January '19

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58 THE SHOP JANUARY 2019 Measure valve stems for wear at three locations: near the top, middle and near the bottom. If deviations are found, replace the valve. A C PRECISION ENGINE Example: If we are dealing with a bore diameter of 4.030 inches, and the appli- cation involves a hot street engine that's naturally aspirated, our top and second ring end gaps will be finalized as follows: • 4.030 x 0.0045 = 0.018 inches top ring end gap • 4.030 x 0.0050 = 0.020 inches second ring end gap If end gaps are too tight, the ends will jam together and the rings will expand and scrape the cylinder walls too severely, resulting in damage to rings, piston lands and cylinder walls. If too loose, you'll sac- rifice some compression and face excessive blow-by and resulting excessive crankcase pressure, as well as oil potentially entering and fouling the combustion chamber. Rings offered by quality manufacturers will likely provide an acceptable end gap when selected for a specific bore size and piston design. Regardless, it's still impor- tant to verify ring end gap before final installation. For optimum results, seasoned builders often choose slightly oversized rings that are designed as file-to-fit, which allows you to tailor your end gaps precisely to your needs. Based on your bore size, the top and second rings will initially feature butted or overlapped ends, requiring filing to achieve the desired gap. Oil rings and oil rails are made for the selected bore size and should not pose an issue. Instead of checking gaps of all top and second rings in one cylinder, it's best to dedicate each ring package to each cylinder, file-fitting the top and second rings to indi- vidual cylinders. This aids in fine-tuning the assembly, eliminating any potential minor cylinder bore diameter variances. Before filing or installing your rings, verify ring side clearance and ring radial depth. If side clearance is too tight, the rings may bind and not be able to move freely within the groove. Sufficient radial depth is needed to ensure that the rings don't bottom-out between the inside base of the groove and the cyl- inder wall. Aftermarket pistons will include specs to verify ring fit. Either install the top ring or slip the outer edge of the top ring into its groove (OD of the ring facing the OD of the piston) and insert a feeler gauge to check side play. Repeat for the second ring. Refer to Note the interference between a rod bolt and block. The area is marked with a pen. Here a slight clearance notch has been ground, providing about 0.085 inches of clear- ance. Counterweight-to-block and rod-to-block clearances need to be at least 0.060 inches. Valve guides can be measured for clearance using a small bore gauge. Here a valve seat is checked for valve concentricity. A mandrel is inserted into the guide, and the gauge is rotated to determine if the seat is concentric to the guide bore centerline.

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