How to Measure a Mortise Cylinder (Storefront & Commercial Doors)

What You Need

A tape measure or ruler marked in 1/16 inch, and ideally a caliper. If the cylinder is still in the door, you will also need a screwdriver to remove the lock faceplate and loosen the set screw.

Step-by-Step: Measuring a Mortise Cylinder

1. Remove the cylinder. On a storefront lock, take off the faceplate on the edge of the door, loosen the cylinder set screw a few turns and unscrew the cylinder counterclockwise with the key inserted.
2. Identify the measurement endpoints. Find the manufacturer and model, and check its dimension drawing. Locate the underside of the front flange and the specified rear endpoint before measuring.
3. Measure the specified length. Use a ruler or caliper between the drawing’s endpoints. Record the body measurement and the measurement to the back of the cam separately if you cannot identify the model, then ask your supplier to confirm the ordering size.
4. Match the manufacturer’s listed size. Common lengths include 1 inch, 1-1/8 inch, 1-1/4 inch, 1-3/8 inch and 1-1/2 inch. Do not round an uncertain measurement and order on that basis alone; confirm the model, thread specification and required fit.
5. Note the cam and keyway. Take a photo of the cam from the side and the keyhole from the front. Length alone is not enough to order the right cylinder.

Common Mortise Cylinder Sizes: 1 Inch vs. 1-1/8 Inch

Use the table as a starting point, not a fit guarantee. Check the exact model’s dimensions and cam options; the GMS cylinder catalog provides manufacturer dimension drawings for its mortise cylinders.

Measuring Without Removing the Cylinder

The exposed portion of an installed cylinder does not reliably identify its full length. Photograph the cylinder face, door-edge lock markings and any trim or guard ring, then check the hardware specifications. If the model cannot be identified, have the cylinder removed and measured before ordering a replacement.

Mortise Cylinder Measuring FAQs

Does the cylinder length include the cam?

Check the exact manufacturer’s drawing. Record which endpoints you measured rather than assuming a body-only measurement is the catalog size.

Will a 1-1/8-inch cylinder replace a 1-inch cylinder?

Not automatically. Confirm the lock and trim requirements, cam and keyway. A longer cylinder can protrude too far, and a trim ring is not a substitute for checking compatibility.

Is a rim cylinder measured the same way?

No. A rim cylinder uses mounting screws and a tailpiece rather than a threaded mortise-cylinder body and cam. Identify the cylinder type before selecting a replacement.

When to Call a Chicago Commercial Locksmith

If the housing rotates while you turn the key, read our spinning storefront cylinder troubleshooting guide before ordering parts. For multiple doors or a broader security project, see our commercial locksmith services in Chicago. You can also explore all Omega Locksmith services.

If the cylinder spins, will not come out, or the door needs to be keyed to match other locks, Omega Locksmith can measure, supply and install the right cylinder on site. See our full storefront lock cylinder service for rekeying, keyed alike cylinders, guard rings and CX5 high-security upgrades.