Posted on November 25, 2020 linda strong aviation
One of the most important aspects of building an engine is the verification and adjustment of bearing clearance. Doing this helps establish a solid and reliable rotating assembly foundation. Unfortunately, there is no quick and easy shortcut to checking bearing clearances, but this blog will cover the basics of how to measure bearing clearance while informing you of how to avoid some common mistakes.
To check for bearing clearances, start by measuring the main bearing journal. It is best to measure in at least two different planes to establish both a diameter and roundness. Ideally, there should be zero out-of-roundness but it’s possible to see a variation of 0.0001-inch, which could simply be a function of measurement accuracy. Varying based on the application, new crankshaft specifications call for runout and taper that does not exceed 0.0002-inch for both rods and mains.
Next, measure the crankshaft journal and record the diameter on a sheet for all journals. In a new crankshaft, the rods and mains will probably differ no more than 0.0001 inch one way or another. Once the journals are measured, the next step is to set up a dial bore gauge to determine the inner diameter of the rod bearings. Begin by setting the dial bore gauge at just above 2.100 inches to establish a load on the gauge. Next, set the micrometer at 2.1000 inches and place it in a protected vise to keep it in place while the dial indicator is set to read zero at this specification. To set zero clearance on the dial bore gauge, it is best to use the same micrometer to read the journals.
When this is done, place a standard set of rod bearings in a connecting rod and tighten the bolts to the required rod bolt stretch figure. After the bolts are stretched, the dial bore gauge is then set to read the vertical oil clearance directly in line with the rod. Measuring the oil clearance line in the true vertical plane is important because all bearings are designed with an eccentricity to produce additional clearance at the bearing parting line. This is done to compensate for the load, because the sides of the bearing housing will pinch inward when placed under stress.
Performance bearing manufacturers offer a wide range of bearing shells in various over- and under-sizes, allowing the builder to customize their clearances. There are several ways to set clearances exactly where you need them. For instance, adding a full 1X bearing set would add 0.001 inch to the clearance, widening it to 0.0026 inches. This is acceptable as long as you do not mix bearing shells of more than 0.0001 inch differences. For example, combining a 0.001 inch oversize bearing with a 0.001 undersize bearing is not possible and the two will be incompatible.
When mixing shell halves, place the thicker shell half into the loaded side of the housing bore. This is done so that in case of a main bearing, the thicker shell half will be placed in the main cap while in a connecting rod, the thicker half will be placed in the upper position of the rod. This creates a situation where under pressure, the oil clearance decreases on the loaded side so the thinner shell half allows more room for the oil to enter the bearing area and maintain lubrication. Setting bearing clearances carefully is not an especially difficult task, but it is critical nonetheless.
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