Our friends over at Mahle Motorsports produce pistons and rings for all types of racing and street-based performance engines, and one of the most common phone calls they receive on their technical assistance line is how to measure piston to cylinder bore clearance. So–you guessed it–that’s the focus of this particular video, hosted by Mahle’s main man, Trey McFarland.
With every piston set they provide, there are recommended minimum and maximum clearance dimensions, along with the gauge point on the piston itself. You did know that there’s a particular point where you’re supposed to measure, correct? If not–now you do.
Once the piston dimension is checked in the proper location and the measurement determined, the cylinders need to be checked with a dial bore gauge as the second part of the process. In this particular case, the piston measured at 4.0370-inch, and the bore of the engine block at 4.0402-inch, giving a piston-to-wall clearance of .0032–right in the range of what is acceptable for the piston design in question.
This clearance will vary with different piston materials, as 4032 alloy and 2618 aluminum alloys expand and contract at different rates; the use of a power-adder like a turbocharger or nitrous oxide will also affect where the manufacturer will recommend the clearances to be set, as the increased heat produced in those applications will cause the piston to “grow” more than in a typical naturally-aspired application.
McFarland also discusses a number of other things to note, such as coatings, material temperature, and more.
For more information on the complete line of products offered by Mahle Motorsports, check out their website.