Question 22 MODE01 - Chief MODU Engineer

The Correct Answer is C **Why Option C is Correct:** Knurling the piston skirt surface is a widely recognized and effective salvage technique used in engine repair, particularly for aluminum pistons that are slightly worn or collapsed (smaller than specification). The process involves rolling a hardened steel tool (a knurl) across the piston skirt. This action displaces and raises the aluminum material, effectively increasing the diameter of the skirt back toward or slightly above the required specification (oversizing). This allows the piston to achieve the necessary tight clearance within the cylinder bore, restoring proper sealing, reducing piston slap, and extending the service life of the component, especially in situations where replacement parts are unavailable. **Why the Other Options are Incorrect:** * **A) building up the piston skirt with a liquid epoxy material and then re-machining:** While epoxies (like specialized metal-filled putties) are sometimes used for minor repairs, they lack the structural integrity, heat resistance, and wear resistance required for the high-stress, high-temperature environment of a diesel engine piston skirt. Such a repair would quickly fail and potentially cause catastrophic engine damage. * **B) increasing the dimensions of the ring land grooves:** The ring lands define the crucial sealing interface between the piston rings and the cylinder wall. Increasing their dimensions (i.e., making them wider or deeper) would degrade the function of the piston rings, compromising compression and oil control. This modification addresses the ring fit, not the dimensional wear of the piston skirt, which is the cause of piston slap and excessive clearance. * **D) turning down the piston skirt to concentric values:** Turning down (reducing the diameter of) the piston skirt would intentionally decrease its dimensions, making the piston even smaller than the worn cylinder bore. This would dramatically increase the running clearance, worsening piston slap, wear, and likely leading to rapid failure rather than extending service life. This procedure is the opposite of what is needed to salvage an undersized component.