What's inside
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Laying out a narrow door mortise is a job where a few thousandths of an inch matter. A mortise that is slightly off-center leaves a thin cheek on one side, weakens the stile, or forces the lock case into a hole that is too tight. A good mortise gauge makes the layout repeatable: one cutter marks the mortise edges while the fence registers against the door edge.
For most carpenters, the best choice is a traditional wooden mortise gauge with two adjustable cutters. It is simple, quick to set, and gives you both sides of the mortise in one pass. A marking knife or combination square still has a role, but the gauge establishes the parallel lines that determine whether the lock will sit straight.
What a mortise gauge does
A mortise gauge has a stock, a fence, and two marking points. The fence rides against the edge of the door stile. The points scribe two parallel lines for the mortise sides. On better gauges, the cutters are mounted on a beam and can be adjusted independently, allowing you to center a mortise or place it deliberately off-center.
Do not confuse this with a standard marking gauge, which normally has one cutter or pin. A single-cutter gauge can mark one side of a mortise, but you must reset it for the other side. That introduces another opportunity for error, especially when working on several doors.
For a typical interior door lock, first measure the lock case width. A 5/8-inch-wide case, for example, needs layout lines spaced slightly wider than 5/8 inch if the mortise must accept it without hammering. Mark the case length and depth separately with a rule, square, and knife.
Features that matter on a narrow door mortise
Two independently adjustable cutters: This is the main requirement. The cutters should slide smoothly but clamp firmly enough that they do not move when the gauge is pushed along a rough stile. A gauge with a single fixed relationship between cutters is less versatile.
A substantial fence: The fence should be tall enough to stay square against the door edge, usually around 1 to 1-1/2 inches. A short or narrow fence can rock, causing lines that wander toward one another at the ends of the mortise.
Sharp, narrow cutters: Knife-style cutters sever fibers more cleanly than round pins. This is particularly important in oak, maple, and other hard woods. A blunt cutter tends to crush the edge, making the layout line wider than intended.
Positive adjustment: Fine threads, separate thumbscrews, or a reliable wedge make setup easier. Decorative brass hardware is not enough; what matters is whether the cutters stay put while you make repeated passes.
Comfortable stock: A gauge that fits the hand and has a smooth fence is less tiring during multiple doors. Rough castings or sharp corners can catch on the work and make it harder to maintain steady pressure.
The main types compared
| Gauge type | Best use | Advantages | Limitations |
|---|---|---|---|
| Traditional wooden mortise gauge | General door and furniture work | Two cutters, quick setup, good feel | Wood can swell or loosen if poorly made |
| Metal mortise gauge | Heavy site use and repeated layouts | Rigid, durable, less affected by humidity | Often heavier and more expensive |
| Combination marking/mortise gauge | One tool for occasional work | Versatile and compact | More setup time; cutters may be less refined |
| Single-cutter marking gauge | Light layout or one-off repairs | Cheap and widely available | Requires two settings for a mortise |
A decent wooden dual-cutter mortise gauge is the sensible starting point for most users. The cheaper option is fine when you are cutting one or two softwood doors and can check each line with a square. Avoid the very light gauges whose cutters slip under moderate pressure.
How to set and use the gauge
Begin by marking the lock case width on a scrap of the same material, or use the case itself as a reference. Set one cutter on each side of the required width. For a snug but workable mortise, make the layout roughly 1/32 inch wider than the metal case. That small clearance prevents a split or damaged lock case when the hardware is driven into place.
Set the fence so the mortise is centered across the stile, unless the lock instructions specify otherwise. On a 1-3/4-inch door stile, a 5/8-inch mortise centered across the width leaves about 9/16 inch of wood on each side. Do not assume the stile is perfectly consistent; measure from the actual edge where the gauge will run.
Make a light first pass with the cutters. Keep the fence firmly against the edge and push in the direction that keeps the cutter slicing rather than levering out fibers. Make two or three progressively deeper passes instead of trying to cut the full line at once. On open-grained wood, one heavy pass can tear out a strip beside the line.
After marking, darken the layout lightly with a pencil if needed, then chop the mortise with a sharp chisel. Use the gauge lines as boundaries, not as a substitute for depth layout. Check the lock case frequently. A mortise that is 1/16 inch too deep is inconvenient; one that breaks through the opposite face is a ruined stile.
What to buy for site and shop use
For occasional DIY work, a basic adjustable wooden mortise gauge is usually enough. Before buying, check that both cutters can be locked independently and that replacement cutters are available. A low-cost gauge with proper adjustment is better than a premium-looking gauge with fixed pins.
If the tool will live in a damp van or be used on several doors each week, consider a metal dual-cutter mortise gauge. Its extra rigidity helps when laying out repeated work, although the additional weight does not improve accuracy by itself.
Store the gauge with the cutters backed off or protected. Touch up knife cutters with a fine stone when they begin polishing rather than cutting the fibers. Before laying out the real door, test the setting on a scrap and confirm the lock case fits between the lines. That two-minute check prevents most expensive mistakes.