Question 21 MODE01 - Chief MODU Engineer

The Correct Answer is C. ### Why Option C ("improve skirt lubrication") is Correct Knurling the piston skirt involves creating a cross-hatched pattern of small indentations (like fine diamond-shaped grooves) on the surface. This process is primarily performed during piston repair or reconditioning (sometimes also done on new pistons). These small indentations act as tiny reservoirs or pockets that hold lubricating oil. When the piston moves up and down the cylinder bore, this stored oil is retained and distributed more effectively across the skirt surface, significantly improving lubrication and reducing friction and wear between the piston skirt and the cylinder liner wall. ### Why the Other Options are Incorrect * **A) transmit forces evenly:** Knurling is a surface modification technique and does not significantly impact the fundamental structural ability of the piston to transmit combustion forces evenly to the connecting rod and crankshaft. The force transmission is determined by the piston's design and mechanical connection points (piston pin boss). * **B) improve the piston seal:** The primary sealing function in an engine is handled by the **piston rings** (compression rings and oil control rings), not the piston skirt. While good lubrication on the skirt indirectly supports ring health, knurling is specifically designed for oil retention and reducing friction on the skirt, not for gas sealing. * **D) allow for heat expansion:** While piston clearance and design must account for heat expansion, knurling is not the method used to manage this clearance. Managing thermal expansion is achieved through specific piston materials (like aluminum alloys) and carefully calculated skirt diameter tolerances and profiles (e.g., tapered or cam-ground shapes) that ensure proper fit when the engine reaches operating temperature. Knurling is a repair/lubrication modification.