1.Describe cam ground and tapered pistons, and explain how they enhance engine
performance. 2. Explain why piston pins are offset in the pistons. 3. Describe the three rings and their purposes. 4. Explain the advantages and disadvantages of cast-iron, chrome, and moly rings. 5. Describe the design and purpose of the connecting rod. 6. Undersize bearings
A. have a smaller outside diameter.
B. are used when the block has been line bored.
C. have a thinner lining than standard bearings.
D. are used when the crankshaft has been ground.
The correct answer and explanation is:
1. Cam Ground and Tapered Pistons
Cam ground pistons are designed with an oval shape that becomes round when they reach operating temperatures. This design accounts for uneven thermal expansion, ensuring a tighter fit in the cylinder at operating conditions. Tapered pistons are slightly conical, with the crown (top) having a smaller diameter than the skirt (bottom). This compensates for greater heat exposure and expansion at the crown. Both designs reduce friction, improve heat dissipation, and prevent scuffing, enhancing overall engine efficiency and longevity.
2. Offset Piston Pins
Piston pins are offset slightly from the piston centerline to reduce noise and wear during operation. The offset ensures the piston transitions smoothly from one side of the cylinder wall to the other as it moves. This minimizes the “slap” that occurs when the piston changes direction, reducing vibration and improving engine durability.
3. The Three Piston Rings and Their Purposes
- Compression Ring: Located at the top, it seals the combustion chamber, preventing gas leakage and maximizing power.
- Second (Scraper) Ring: Below the compression ring, it aids in secondary sealing and helps scrape excess oil from the cylinder wall.
- Oil Control Ring: The lowest ring, it ensures proper lubrication by spreading oil evenly on the cylinder wall and scraping excess oil back to the crankcase.
4. Advantages and Disadvantages of Ring Materials
- Cast-Iron Rings: Durable and adaptable to cylinder imperfections, but they wear faster.
- Chrome Rings: High wear resistance and longevity, but poor initial sealing (requires longer break-in periods).
- Moly Rings: Offer excellent wear resistance, heat dissipation, and sealing, but they are more expensive and fragile during installation.
5. Connecting Rod Design and Purpose
The connecting rod links the piston to the crankshaft, transferring the force from combustion to rotate the crankshaft. Its robust design ensures durability under high stress and temperature, while its lightweight construction minimizes inertia.
6. Undersize Bearings
Correct Answer: D. Are used when the crankshaft has been ground. When a crankshaft is ground to repair damage or wear, its journal diameters become smaller. Undersize bearings compensate for this reduced diameter, ensuring proper fit and operation.