Engine assembly requires the measurement of several components to ensure adequate clearances. Piston to cylinder bore clearance is one of those critical areas. Although the clearance is typically set by the machinist during cylinder honing, it is imperative that the assembler knows how to take the measurement to double-check the machinist. Mahle Motorsports has put together this informative how-to video on the process.
Specific tools are required to measure piston to bore clearance. The list includes the following: micrometer, dial caliper, and bore gauge. Micrometers have a one-inch range and will be used to measure the piston. Select a micrometer with a range that includes the piston diameter. The dial caliper is used to locate the measuring point on the piston. A zero to six-inch caliper will be sufficient.
Finally, the bore gauge will be used to measure the cylinder bore diameter. Like the micrometer, bore gauges cover a certain range. However, the bore gauge comes with a range of extensions and spacers to cover a larger spread such as two to six-inches.
Once all the tools have been acquired, the clearance specification and the measuring point for the piston need to be located. Both of these are provided by the piston manufacturer. Mahle supplies an online version of their application guide via their website.
Variables That Affect Piston To Cylinder Bore Clearance
Precision measurements like bore size and piston diameter are critical for engine survival. Consider any outside factors that may skew the measurement. Mahle pointed out two common ones.
First, temperature will cause the cylinders and pistons to expand and contract. It is imperative that both the cylinder block and the pistons are at room temperature. Small differences in temperature can change measurements considerably considering the micrometer and bore gauge are measuring in one ten-thousandths of an inch increments.
Measuring tools should also be at room temperature. Keep hand contact with the tools to a minimum. Take a measurement, then set the tool down to record the reading. Body heat from holding the gauge in your hand will move the needle two-tenths. Hand warm the gauge before zeroing and taking measurements as an alternative.
As mentioned earlier, cylinder honing sets the piston to bore clearance. Provide the engine machinist with the pistons so the correct clearance can be obtained. The steps laid forth by Mahle will allow the assembler to double check the machinist work.