Pistons – Rings - Pins

1. Pistons – Rings - Pins Detroit Basketball? Page 254-257

2. Piston Construction • Pistons are normally cast or forged from an aluminum alloy • Cast pistons • relatively soft, used in slow-speed, low-performance engines • Forged pistons • used in fuel-injected, turbocharged, and diesel engines

3. Piston This piston is for a diesel engine and has a groove that allows an oil spray to help cool the piston

4. Cam-Ground Piston • Slightly out-of-round when viewed from the top • Machined a few thousandths of an inch larger in diameter perpendicular to the piston pin centerline • compensates for different rates of expansion due to differences in metal wall thickness

5. Cam-Ground Piston

6. Cam-Grind Theory • As the piston is heated, the thicker area around the pin boss causes the piston to expand more parallel to the piston pin • The piston becomes round when hot • A cam-ground piston maintains the correct piston-to-cylinder clearance when cold and at operating temperature

7. Piston Taper • Used to maintain the correct piston-to-cylinder clearance • The top of the piston is machined slightly smaller than the bottom • Since the piston head gets hotter than the skirt, it expands more • The piston is almost equal in size at the top and bottom at operating temperature

8. Piston Taper

9. Piston Shape • Refers to the contour of the piston head • Piston head is shaped to match and work with the shape of the combustion chamber • Piston may have a flat top or a domed head

10. Piston Shape This is a piston for a diesel engine having a direct injection nozzle

11. Slipper Skirt • Produced when the portions of the piston skirt below the piston pin ends are removed • Provides clearance between the piston and the crankshaft counterweights • Piston can slide farther down in the cylinder without hitting the crankshaft

12. Slipper Skirt Piston

13. Piston Ring Construction • Automotive pistons normally use three rings: • two compression rings • one oil ring

14. Compression Rings • Prevent pressure leakage into the crankcase • Wipe some of the oil from the cylinder walls • Usually made of cast iron • An outer layer of chrome or other metal may be used to increase wear resistance • Needs to be installed with correct side up

15. Compression Rings

16. Oil Rings • Keep crankcase oil out of the combustion chambers • Available in two basic designs: • rail-spacer type (three piece) • one-piece type

17. Oil Rings • Rail-spacer ring (most common) • One-piece ring made from cast iron

18. Piston Ring Dimensions

19. How Oil Rings Work

20. Piston Ring Gap • Distance between the ends of the ring when installed in the cylinder • Allows the ring to be installed on the piston and to “spring” outward in its cylinder • Allows the ring to conform to any variation in the cylinder diameter due to wear

21. Piston Ring Gap Most piston rings use a butt joint

22. Piston Ring Coatings • Soft ring coatings • porous metal, such as iron • help the ring wear in quickly • the outer surface will wear away rapidly so the ring conforms to the shape of the cylinder • Hard ring coatings • chrome or moly • increase ring life and reduce friction • used in new or freshly machined cylinders

23. How Rings Work

24. Installing Rings Check Ring to piston side clearance Check END gap in cylinder Don’t line up end gaps

25. Piston Pin Construction • Piston pins are normally made of case-hardened steel, which increases the wear resistance • A hollow piston pin is machined and polished to a very precise finish

26. Piston Pins • Piston pins are held in the piston by one of two means: • snap rings (full-floating piston pin) • press-fit

27. Full-Floating Piston Pin Secured by snap rings Free to rotate in both the rod and piston

28. Press-Fit Piston Pin Forced tightly into the connectingrod’s small end and free to rotate inthe piston pin hole

29. Piston Assembly The piston notch indicates the front of the piston

30. Piston Assembly This piston has afull-floating piston pin