Chapter 12
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Presentation Transcript
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 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
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 • Height of ring • Radial thickness • Depth of the ring into groove • Flutter • Inertia from high speed causes ring to stay against top of ring groove
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
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
Wrist Pins • Piston 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
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
