Question 30 AEL01 - Assistant Engineer - Limited

The Correct Answer is B **Explanation for B (Replacing the plunger and the barrel):** Diesel engine fuel oil injection pumps utilize a matched set consisting of the plunger and its corresponding barrel (or sleeve). These components are manufactured together with extremely precise, close tolerances (often measured in micrometers) to ensure a perfect sealing fit. This fit is crucial for generating the extremely high pressure required for injection and for accurate fuel metering. Because the fit relies on the precise relationship between both components, wear occurs on both the plunger and the inner surface of the barrel. Therefore, when the assembly wears out and fails to maintain sufficient pressure or metering accuracy, both the plunger and the barrel must be replaced as a matched unit. **Why other options are incorrect:** * **A) Grinding the spare plunger to the barrel:** Grinding or lapping a new plunger to an old, worn barrel (or vice versa) is impractical and would destroy the precisely manufactured fit and tolerances required for high-pressure sealing. Modern fuel pump components are not designed to be customized or fitted manually. * **C) Highly polishing both the plunger and barrel:** While these parts are highly polished initially, simple polishing is insufficient to correct the dimensional wear and material loss that occurs over time. Polishing would not restore the critical factory tolerance necessary for effective sealing and pressure generation. * **D) Replacing plunger only:** Since the high-pressure sealing depends on the extremely close fit between the plunger and the barrel, replacing only the plunger while keeping a worn barrel will result in continued leakage, poor compression, and inadequate fuel delivery performance. They must be replaced as a matched pair.