Outside Mikes----outside micrometers are for measuring outside dimensions such as value stems and crankshaft journals or material thickness such as valve spring shims or head gaskets. They measure in a 1-inch range starting from zero , one., two, three, fourâ€¦and are designated 0-1 in. , 1-2 in. , 2-3 in. , 3-4 in. , 4-5 in. â€? respectively. For example , a 3-4 in. micrometer measures 3 and 4 inches and all points in between to within 0.0001 in.. So if that youâ€™re measuring is smaller than 3 in., or greater than 4 in. , youâ€™ll need another micrometer.
If you expect to do a complete and accurate check of your engine, you will need at least three micrometers. The ones youâ€™ll need depend on the dimensions of the components youâ€™ll be measuring such as valve stems, bearing journals and cylinder bores.
Check the critical dimensions of your engine to determine which micrometers youâ€™ll need. Chances are youâ€™ll need a 0-1 in., a 2-3 in. and a 3-4 in. micrometer if your engine is typical . Once you decide on the ones you need, shop for quality micrometers first, then look for the lowest price. And quality doesnâ€™t mean gizmos such as battery-powered digital readout micrometers that hook up to a video display. What you need are basic durable micrometers that will give you an accuracy within 0.0001 in. , or 0.001 mm if you choose a metric micrometer.
Features to look for include a vernier scale on the hub, a checking standard, a ratcheting thimble and a lock. The vernier scale allows you to read measurements within 0.0001 in. whereas the resolution scale by itself will only allow you to read within 0.001 in., or 0.01 mm. Anything in between will be a guess. The checking standard, or gauge block as some call it , is an accuracy machined rod that is used to check and set the accuracy of the micrometer. Note: A 0-1-in./0-25mm micrometer wonâ€™t need a standard since it should read 0.0000 in. /0.000mm when the micrometer is closed. The others wonâ€™t close up, so they will need standards. All micrometers need a wrench for readjusting their accuracy.
A ratcheting thimble is more of a convenience than a necessity. In fact, most professionals donâ€™t use them. If, however, youâ€™re not experienced with using a micrometer, a ratcheting thimble can be helpful. It allows you to tighten the micrometer just right when youâ€™re taking a reading. When the thimble ratchets or slips, the micrometer is tightened correctly. If it is too tight, the micrometer reading will be too low; too loose and it will read high. The lock keeps the thimble from rotating so your reading wonâ€™t change while youâ€™re handling the micrometers.
A final note about outside micrometers: Make sure a ball adapter is included with the 0-1-in./0-25mm micrometer. This attachment allows you to measure curved components such as the thickness of a bearing insert. The ball fits on the end of the spindle and bares against the inside curve of , say, a bearing inset . Just remember to subtract the 0.200 in./ 5.08mm from each reading or whatever is added by the ball.
For a micrometer that doesnâ€™t have a ball adapter find a ball from an old ball bearing thatâ€™s about the same diameter as the micrometer spindle. To use , fit the ball to the spindle and ball, Mike the ball and subtract that number from whatever youâ€™re measuring.