1. Pistons, Rings, and Connecting Rods
Chapter 12
Pistons, Rings, and Connecting Rods

2. Objectives Analyze wear and damage to pistons, rings, pins, and rods
Select most appropriate repairs to perform
Describe related theory of pistons, rings, wrist pins, connecting rods, and related parts

3. Cast and Forged Pistons
Cast are most common
Forged are used for heavy duty and high performance application
Forged pistons are 70% stronger than cast
Cast pistons should not be used above 5,000 rpm

4. Piston Head and Ring Grooves
Diameter of piston head is typically .022" less than diameter of skirt
Top piston ring is placed as high as possible to prevent piston slap
Top ring groove suffers most abuse
Typically have three ring grooves

5. Piston Head and Ring Grooves

6. Piston Expansion Control
Aluminum piston expands at about twice the rate of the cast iron block

7. Piston Expansion Control
To help prevent expansion:
Most piston skirts are tapered
Skirt is cam ground to allow for a tighter cylinder clearance
Cast pistons have a spring loaded steel cast
This helps the piston expand along wrist pin during warm up

8. Piston Wear Problems
Scuffing on the piston skirt caused from excessive idling or by lugging the engine
Scuffing caused by cylinder wall hot spots

9. Piston Wear Problems
Overheated piston because of cooling system problems or combustion problems
Piston head wear above top ring due to excessive cylinder clearance
Wear above wrist pin due to a bent rod

10. Piston Service Pistons should be removed from the connecting rod
Wrist pin bores need to be protected
Use a soda blaster to remove carbon
Excessive blasting can distort ring grooves

11. Cleaning Ring Grooves Removal of carbon from ring grooves:
Soda blaster
Piece of broken ring
Ring groove cleaner

12. Cleaning Ring Grooves
Carbon must be removed or new rings might not compress enough to enter cylinder

13. Cleaning Ring Grooves

14. Piston Rings Most engines use two compression rings and one oil ring
Seal compression helps cool piston and control oil consumption
Top sealing ring is exposed to pressures of 1,000 psi

15. Ring Size Terminology Width Radial thickness Flutter Height of ring
Depth of the ring into groove
Flutter
Inertia from high speed causes ring to stay against top of ring groove

16. Ring Size Terminology

17. Compression Ring Design
Napier Rings
Steeped ridge provides “squeegee” edge
Torsional Twist Rings
Chamfered on top inside edge
Reverse Twist Rings
Machined in lower inner corner
Barrel-Faced Ring
Less likely to cause ring ridge

18. Piston Ring Materials and Coatings
Cast iron rings
Moly rings
Chrome rings
Premium ring combination

19. Piston Ring Materials and Coatings
Ductile iron rings
Steel rings
Plasma ceramic
Pressure-balanced rings

20. Oil Control Rings Oil ring failure
Plugged because of blow by leaking past compression rings
Most engines have only one oil ring located below compression rings
Oil ring contributes most friction to engine

21. Oil Control Rings

22. Ring Wear Main cause of ring wear is abrasion
If wear is due to dirty air entering the engine
Top compression will show more wear
If ring wear is due to abrasives in the oil
Lower rings will show more wear

23. Installing Rings on Pistons
Use a ring expander to install rings
Identification marks facing up
If one compression ring is installed upside-down
Oil consumption may double
Gaps should not line up with the gaps in other rings

24. Installing Rings on Pistons

25. Wrist Pins Piston pin Lack of lubrication can damage pin
Used to attach piston to connecting rod
Lack of lubrication can damage pin
Lubricated from either a hole in the top of the pin boss or through an angle-drilled hole that runs from the wrist pin
Can be pressed in or held in with retainers

26. Wrist Pins

27. Connecting Rods Made from forged or cast steel
Some racing rods are made from forged aluminum
Generally made in an I-beam shape for strength

28. Rod Service Inspect rods for any obvious cracks, bends, or heat damage
Check rods suspected to be bent or twisted

29. Rod Service