Class 4 (4th Engineer) MEKM 📅 Dec 2024

Exam Question

With reference to engine inlet and exhaust valves:

(a) State why tappet clearance is essential and why it must be periodically checked. (6)

(b) List with reasons the likely consequences of excessive and insufficient tappet clearance. (4)

(c) State how carbon build-up can occur during engine running and indicate the possible effects on cylinder performance. (6)

Reference Answer

### (a) Tappet Clearance: Necessity and Periodic Checks
**1. Essential Nature of Tappet Clearance (Why it is provided):**
Tappet clearance, also known as valve lash, is the small, specified gap in the valve train mechanism, typically measured between the rocker arm tip and the valve stem tip when the valve is fully closed and the engine is cold. It is a critical design feature for the following reasons:
* **Compensation for Thermal Expansion:** The primary reason for tappet clearance is to accommodate the thermal expansion of valve train components (valve stem, pushrod, rocker arm) as the engine warms up from ambient to its normal operating temperature. Without this clearance, the expansion would cause the valve to be held partially open, preventing it from seating correctly on the valve seat.

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