By Titus Bloom:
Gibtec, makers of custom billet competition pistons reveals…
Below their ring lands, pistons are neither round nor do they have parallel sides. Instead they have a larger diameter towards the bottom, between the base of the piston and the piston pin hole. They become progressively smaller toward the top of the piston, which is round because, to achieve proper sealing, round rings operate in round cylinders . The smaller diameter piston crown allows for greater expansion in a region closest to the heat source.
Ovality, which means out of roundness, is necessary in all pistons to allow for thermal expansion. It also compensates for deflection of the piston skirt caused by side loads. Pistons are manufactured with single, double or triple ovality and the ovality is best measured by a precision roundness form measuring machine.
Ovality in simple terms means the piston is narrower along the piston pin axis (the minor axis) than the thrust axis (the major axis). Because ovality is a function of heat, and therefore the growth the piston is likely to encounter, it is the amount of ovality assigned to a piston that’s the clever part. Ovality values differ, depending upon whether the engine is naturally aspirated, supercharged, turbocharged, or charged with nitrous oxide. Obviously, heat, cylinder pressures and side loadings are much less in the naturally aspirated engine and hence less piston ovality is required.
Correct ovality usually manifests itself in the form of a bearing area. This is the area in the middle of the piston skirts that rides in the cylinder and it’s surrounded by a boundary of piston skirt that should not touch the cylinder wall. If the boundary area does make contact it creates friction. A similar situation exists higher on the piston at the ring belt, which is the thicker area that accommodates the piston rings.
Nick Plantus’ life has been a series of bright ideas, mainly in racing piston design. A career that stretches from 1968, when he co-founded Diamond Racing, today he’s the design engineer at Gibtec Pistons in Denver, carefully considering the myriad shapes of today’s racing pistons. “The design and production of single sets of custom billet pistons is what we do well,” says Plantus.
“In the late 1960’s, Diamond was at the cutting edge,” he recalls. “And in those days, forty-seven years ago, our best piston shape was the tapered skirt, then the highest technology our machines could produce.
“With regard to materials, 2618 is still in widespread use. For some applications, though, like NASCAR and some road racing cars where fatigue life is an issue, we recommend a blueprinted 2618A.
“We used to do a ton of 4032 but stopped using it except in some street applications. It is an alloy with good wearing properties, but as engine speeds and pressures increased we found it unreliable in engines producing over 600hp.”
To questions relating to ring grooves, he replies, “They last longer today because ring manufacturers achieve better surface finishes on the sides of the rings. Also piston makers can now not only achieve flatter ring grooves than before but also much better surface finishes than 10 years ago. With today’s thin rings most Sportsman Drag racers don’t run an engine longer than a season.”
On the topic of conventional full-round piston designs, Plantus will tell you that they produce them daily, but unlike inboard racing pistons they contain a lot of unproductive weight without any additional stability.
Plantus like Rob Giebas, head of Gibtec, are intelligent and straightforward men. With twelve years of Pro Stock championship-winning piston-making in their background they will withhold certain information and if you challenge them they’ll confess to withholding it because they have to protect the interest of the teams.