Among the shiny objects that emerge from a new box of high-performance pistons, you are also presented with a spec sheet detailing the piston’s critical dimensions and, among other things, the all-important piston-to-cylinder wall clearance. It’s the core specification that engine builders always target to ensure trouble-free operation of the engines they build.
Most end users regard the recommended piston-to-wall clearance as an all-inclusive indicator of the piston’s optimum fitment in the cylinder bore for safe operation.
In the overall sense they are correct, and careful attention to the recommended fit will almost always ward off the dreaded evils of excessive friction, piston slap, ring damage, and attending failures.
The recommended measurement point on the piston is the largest diameter point on the piston, thus it must be fitted with the proper manufacturer’s clearance. Think of it as the safety point manufacturers provide to prevent improper fitment and subsequent engine damage. But there is more to it. When the engine is running at operating temperature, every point on the piston skirt and the ring land area has a specified clearance designed to ensure proper function of the piston and associated ring pack.
In determining the optimum piston-to-wall clearance, designers consider the entire physical and thermal operating environment of the piston by evaluating the following factors and how they inter-relate for each piston design.
- Block Type (material)
- Piston Material (alloy)
- Type, (cast, forged, hypereutectic}
- Piston Size
Different applications present varying requirements. Engine speed, cylinder pressure, skirt loading, rod angularity and other factors all play a role in the designer’s assessment of a piston’s final clearance requirement. Many low-speed production engines still use inexpensive cast pistons with very controlled expansion characteristics. They can be fitted very tightly in the bore and last a long time under normal service. They don’t rattle on startup, which is a major concern to automakers, and they provide smooth, trouble-free operation year after year.
Block Type and Material
Cast iron and aluminum are the predominant materials from which cylinder casings are manufactured. These materials exert considerable influence on piston-to-wall clearance, primarily due to their thermal expansion characteristics. Cast iron blocks expand less than aluminum blocks with cast iron cylinder liners and are thus more thermally stable.
Some blocks incorporate Nikasil-plated aluminum cylinders without liners––these expand even more. In every case, dimensional changes due to heating must be addressed to arrive at a proper piston clearance. And that includes accounting for the thermal characteristics of the piston material as well. Bore distortion due to cylinder head fastener clamping load also influences final piston clearance figures. Depending on the engine and the construction of the block, it is also possible for other fasteners to distort the bore. These might include motor mounts, pumps, bracketry and so on.
Cast pistons with an integral expansion strut were commonplace for many years and they still provide very reliable service in low-power, low-rpm situations. Close to a century ago, the addition of 12 percent silicon as an alloying constituent was found to significantly stabilize the expansion of aluminum components, such as pistons.
Known as eutectic-aluminum-silicon alloy, it permitted the development of cast, high-silicon pistons with up to 20 percent silicon alloy. These are known as hypereutectic pistons and their chief advantage is a very low expansion rate. They can be installed with as little as 0.0005-inch piston-to-wall clearance on the major diameter.
Interestingly, when a modern forged piston with more initial cold clearance reaches operating temperature, the difference in running clearance is less than might be surmised. For example, Wiseco uses 2618 and 4032 alloys for all its forgings. While the expansion rates are different for each alloy, Wiseco has made pistons from each alloy for the same engine operate successfully at nearly the same running clearance. The higher-expanding 2618 piston may have a larger initial clearance than a 4032 piston, but once the engine reaches operating temperature, both pistons will have similar running clearances.
The piston profile plays an important role in determining clearance. Tighter clearances tend to reduce piston slap (clatter) on cold startup and they provide a more stable fit to promote good ring seal.
Pistons with full radius skirts (as opposed to barrel shaped profiles) are claimed to be capable of a tighter fit. The reality is such that the full skirt piston, because it has a full radius profile, is measured at the very bottom and has a much greater clearance everywhere but at the measuring point. This is an example of the overall clearance of the piston differing significantly from the published clearance specification.
Larger pistons typically require more clearance than smaller pistons. A comparison of extremes illustrates the point if we consider the difference in two unrelated pistons used for flying. The thimble sized piston from a Cox .049 model airplane engine operates just fine with such minimal clearance that it does not even require any form of piston ring to seal the combustion gas.
Conversely, a 5.400" diameter piston from the Merlin V-12 engine that powered the P-51 WWII fighter requires .012" to .014" clearance for satisfactory operation. Here, we also note that the thermal loading from friction is far greater in the Merlin engine than in the Cox engine. Under the extreme conditions of airplane racing, massive heat loading occurs in the Merlin engine with the pistons expanding accordingly.
Oil on the cylinder wall adapts to local conditions at operating temperature, but the piston clearance must allow some room for the lubricant film to do its job. The oil film is supplied by splash coming off the rapidly rotating crankshaft. In simplified terms, oil bleeding out of the rod and main bearing side clearances is flung onto the cylinder walls and controlled in a thin film by the oil ring. The oil film can be less than 0.001-inch and is accounted for in the final piston clearance. An oil film must be present to not only lubricate the surfaces, but to transfer heat from the piston to the cylinder block and then the cooling system.
Cooling System Type
Considerable difference exists in clearance requirements for air cooled engines versus liquid cooling. Air cooled applications such as found in Volkswagen or Porsche cars, are essentially non-regulated systems at the mercy of air flow conditions. They are more prone to bore distortion and uneven expansion. Air cooling is more finicky due to broad fluctuations in air flow. In an aircraft engine for example, cooling also diminishes with altitude because the air is thinner and carries away less heat.
Liquid cooling provides greater consistency with an easily regulated system and quicker warm up as a bonus. The cooling medium is transferred in and out as required by thermal conditions and regulation provided by the thermostat. These factors affect the resultant piston-to-cylinder bore clearance.
Engineers go to great lengths to determine proper piston to wall clearance. This includes real world testing on running engines with various skirt configurations and different clearances to determine the suitability of each piston for a particular application. When a design is finalized, the clearance and checking location are specified in the instructional paperwork that accompanies the pistons. If these recommendations are strictly followed you can expect trouble free operation from your new pistons and optimum cylinder sealing for maximum power.