Why shouldn't the drain plug be over-tightened?

[SanIsabella]

Because over-tightening the drain plug can easily cause thread stripping. Generally, the torque specification for engine oil drain bolts is 35Nm, and it only needs to be tightened until there's no oil leakage. Here's some relevant information about drain plugs: 1. Consequences of over-tightening: If tightened too much causing thread stripping which leads to oil leakage, or if tightened excessively causing bolt breakage, in severe cases it may require replacing the oil pan to resolve. 2. Drain plug tightening technique: Typically a washer needs to be added before tightening the drain plug, as the washer serves a sealing function. Since the contact surface between the drain plug and oil pan cannot be perfectly flat, the two won't fit together tightly when in contact, and the washer can compensate for the gap between the drain plug and oil pan to achieve sealing.