Measuring right: calipers, micrometers and dial indicators

Race engines are built to tight clearances. Bearing clearance and piston fit are measured in thousandths of an inch.

One thousandth of an inch is written 0.001 inch. Machinists call it one thou.

Every good reading starts with a clean tool and a clean part. A chip or a speck of dirt under a jaw can add a thou or more.

A caliper measures outside size with the big jaws, inside size with the small upper jaws and depth with the rod at the end. Wipe the jaws, close them and check that the caliper reads zero before you measure.

On a dial caliper, the beam scale is marked every 0.100 inch. One full turn of the dial needle equals 0.100 inch. Each mark on the dial is 0.001 inch. Add the whole inches, the tenths on the beam and the dial number. One inch plus four tenths plus 37 on the dial reads 1.437 inch.

On a common inch vernier caliper, the beam is marked every 0.025 inch, with a number at every 0.100 inch. The sliding vernier scale has 25 lines. Each vernier line is worth 0.001 inch. Read the beam up to the vernier zero. Then find the one vernier line that lines up exactly with a beam line and add that many thousandths.

A digital caliper shows the number for you. Close the clean jaws and press zero before you start. Check that it is set to inches and not millimeters.

An outside micrometer, or mic, is more precise than a caliper. Each mic covers a one inch range, such as 0 to 1 inch or 1 to 2 inch.

The mic spindle has 40 threads per inch. That makes each line on the sleeve worth 0.025 inch. Every fourth line is numbered. Each number means another 0.100 inch. The thimble has 25 marks. Each thimble mark is 0.001 inch. One full turn of the thimble moves the spindle 0.025 inch.

To read it, take the largest number showing on the sleeve. Add 0.025 inch for each line past that number. Then add the thimble mark that lines up with the sleeve line. A sleeve showing 1 plus three lines, with the thimble on 3, reads 0.100 plus 0.075 plus 0.003. That equals 0.178 inch.

Close the mic with the ratchet, not by twisting the thimble hard. The ratchet slips at the same pressure every time. Check zero on clean faces before you trust a mic. Larger mics are checked on the standard rod that comes with them. If zero is off, the small spanner wrench turns the sleeve to line it back up.

Hold the tool square to the part. On an outside measurement, a tilted caliper or mic reads bigger than the part really is. Measure close to the beam of the caliper, deep in the jaws, not out at the tips.

Look at a scale straight on. Viewing it from an angle shifts the reading. This error is called parallax.

Precision measurements are referenced to 68 degrees Fahrenheit. Parts grow when they get warm. A steel part 4 inches across grows about a quarter of a thou when it warms up 10 degrees Fahrenheit. Aluminum grows about twice as much as steel.

Let parts cool to shop temperature before you measure them. Handle the tool and the part as little as you can. Heat from your hand changes the reading.

A dial indicator shows movement, not size. You use it to see how much something wobbles or moves. Mount it on a rigid base that cannot flex.

Point the plunger straight at the surface, at 90 degrees to it. An angled plunger gives a false reading. This error is called cosine error. Push the plunger in part of its travel before you zero it, so it can read movement in both directions. Then turn the bezel until the needle sits on zero.

To check runout, turn the part one full turn slowly and watch the needle. Note the highest and lowest readings. Runout is the high reading minus the low reading, called total indicator reading. A needle that swings 3 thou one way and 2 thou the other way from zero shows 0.005 inch of runout.

To check gear lash, set the plunger at 90 degrees to the face of a ring gear tooth. Hold the pinion still and rock the ring gear back and forth. The needle travel is the backlash. Check it in three places around the gear and compare each reading to the spec.