Reach into bores to measure the precise size and location of grooves. These micrometers have two discs on their measuring spindle to find a groove’s edge. Slide the spindle into your bore, press the discs against the wall and into the groove, and twist the thimble to measure the distance between the two discs. A speeder knob moves the measuring spindle more quickly toward or away from your groove, without turning the thimble repeatedly. When the spindle touches the groove you’re measuring, it will stop tightening, but the speeder knob will continue turning with a clicking sound. The spindle doesn’t rotate, so the contact points won’t spin or slip out of a groove.

Measure the inside diameter of cylinders and rings. These micrometers have a plain thimble that turns freely without friction or a stopping device. It relies on user feel for accurate measurements. All include steel extensions that you can attach to one end of the micrometer head.

Find the inside diameter of cylinders and rings. Rotate the plain thimble until it fits snugly inside the space you’re measuring. These micrometers rely on the operator to determine a secure fit to take accurate measurements. Attach one of the included extension rods to the micrometer head to measure in hard-to-reach spaces.

The jaws on these micrometers work like caliper jaws to precisely measure inner diameters. These micrometers have a friction thimble with a constant-force mechanism, so they stop tightening once both jaws touch the sides for repeatable measurements. When the jaws stop tightening, the thimble continues turning with resistance, preventing overtightening. Use the included setting ring to calibrate and verify the accuracy of these micrometers.

With an LCD screen, these micrometers make it easy to read inside diameter measurements taken from cylinders and rings. Place the micrometer inside your bore, then turn the thimble until the spindle makes contact with both sides. For an accurate reading, stop turning as soon as you feel resistance on the thimble.