Vernier calliper

A vernier calliper (in Spanish, calibre or pie de rey) is a precision measuring instrument with two jaws, one fixed and one sliding, that measures lengths, thicknesses and diameters to the millimetre or even finer. There are versions with an engraved scale and digital ones.

The locksmith uses it every day: to measure a lock cylinder from the cam to each end, the thickness of a door leaf, the spindle of a handle or the centre distance of a lock. A cylinder of the wrong size sticks out, and a protruding cylinder is easier to snap or pull out.

If you want to know the size of yours before changing it, you can measure it with a ruler and use our calculator to measure the cylinder. For the change itself, see cylinder replacement.