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Getting to Know Pistons

On my farm, I grow fresh market sweet corn, but that doesn’t hinder me from learning about practices employed for other crops. I’ve always believed that anything you can learn has value. An ancillary benefit of new knowledge is that I can apply it to something else in my operation. The goal of this article is to provide you with information so if you ever have to buy a new piston for an engine, you can do it as an educated consumer. 

The anatomy of a piston

Every reciprocating engine uses a piston to transfer the chemical energy from combustion to the mechanical work performed by the crankshaft. The expansion of the fuel and air mixture uses the piston as a platform for cylinder combustion pressure to work against, which then transfers the energy to the crankshaft via the connecting rod. Due to the shape of a piston, engine experts often call it a slug.

The following terms describe the parts of a piston.

Crown. This is the top of the piston that faces the combustion chamber in the cylinder head. The crown can be flat with or without reliefs cut for the valves. This is a flat-top piston. If the crown is recessed, then it’s a dished piston. Depending on the design, the entire crown may be recessed or just the area that mirrors the combustion chamber shape. The converse of a dished piston is a pop-up or domed piston. This style has an arch facing the combustion chamber of the cylinder head. Domed pistons are rarely, if ever, used in ag engines.

Many diesels have a flat deck on the cylinder head. Thus, the entire combustion chamber is in a recessed dish in the piston crown. A gas engine always has the combustion chamber in the cylinder head, but modern designs mirror image a significant percentage of that region in a slightly recessed dish. This is done to improve mixture motion and to increase flame speed.

Ring land. This is the area on the side of the piston from the crown to the top piston ring and then the material between the rings on the piston’s side. The ring land from the top ring to the crown is also identified as a crevice volume. It is a region where the air/fuel mixture will travel to, but the flame will not.

Modern gas engines have the top ring very close to the crown (a thinner ring land) so that the crevice volume is decreased (which is a major contributor to hydrocarbon emissions). Older engines normally had the top ring farther from the crown (increased ring land) to keep the heat of combustion away from the ring for durability. If an engine is detonated (pinged) badly under load, the top ring land will fail and break off, damaging the cylinder.

Ring groove. This is the groove machined into the piston where the rings reside. They are made to a defined dimension, and the ring set you use will need to coincide with that.

Compression height. This is the location of the center of the wrist pin bore in relation to the crown. It will be a dimension such as 1.150 inches. This means the center of the pin bore is 1.150 inches when measured from the crown. The compression height of the piston is a function of the length of the connecting rod, the stroke of the crankshaft, and the deck height of the block (the distance from the crankshaft centerline to where the cylinder head attaches).

Skirt. This is the area below the pin bore to the bottom of the piston that resembles a woman’s skirt when viewed from the side. Its purpose is to guide the piston as it travels in the cylinder bore and to stop it from rocking as the crankshaft spins in its arc of rotation and the thrust load forces it against the cylinder wall. The bottom of the skirt is also where the piston is measured. This dimension determines the cylinder bore sizing. It is called the piston-to-wall clearance.

3 Piston designs

Pistons are made from aluminum and can be considered cast (poured into a mold), forged (pressed into shape), or hypereutectic (aluminum mixed with silicone).

Cast pistons are considered the weakest design but are the least expensive to produce due to a low rate of thermal expansion. Cast pistons are fitted with less clearance to the cylinder wall. This results in less engine noise (especially when the engine is cold) and in lower oil consumption during warm-up. 

Forged pistons are the strongest design but more expensive to produce. These pistons also require more clearance since their rate of expansion is higher. The sound a forged piston makes when cold is called piston slap. Oil consumption during short run cycles with forged pistons is much higher than with cast designs.

Hypereutectic pistons are relatively new (over the past 20 years) and offer a stronger design than a cast piston. Hypereutectic pistons produce less noise and consume less oil vs. forged pistons. This is due to a fitment close to that of a cast-style piston. Some studies have shown that the hypereutectic pistons may be more brittle than either a cast or forged piston, making them a poor candidate to endure a bout of severe detonation without lifting a ring land.

Piston sizes

The industry offers bore sizes of 0.020, 0.030, 0.040, and 0.060 inch. A standard bore is the dimension the engine was originally made to.

If the cylinder is worn or damaged, it will first need to be overbored to a dimension and then finished honed to the final size to use a new piston. If a sleeve is installed in the damaged bore, then a standard size piston can be used.

Custom manufacturers can produce a piston to exact requirements. These pistons will always be a forged style and are many times the price of a mass-produced piston in the commonly offered bore dimensions. 

When rebuilding an engine, your goal is to keep the cylinder wall as thick as possible. Therefore, you will want to choose a piston with the smallest increase in diameter while still producing the desired cylinder bore correction.

Choosing a replacement

The logic I like to apply when buying a piston is to see what method of construction was used by the original manufacturer. If it was a forged design, then I would use that style of piston. When the engine is factory fitted with a cast piston, you can either go back to that design or upgrade to a forged or hypereutectic piston. If the engine will see severe duty for extended periods such as running an irrigation system, then I would move up to a forged piston for strength and not be concerned with cold operation noise or oil consumption.

In contrast, if the engine is going to be cold-started often and then only run for short periods of time and never building up much operating heat (such as a truck used to feed cattle), I would suggest buying as a replacement piston a cast-type design, as it provides a tight fit and lower oil consumption. Bear in mind that when oil goes past the rings (as it can with a loose-fitting piston), it builds carbon on the land and ring grooves, which over time will lead to sticking oil and compression rings.

Hopefully you’ll never need to buy new pistons. If you do, making the proper choice will determine the success of the rebuild. 

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