Performance & Hotrod Business - January '15

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10 n PRECISION ENGINE n January 2015 S omething that most hot rodders don't realize is that the cylinder bores in their engines don't remain perfectly round at all times, even though they may have been machined round during engine block preparation. It's a fact of life that an unavoidable degree of cylinder bore distortion is likely to occur under dynamic stress, due to fas- tener tightening, operating temperatures and cylinder pressures during the combus- tion cycles. In addition, as the block ages and/or is exposed to thermal changes, the casting's molecular structure will change, however slightly, which will affect bore geometry. Simply put, cylinder bores are some- what "alive" due to the stresses imposed. The challenge is to understand how these changes take place and to establish proce- dures that will attempt to minimize these changes. Bore Shape Changes in bore shape will take place as the block is assembled. The clamping forces that result from the installation of cylinder heads will, in most cases, cause the bores to slightly alter their static pro- file as the fasteners pull and squeeze metal adjacent to the bores. Sometimes these changes are insignificant, while in other situations, the change can be so dramatic as to cause measurable ring dragging and subsequent loss of power, fuel economy and sealing due to both out-of-roundness and frictional heat. In some engines, these problems may be compounded if the specific block is affected by additional distortion that results from clamping forces caused by bellhousing bolts, water pump bolts, motor mount bolts, etc. Honing pressure guidelines must be carefully followed to prevent uneven or concentrated dwelling that can result in barrel-shape wall profile. When a cylinder is honed, the heat that is generated will transfer to the adjacent bore, which can affect the honed size/profile. Avoid honing bores in sequen- tial order. For example, after honing cylinder number 1, hone cylinder num- ber 5 next, followed by 3 and then 7. Cylinder Bore Distortion How stresses affect bore shape. By Mike Mavrigian All photos by author unless otherwise noted PRECISION ENGINE

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