If blue smoke is seen at the tailpipe when an engine is started, which part(s) is/are most likely worn out?

Blue smoke from the tailpipe upon starting an engine is a classic sign of oil burning in the combustion chamber. The most likely culprit for this issue is worn valve stem seals. These seals are designed to prevent engine oil from leaking into the cylinders. When they wear out, they can no longer effectively seal the area where the valve stem enters the cylinder head, allowing oil to seep into the combustion chamber. This oil mixes with the fuel and air, resulting in the characteristic blue smoke as it burns off.

In contrast, components such as a thermostat regulate engine temperature and are not directly involved in oil management, making them less relevant to this symptom. The oil pan gasket mainly keeps engine oil from leaking out of the oil pan and does not contribute to oil entering the combustion chamber. A timing chain controls the timing of the engine's valves but does not affect oil consumption or combustion directly. Therefore, the observation of blue smoke strongly indicates that the valve stem seals are the likely issue, leading to oil entering the combustion process.