A cylinder machine is used to bore an engine block oversize. Often called a boring bar, a cylinder machine is used to rid a cylinder wall of scratches and nicks and to make a straight cylinder wall. Acting like an internal metal lathe, the cylinder machine cuts material from the cast iron cylinder liner. In some cases of extreme cylinder wall damage, the cylinder machine bores the engine block until a replacement cylinder sleeve can be inserted and locked into place.
As a piston rides up and down within a cylinder wall, it often begins to create a dish near the top and bottom of the stroke. This is due to the piston rocking as it changes direction. By boring the cylinder to a slightly larger size, the cylinder machine rids the cylinder of these depressions. The piston rings also create a ridge near the top of the cylinder wall. This is the result of the piston rings scraping the cylinder wall of oil and the wearing not only of the cylinder wall, but also of the piston rings themselves.
Typically when a cylinder is bored oversize, it is not bored to the exact size by the boring bar. The final sizing of the cylinder wall usually is completed with a cylinder hone. The cylinder machine will be set to hone the cylinder to the final size, with the machinist taking critical measurements in three places within the cylinder wall. This is done to ensure a straight and true cylinder wall at the bottom, middle and top of the cylinder.
To replicate the affect that torque has on the roundness of a cylinder wall, most reputable machine shops will fasten a deck plate to the top of the block prior to machining it. The deck plates replicate the torque that the cylinder head and its fasteners place on the engine block. This is the most accurate method of maintaining a straight and square cylinder bore. After the machine work has been completed, the engine block is removed from the cylinder machine and thoroughly washed.
Washing the block removes any metal chips and abrasives remaining from the cylinder machine. Failure to wash the block could result in a catastrophic engine failure. It takes only one metal chip left inside an oil passage to cause damage to an engine. The coolant used in the cylinder machine typically washes most chips away as they are created. The power wash is to add insurance that all of the chips have been rinsed away. Blowing out the oil galleys with compressed air is the final step in cleaning the engine.