A look at static and dynamic cylinder bore shape issues
Even with the best of intentions and preparation, an unavoidable degree of cylinder bore distortion is likely to occur under dynamic stress (heat and pressure). The challenge is to understand how these changes take place and to establish procedures that will attempt to minimize these changes. 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.
In addition, distinct 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 “morph” 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 severely affected by additional distortion that results from clamping forces caused by bell housing bolts, water pump bolts, motor mount bolts, etc.