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.

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.