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Measuring concentricity requires an accurate bore dial gauge or ID micrometer. The roundness of the bores is determined by measuring each bore parallel to the direction of the crankshaft, then measuring the same bore again perpendicular 90 degrees to the crankshaft.
If both dimensions are identical, you have a nice round hole. If the numbers differ, the greater the difference the more out-of-roundness exists in the hole.
Taper is another critical dimension to consider. Taper in the bores will cause the rings to flex in and out as the rings slide up and down in the cylinders. Over time, this can lead to metal fatigue and ring breakage. Taper is determined by measuring the bore ID near the top but below the ring ridge where wear is greatest, then measuring the bore ID near the bottom where wear is minimal.
Subtract the small number from the larger to calculate the amount of taper in the cylinder. In late model engines, the maximum amount of taper allowed is not much at all. On a Mustang 4. By comparison, the maximum amount of taper allowed in many older passenger car engines was as much as 0. To achieve the best possible bore geometry, late model engines with thinwall castings and performance blocks should always be honed with a heavy steel torque plate deck plate and head gasket bolted to the block.
The plate simulates the loads placed on the block when the head is installed. Honing the block in this condition will result in better bore geometry when the engine is assembled and run. Good bore geometry also requires good equipment. If you are honing with conventional vitrified abrasives, the operator or equipment must compensate for stone wear to keep the cylinders straight. Diamond honing stones experience minimal wear and will generally produce straighter bores and greater repeatability hole to hole.
A honing machine that offers variable speed stroking and can dwell in the bore while maintaining the same loading reading will produce better bore geometry than a machine that lacks these features. Using a coolant that is compatible with your honing stones will also improve bore geometry.
The coolant flushes away debris while helping the stones maintain a consistent temperature. Surface finish recommendations for late model engines vary depending on the application, but are often in the 15 to 20 microinch range Ra roughness average.
For performance engines, the recommendation may be even smoother, say 10 to 15 Ra. Regardless of the Ra number, most rings seal best with a plateau finish in the cylinders. There are as many recipes for plateau finishing as there are for making pizza. The objective is to create sufficient crosshatch depth in the cylinder wall to retain oil with a relatively flat, smooth flat surface area between the grooves to support the rings.
A plateau finish will essentially mimic a broken-in cylinder. This will drastically reduce the time it takes for new rings to seat, and also minimize ring wear during the break-in process for longer overall ring life.
Rings manufacturers say the best surface finish is often achieved by rough boring a cylinder to within. Brushing the cylinder with a plateau honing tool as the final step removes loose and folded surface debris, and does not alter the dimensions of the bore. Accurately measuring the surface finish in a freshly honed cylinder requires a special instrument called a profilometer. The profilometer drags a tiny diamond-tipped stylus across the surface to calculate a number of important parameters.
Position the boring cutter so that it is parallel with the cylinder. Allow the boring cutter tool to lower into the cylinder and perform the boring work, producing a round hole in the cylinder material. Finish the cylinder ports with a hand-held rotary or grinding tool by chamfering the intake and exit port edges inside the cylinder only applies to two-stroke cylinders.
Use a rotary burr for the initial chamfer work and a sandroll bit for the final grind. Attach the proper-size honing tool for your cylinder to a power drill or similar tool that will allow the honing tool to rotate at high speeds.
Use a screwdriver to adjust the tension on the honing tool so that it properly fits the cylinder when inserted. Apply engine oil to the honing tool end so that it is fully covered with oil. Insert the tool into the cylinder and turn it on to maximum speed, holding it firmly. Move the honing tool in and out of the cylinder quickly so that it doesn't grind one spot too long.
Do not pull it all the way out or you will damage the cylinder edge. Pull the tool out and check the work.
Reinsert the tool and spin it the opposite direction. Pull the tool out again and ensure that the inside of the finished cylinder displays a cross-hatch pattern.
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