Question 67 3AE01 - Third Assistant Engineer

The Correct Answer is B. **Why Option B ("Replacing the plunger and the barrel") is correct:** Diesel engine fuel injection pumps operate using extremely high precision components (the plunger and barrel/bushing) that are matched sets. They are manufactured together with microscopic tolerances to ensure a tight hydraulic seal (necessary for generating the required injection pressure) and precise fuel metering. Because of these close tolerances, the plunger and barrel wear together. If the fit is worn to the point where replacement is necessary, it is impossible to simply replace one component (the plunger) and expect it to mate perfectly with the worn surface of the older component (the barrel). To restore the required sealing properties and operational efficiency, both the plunger and the barrel must be replaced as a calibrated, matched assembly. **Why the other options are incorrect:** * **A) Grinding the spare plunger to the barrel:** Grinding is typically used to remove material. The precise, matched surfaces of these components are created during manufacturing using specialized lapping processes. Attempting to "grind" a spare plunger to fit a worn barrel in a standard workshop environment would destroy the precise clearances, leading to leaks, poor performance, or complete pump failure. Furthermore, the worn barrel surface cannot be easily compensated for by grinding the new plunger. * **C) Highly polishing both the plunger and barrel:** Polishing is part of the manufacturing process (lapping) that creates the original precise fit. Once the components are worn past their service limit, simple polishing cannot restore the lost material or correct the dimensional changes (out-of-roundness or taper) caused by wear. Polishing would likely just slightly increase the already unacceptable clearance. * **D) Replacing plunger only:** As explained above, the plunger and barrel are matched sets. A new plunger will have perfect, unworn dimensions, but it cannot seal effectively against the imperfect (worn) internal surfaces of the old barrel. This mismatch would result in excessive fuel leakage (blow-by), inability to achieve required injection pressure, and loss of engine power.