🔍 Key Concepts

• Tappet (valve) clearance and its purpose in a diesel engine valve train • How thermal expansion of metal parts at operating temperature affects valve timing • Relationship between clearance size and when the valve starts to lift off and returns to the seat

💭 Think About

• Think about what happens to valve movement if there is LESS clearance at the tappet when the engine is cold: does the cam contact the valve train sooner or later? • Consider how the valve timing diagram would shift if the valve starts to move with a smaller gap: do opening and closing both move earlier, both later, or one earlier and one later? • At normal operating (hot) temperature, the parts expand and clearance changes. With too little cold clearance, what risk does this create for how long the valve is actually off its seat?

✅ Before You Answer

• Be clear on the definition: tappet clearance is the gap between the rocker arm (or cam follower) and the valve stem when the valve is fully closed and the engine is cold. • Visualize the cam lobe rotating: with reduced clearance, does the lobe take more rotation or less rotation before it starts lifting the valve? • Check which choice best matches the idea that the valve motion will begin with LESS lost motion due to a smaller gap.