JE Pistons On Piston Design Basics - CHP Insider

Sean Crawford, Stephen Golya, And Alan Stevenson Of JE Pistons Break Down The Basics Of Piston Design

Stephen Kim Sep 1, 2009 0 Comment(s)

Oil Control
The benefit of maximizing connecting rod length is debatable, but the practice persists nonetheless. While long-rod motors push the wristpin into the rink package, JE says that it isn't as detrimental to oil control as some people claim. "With the use of an oil rail support, there should be no problems maintaining oil control when the pin bore intersects the oil ring groove," says Crawford. "In fact, the extra openings can actually help the oil drain properly. Some piston designs allow the pin bore to intersect the third ring land, which can cause oil control problems since oil is fed above the oil control ring. For this reason, all of our pistons are designed with the pin bore below the third ring land to prevent this issue."

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Some people tend to overlook the role of the wristpins in the overall piston equation. However, since they endure such heavy loads, they're a critical link in the rotating assembly. The wristpin sees the loading that each piston puts upon its respective crankshaft big end journal, a loading that includes inertial forces as well as the effect of combustion pressure.

The pin is loaded both by the rod and by the piston in a complex combination of forces varying both in magnitude and direction, the result of the applied and reactive loads. The loading on the pin promotes bending along its axis and also ovalization, and the combination of both can lead to frictional binding and twist.

Pin stiffness is a vital consideration. It not only impacts its ability to function as a journal; it also influences the stiffness of the entire piston and pin assembly. Increased pin stiffness can actually translate into a more stable ring platform, resulting in improved oil control and reduced blow-by.

A fully floating pin is able to rotate about its main axis and slide along that axis. Stock pins are often an interference fit in the small end. An interference fit means there must be heating of the small end each time the pin needs to be removed, which is not very practical for a race engine that is frequently rebuilt. Where there is an interference fit, the pin material upon which bearing and bending loads are imposed is subject to greater fatigue effects because it is always the same fibers being cyclically loaded and unloaded. Conversely, with a floating pin the fatigue cycles are more evenly spread around the outer surface fibers of the pin. In addition, if the pin is allowed to rotate, its velocity relative to the individual bearing surfaces will be lower. Rotation also has the effect of moving the oil around within the pin bores, reducing the possibility of dry spot formation.--Alan Stevenson

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Gas Porting
Gas ports are small holes that feed cylinder pressure into the top ring groove. Their purpose is to allow pressure from behind the top ring to increase the sealing effect. Without gas ports, the top ring seals itself primarily from the pressure acting on its top face. Gas ports are usually needed in engines with high cylinder pressure or in conjunction with very narrow top rings. JE offer two types of gas ports, vertical and lateral. Vertical gas ports are most popular in drag race application where maximum pressure behind the top ring is desired. Lateral gas ports provide slightly less pressure on the ring and are more desirable in endurance applications. Both styles of gas ports significantly reduce ring life and are not recommended for street use. In addition to gas ports, JE also offers "gas distribution grooves." This is a small groove that intersects the entire upper half of the top ring groove and helps evenly distribute pressure around the circumference of the top ring.--Stephen Goyla


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