Pistons, Rings, and Connecting Rods

Pistons The piston's primary responsibility is to take thermal energy created by the ignition of fuel and air, and transform it into linear motion. Linear motion acts on the crankshaft journal and becomes rotary motion.

A typical piston illustrating the various parts and the names.

Piston Heads (Crown) Domed Flat Top Recessed (valve reliefs) Dished

A domed piston with valve reliefs or valve pockets.

A Flat Top piston

A Flat Top piston with valve reliefs or valve pockets.

A Dished piston with valve reliefs or valve pockets.

Piston Skirts Moly Graphite

Piston Skirts Finished perpendicular to the head Scuffing promotes blow-by

A low-friction moly coating on the skirt of this piston helps prevent piston scuffing when the engine is cold.

Piston Composition @ Process Iron – heavy machinery or “old school”

Piston Composition @ Process Aluminum - cast Pour aluminum into a mold Light-weight economical Some silicone added General usage Brittle Somewhat unpredictable expansion qualities

Piston Composition @ Process Hypereutectic Cast aluminum with a high silicon content Light-weight Higher performance Less brittle Predictable expansion qualities

Piston Composition @ Process Aluminum - Forged Can be made lighter weight (smaller) than cast because it’s stronger Can withstand abuse Newer designs have predictable expansion qualities Silicon & Nickel added Greater piston to wall clearance

Notice the temperature difference between a forged piston and a cast piston.

Pistons are often cam-ground to produce the elliptical shape when the piston is at room temperature.

Piston diameter being measured using a micrometer.

Piston (wrist) Pins High-Quality steel Usually Hollow Cross-sectional piston pins. Most piston pins are hollow to reduce weight and have a straight bore. Some pins use a tapered bore to add strength.

Piston pin is offset toward the major thrust surface. TECH TIP

Engine rotation and rod angle during the power stroke causes the engine to press harder against one side of the cylinder, creating a major thrust surface. In this clockwise-rotating engine, as viewed from the front of the engine, the major thrust surface is on the left side.

Piston Pin Retaining Methods Full Floating Lock Rings Circlips or snap rings hold full-floating piston pins in place.

Piston Pin Retaining Methods Interference Fit Pressed

Piston Rings Compression Oil – usually unidirectional Ring expander

Compression Ring Composition Pearlitic Nodular Iron Ductile Iron – flexible Cast Iron Chromium - .0004” - durable Molybdenum – reduced scuffing Chrome-moly

The preferred material for compression rings is a low-alloyed, heat-treated nodular cast iron (KV1/GOE 52). This material is characterized by a high bending strength of min. 1300 MPa and a high modulus of elasticity attributable to a martensitic microstructure and spherulitic graphite structure.

In the 2nd groove, alloyed grey cast irons are used in a heat-treated condition. Besides having a high bending strength and modulus of elasticity, an increased hardness of 320 to 470 HB is produced in order to obtain the required wear resistance in the uncoated condition. The demand for high wear strength is also met by the use of a tempered, alloyed cast iron (GOE 44). This has the benefit of a high bending strength of min. 800 MPa and high modulus of elasticity. The good wear resistance results from the combination of a fine-pearlitic matrix structure and finely dispersed, precipitated secondary carbides.

Unalloyed grey cast iron is used for 2-piece oil rings in the 3rd groove. These ring materials (STD / GOE 12, GOE 13) are characterized by a fine-lamellar graphite structure in a pearlitic matrix and have good conformability due to a relatively low modulus of elasticity.

Reduced width piston rings in gasoline engines to match reductions in the overall height of pistons, and increasing combustion pressures in diesel engines call for materials with increased strength characteristics. These challenges are met by the use of high-chromium alloyed steels and spring steels. The greater durability under increased stresses is demonstrated by the improved fatigue strength manifested as form stability in a comparison of S/N curves for different piston ring materials (spherulitic, heat-treated cast iron versus heat-treated 18% chromium steel).

The wear resistance derives from finely distributed chromium carbides of the type M23C6 and M7C3 embedded in the tempered martensite matrix. For improved wear resistance these steels are mainly used in a nitrided condition or with a peripheral coating. The steels mentioned are used chiefly as compression ring materials for gasoline engines and truck diesel engines as well as for the steel rails and expander-spacers of oil control rings and for 2-piece profiled steel oil rings.

Pearlitic Rings Pearlite is a two-phased, lamellar (or layered) structure composed of alternating layers of alpha-ferrite (88 wt%) and cementite (12%) that occurs in some steels and cast irons. The eutectoid composition of Austenite is approximately 0.8% carbon ; steel with less carbon content will contain a corresponding proportion of relatively pure ferrite crystallites that do not participate in the eutectoid reaction and cannot transform into pearlite. The appearance of pearlite under the microscope resembles mother of pearl (also a lamellar structure), from which it takes its name.

Nodular (ductile) Iron Ductile iron, also called ductile cast iron, spheroidal graphite iron, nodular cast iron, is a type of cast iron invented in 1943 by Keith Millis. While most varieties of cast iron are brittle, ductile iron is much more flexible and elastic, due to its nodular graphite inclusions.

Chromium rings Chromium is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is odourless, tasteless, and malleable.

Chromium facing can be seen on the right side of the sectional view of the piston ring.

Molybdenum Rings Molybdenum (pronounced /məˈlɪbdənəm) It has the sixth-highest melting point of any element, and for this reason it is often used in high-strength steel alloys. Molybdenum was discovered in 1778 by Carl Wilhelm Scheele and first isolated in 1781 by Peter Jacob Hjelm.

Molybdenum facing can be seen on the right side of the sectional view of the piston ring. TECH TIP

This typical three-piece oil control ring uses a hump-type stainless steel spacer-expander. The expander separates the two steel rails and presses them against the cylinder wall.

Ring Gaps Ring gap must be checked prior to engine assembly Butt gap Loose Tight Butt gap Gapless?

The gapless ring overlaps, while the conventional ring design uses a gap. Frequently Asked Question

Gapless Ring

Combustion chamber pressure forces the ring against the cylinder wall and the bottom of the ring groove. These are the two sealing surfaces that the top ring must be able to seal for maximum engine power.

Fitting Piston Rings The piston rings must have the specified side and back clearance.

The rectangular and the barrel face are the most commonly used top compression rings because they provide the best seal.

The taper face ring provides good oil control by scraping the cylinder wall. If this design ring were accidentally installed upside down, the tapered face would pump oil into the combustion chamber.

Torsional twist rings provide better compression sealing and oil control than regular taper face rings.

Cutting Rings

Some designs utilize spit holes or bleed holes Connecting Rods Some designs utilize spit holes or bleed holes Cast iron – general duty Forged steel – stronger – generally heavier

Connecting Rods Cast aluminum Forged aluminum Lightweight Stretch easily

Connecting Rods Powdered metal Pre-balanced Fractured parting line

Connecting Rods H-Beam Design

A typical connecting rod and related engine parts A typical connecting rod and related engine parts. The connecting rod is probably the most highly stressed part in the engine. Combustion forces try to compress it and when the piston stops at the top of the cylinder, inertia forces try to pull it apart.

Some connecting rods have balancing bosses (pads) on each end of the rod. Rod caps are unidirectional and must be reinstalled in the same rod position.

The rod bearing bores normally stretch from top to bottom causing the rod bearing to wear most near the parting line.

Rod Bore Measurement Use a bore gauge and rod fixture in vise Check out-of-round Rod caps must be torqued to mfg. specs.

Connecting Rod Installation The chamfered side of the bore will always lead towards the crankshaft side on a v-type motor.

A press used to remove and install connecting rods to the pistons.