screw kit, which is usually a separate part number rather than something waiting in the box.

Why a self-centering jig beats a steady hand

A paper template taped to the door and a punched center mark is how most deadbolts are meant to be installed, and it works for as long as the drill stays perpendicular. It rarely does. A 2-1/8 inch hole saw is a wide cutter with a lot of leverage, and two or three degrees off square across a 1-3/4 inch door is enough to put the exit hole visibly out of place. That turns up later as a bolt running downhill through the edge bore and never quite meeting the strike.

A self-centering jig clamps over the door edge, sets the backset mechanically and holds a hardened bushing that keeps the saw square to the face. Borrowed, rented or bought, it removes the largest single source of error in the job. The boxed kits sold as deadbolt installation kits or door lock installation kits generally pair a jig with both cutters and are the cheapest way in, though what one costs moves around by brand, retailer and season.

Cutting the bores, in order

1. Confirm the construction, the thickness and the backset of the existing knob bore before anything is marked.
2. Wedge the door so it cannot swing. A door moving on its hinges under a hole saw is how the cutter grabs and how wrists get hurt.
3. Set the jig to the backset, clamp it at the marked height, and check that height again from the knob bore on both faces of the door.
4. Cut the face bore from one side, going only until the pilot bit breaks through the far face. Stop there.
5. Move to the other side, drop the pilot into its own exit hole and cut back through. Meeting in the middle is what stops the far face bursting out into splinters around the rim.
6. Swap to the 1 inch bit and drill the edge bore into the middle of the finished face bore, square to the edge in both planes. The jig usually guides this too.
7. Fit the bolt, score around its faceplate with a knife, and chisel the mortise until the plate sits flush with the edge or a shade below it.
8. Work the bolt by hand with the door open, several times, before going anywhere near the jamb.

A sharp hole saw cuts and a blunt one burns. If the door starts to smoke, back the saw out, clear the packed dust from the teeth and let everything cool. Forcing a dull cutter through the last quarter inch is what tears the veneer around the hole.

The strike side, where the security actually lives

A perfect bore in the door and a strike plate held by 3/4 inch screws into soft jamb trim gives you a lock that fails at the frame instead of at the bolt. Manufacturer literature and building trade guidance have been consistent on this point for years: what the strike is fixed into matters more than the grade stamped on the lock in front of it.

Mark the bolt position by coloring the end of the bolt and closing the door onto the jamb. Drill the jamb hole a full inch deep or a little more, so the bolt has room beyond full projection rather than bottoming out. Chisel the strike plate flush. Then set aside the short screws that came in the box and drive 3 inch screws through the jamb, past the shim space and into the framing behind it. That is the cheapest resistance available on any door and it takes about two minutes.

If the bolt binds or sits proud when you test it, adjust the strike rather than filing the bolt. With the bore cut and the strike right, the rest of the installation is the ordinary part of the day.

Tools, cost, and when to hand it over

The outlay for a first install is a drill you probably already own, a boxed jig and cutter kit, a sharp 1 inch chisel, a utility knife, a clamp and a tape measure. On a solid wood or engineered exterior door with the tools to hand, it is an hour or two of unhurried work, and the tools stay yours for the next door.

Hand it over when the door is metal-skinned and nobody can tell you where the lock block sits, when it is fire-rated, when it carries a mortise case or multipoint hardware, or when it belongs to a landlord. Locksmith labor for a fresh bore is normally quoted well above the price of the lock itself, so put the quote next to what the DIY route costs before deciding, and treat both as ranges that vary by market and by the week you ask.

Common questions

What size hole saw do I need for a deadbolt?

A 2-1/8 inch hole saw for the face bore and a 1 inch bit for the edge bore, which is exactly what the boxed installation kits contain. Check the template that came with the lock anyway, because a handful of commercial and imported bodies use different figures.

Can I put a deadbolt on a hollow core door?

Not sensibly. There is a solid block behind the knob bore and cardboard honeycomb above it, so a bolt fitted higher has almost nothing to hold on to and gives the appearance of security without the substance. If that door genuinely needs a lock, the door is the thing that needs changing first.

How far above the door knob does the deadbolt go?

Follow the template in the box, which usually places it five to six inches above the center of the knob bore. Measure from the knob bore rather than the floor, and take the mark from that same reference on both faces of the door so the two holes agree with each other.

What if the door already has an old 1-1/2 inch bore?

It can be opened out to 2-1/8 inches, but a pilot bit has nothing left to bite on in an existing hole, so you need a jig or a scrap of plywood clamped over the face as a guide to keep the cutter centered. If there is old hardware still sitting in that bore, getting it out is its own small argument and worth reading before you start.

The short version

Establish that the door can take a bore at all before anything else, because hollow core, foam-cored steel and fire-rated doors are each a reason to stop. Match the backset to the existing knob bore, take the height from the lock’s own template rather than from the floor, and use a self-centering jig instead of trusting your eye across 1-3/4 inches of door. Cut the face bore from both sides so the far face cannot burst out, chisel the latch mortise, and work the bolt by hand before touching the jamb. Then finish with 3 inch screws into the framing behind the strike, which buys more real resistance than anything else you do that day.