In our example, the micrometer is set at 1.3775. Let's see how we get there. First of all, we're working with a 1- to 2-inch micrometer. Next, note that the rotating barrel has uncovered the main horizontal line well past the 3. Next, we start counting the 0.025-inch hash marks, coming up with three for 0.075 inch. That means we're at 1.3750. Now let's look at the marks on the rotating barrel. Note that the main horizontal line indicates the position between the 2 and the 3, so add 0.002 + 1.3750 = 1.3770. If the main horizontal line had lined up directly with the 2, we'd be done, but since it doesn't, take a look at the lines above the main horizontal line. Here, the 5 lines up perfectly, indicating we should add an additional 0.0005 inch to our reading-0.0005 + 1.3770 = 1.3775. There you have it. Now you know how to read a mic.
Crank endplayCrank endplay is a measure of how far the crankshaft can travel fore and aft in the engine once it is installed, which is determined by the amount of clearance between the thrust bearing and the thrust flange or thrust journal, depending on the engine design. A general guideline for thrust clearance is somewhere between 0.005 and 0.0010 inch, but as always, the exact spec for your engine should be referenced.
With most engines, setting the thrust bearing is a recommended practice during assembly. This typically involves setting the crank in place with the main caps installed but not yet tightened. The crank is then struck with a soft-faced mallet on each end to shift it fore and aft, aligning the two thrust-bearing shells.
Once the thrust is set, the clearance can be measured. On some engines, feeler gauges can be used to determine the amount of thrust, but most engine builders prefer to use a dial indicator with a magnetic base. The dial indicator should be assembled so that the plunger is parallel with the crank's axis; a prybar is then used to shift the crank back and forth in the block. If the dial is set to zero with the crank at one extreme of its thrust travel, when the crank is shifted to the other extreme, the reading should indicate total endplay.
It's fairly common when using worn or even reconditioned crankshafts to find excessive endplay, and correcting this can be difficult. As with rod side-clearance, a little extra endplay can be allowed, but if the crank's thrust movement is too great to ignore, it may have to be repaired or just replaced. If there is too little thrust clearance, the thrust bearings can be sanded to increase the clearance. The recommended procedure for this is to use wet/dry sandpaper with a grit of around 1,000. Using a flat surface under the paper, the bearing is sanded carefully and then reinstalled to check the increase in clearance. This usually will require several test fits, and possibly some initial work with a more coarse grit if the clearance is particularly tight.
Measuring Bearing ClearancesThe most important fact that many enthusiasts miss when measuring bearing clearance is that the housing bore is a big factor in setting clearances. Bearing shells don't vary by more than 0.0001 inch in thickness, but housing bore diameters can easily vary by 0.0005 inch. The simple, easy way is to use Plastigage, but to establish a true bearing clearance, you need to use a micrometer and measure everything.
Let's deal with a connecting rod and rod journal. Start by measuring the connecting rod journal with a micrometer. For the sake of easy math, let's say the journal diameter is exactly 2.0000 inches. It's best to use a mic that indicates to the ten-thousandths of an inch (0.0001 inch). Next, place a set of rod bearings in the matching rod and torque the cap in place. Use a quality dial bore gauge to measure the inside diameter of the connecting rod in the vertical plane. A dial-bore gauge accurately reads the inside diameter of cylinder, main-bearing, or rod-bearing bore. There's a photo of a dial-bore gauge on page 42.