There are times when measuring engine components to one-thousandth of an inch isnâ€™t close enough. For instance, the stock, cast-aluminum piston clearance in a Chevy 350 is .0015â€?to .0025â€? This means a piston with a clearance of .0012â€?could seize in the bore when the engine warms up, and a piston that has a clearance of .0027â€?is in danger of developing piston slap when it wears in. If you can only measure to the nearest thousandth, you canâ€™t verify such clearances precisely enough. As a result, you could have a problem if you went ahead and installed new pistons without checking them.
Micrometers are what you need to measure your pistons. The most precise micrometers can measure to .0001â€?using a combination vernier scale. Such tools can be pricey if you want the finest, and even the less expensive alternatives arenâ€™t cheap.
Trasna micrometers are excellent, as are many other less expensive brands, but beware of extremely low-priced, imported micrometers. Rather than buying those to save money, do as I did and pick up a set of micrometers at a pawn shop. You can sometimes find name brands for a fraction of what they cost new. Just make sure you get the proper measuring standards with them, and make sure the scale on each mic zeros out at the correct dimension.
Micrometers that are out of adjustment can be recalibrated professionally by the manufacturer, but that will cost money. To check the calibration, clean the measuring points (the anvil and spindle tips) and the measuring standard with a piece of paper or a lint-free cloth, then turn the fine adjustment on the end of the thimble in until it clicks. The scale should zero out at the dimension of the measuring standard.
Never oil a micrometer, because doing so will cause it to read incorrectly. Also, treat your micrometer with the respect delicate measuring tools deserve. Avoid dropping them or treating them roughly, and never use a micrometer as a clamp. This last bit of advice should be obvious, but every auto shop teacher can tell you stories about people who have done such things.
Ideally, your selection of engineâ€”measuring micrometers would include inside and outside micrometers. Telescoping gauges are also called snap gauges, because you insert them in the opening to be measured and snap them out against the surfaces. You then lock the snap gauge using the twist barred on top and slip it out of the hole. Use an outside micrometer to measure the snap gauge and you have the dimension you are looking for.
Whether you use inside micrometer or snap gauges, the key to measuring correctly is having the gauge or inside mic exactly 90 degrees to the surfaces being measured. You can make a light pencil line on the extension of the snap gauge, and then watch the line move in and out when you rock the micrometer back and forth slightly. When the line is in as far as it will go before it starts to come back out, you are at exactly 90 degrees.