6. After boring 0.030 inch, the cylinder walls were tested with a Dakota Instruments ultrasonic micrometer. The rule of thumb is that you need approximately 0.200 inch (just over 3/16-inch) of cylinder wall left to support the stresses of a performance engine. A low-po engine can get by with less, perhaps around 0.150 to 0.180-inch. Our metric block tested to an average of 0.250 to 0.270 inches on all cylinders.