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PISTON MATERIAL RING TECH

4032 vs. 2618/Forged Wrought Aluminum RING DIMENSIONS

8

Pistons manufactured from 4032 wrought aluminum Pistons manufactured from 2618 wrought aluminum 1 1
alloy are designed for high performance applications alloy are designed for racing and very demanding 9 10
where a strong and quiet piston is required. These applications. These environments necessitate the 5 7 6 11
pistons require less initial piston to wall clearance and higher strength 2618 aluminum alloy.
are quieter at startup. The perfect street/strip piston! 2
1. FREE GAP: The end gap clearance 7. RING FACE: The section of the ring
when the ring is not compressed that contacts the cylinder wall
Physical Properties of 4032 Physical Properties of 2618
2. END GAP: The end gap clearance 8. BACK CLEARANCE: Distance
Nominal Density 2.68 g/cc .097 lb/in3 Nominal Density 2.81 g/cc .100 lb/in3 when the ring is compressed between the inside diameter of the ring
3 to the bore diameter and the back of the ring groove when
Mechanical Properties of 4032 Mechanical Properties of 2618 3. INSIDE DIAMETER: The inside diam- the ring is flush with the ring land
Tensile Strength, Ultimate 380 MPa 55,000 psi Tensile Strength, Ultimate 440 MPa 64,000 psi eter of the ring at bore diameter 9. AXIAL CLEARANCE: The distance
4. OUTSIDE DIAMETER: The outside between the ring axial height and the
Tensile Strength, Yield 315 MPa 46,000 psi Tensile Strength, Yield 370 MPa 54,000 psi 4 diameter of the ring at bore diameter piston ring groove width
Modulus of Elasticity 79 GPa 11,400 psi Modulus of Elasticity 74 GPa 10,400 psi 5. RING AXIAL SIDES: The top and 10. RADIAL WIDTH: The width of the
Fatigue Endurance Limit 110 MPa 16,000 psi Fatigue Endurance Limit 125 MPa 18,000 psi bottom surfaces of the ring ring in the radial direction
6. TORSIONAL TWIST: The installed 11. AXIAL HEIGHT: The height or the
position of the ring due to a chamfered thickness of the ring in the
area on either ring side that helps the ring axial direction
cross-seal
COEFFICIENT OF THERMAL EXPANSION
When exposed to heat, 2618 aluminum expands approximately 15% more than 4032, therefore the intial piston-to-wall
clearance has to be 15% greater. This difference is most noticeable during a cold engine start. When cold, the 2618 piston
can rock back and forth producing a slight noise (sometimes referred to as piston slap) until the aluminum expands. Both
types of aluminum have approximately the same clearances once the pistons have expanded and the engine is running at
operating temperatures.
RING TERMINOLOGY RINGFACE SHAPES

COEFFICIENT OF THERMAL EXPANSION FOR 4032 COEFFICIENT OF THERMAL EXPANSION FOR 2618 D-wall: A specification established by the Society .182” Barrel Face: Term used to describe the
of Automotive Engineers (S.A.E.) that dictates the curved section of the ring that is in contact
Temperature Range Average Coefficient Temperature Range Average Coefficient radial width of a standard automotive piston ring with the cylinder wall. Used only on top
by the use of the following formula; Bore diameter compression rings.
ºC ºF µm/m • K µin/in • ºF ºC ºF µm/m • K µin/in • ºF
20 to 200 68 to 392 20.2 11.2 20 to 200 68 to 392 23.2 12.9 ÷ 22 = radial thickness. (4.000” ÷ 22=.182”) Dykes: A step cut into a top compression
Back-cut: Used to describe a compression ring that ring that helps to direct gas pressure to the
EXAMPLE: EXAMPLE: has less than S.A.E. standard D-Wall radial thickness. shaded area on the back side of the ring,
When measured at room temperature, a piston designed for When measured at room temperature, a piston designed for Back-cutting is used to reduce natural radial .162” improving ring to cylinder wall seal.
a 4.000” bore might measure 3.9966. If the same piston were a 4.000” bore might measure 3.9960. If the same piston were ring tension. In applications with tight top ring land
measured at 375 degrees F, the piston would measure 4.000”. measured at 375 degrees F, the piston would measure 4.000”. Flat Face: Simple flat rectanglular shape
to piston intake valve pocket clearance problems,
back cut rings allow the rings to be moved up Taper Face: Describes the angled face of the
toward the top of the piston which improves second compression ring that scrapes excess
combustion efficency and provides more power. oil from the cylinder wall surface. Used only
on second rings.
Positive Twist: An asymmetric change in the
ring cross section that causes it to twist in an Napier: A special hooked shape found on
upward direction (towards the piston crown the underside of some second compression
aiding ring sealing of the top and bottom of the rings used to more efficiently remove excess
ring groove. Positive twist is used only on top oil from the cylinder walls.
compression rings.
Piston Crown
Reverse Twist: An asymmetric change in the
ring cross section causing the ring to twist Top Ring Groove
downward (towards the piston skirt) that
enhances the second compression ring’s oil Second Ring Groove
scraping properties.
Oil Ring Groove
Neutral: A term used to describe a piston ring
that has no torsional bias or twist. Piston Skirt

Wrist Pin Bore

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