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

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

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

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

Piston Head and Ring Grooves

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

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

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

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

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

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

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

Cleaning Ring Grooves

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

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

Ring Size Terminology

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

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

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

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

Oil Control Rings

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

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

Installing Rings on Pistons

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

Wrist Pins

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

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

Rod Service