Winter has arrived, and for many of us, now is a great time to freshen up those top-ends and replace the heart of those hard-working thumper engines – the piston. The piston is critical to your engine’s performance, as it seals and transfers the power created in the burner of the internal combustion engine. The air fuel mixture enters the cylinder; the piston and rings compress the charge; and the charge ignites and accelerates the piston down the stroke, transferring the power to the crank, through the transmission, then ultimately to the rear axle. If the piston doesn’t do its job properly, valuable power and efficiencies can be lost.
To some, a piston is a round aluminum part that simply fills a hole in your engine, moving up and down in daily service. To others, a piston is a complex component, made with space-age alloy, machined to a high tolerance to seal combustion, transfer power efficiently, and operate correctly.
Modern day four-strokes, such as 250Fs and 450Fs, are high-RPM, state-of-the-art, single-cylinder Formula 1 engines. They reach RPMs higher than 13,500 and combustion temperatures over 600 degrees. The piston lives at the heart of this highly stressed power plant. When something goes wrong at 13,500 RPMs, costly repairs generally follow. Four-strokes have developed a reputation of being expensive to own and maintain compared to two-strokes, and of course this is true in many ways. There are more moving parts in a thumper engine, which means more parts to wear and go out of adjustment. However, four-strokes don’t have to cost $2,000 to repair every time, if you stay ahead of the wear and adjustments, and invest in cheap insurance – a fresh top end.
When you jump on the Internet or open a parts catalog, you will find endless piston options—from standard bore to big bore, standard to high-compression, cast, forged, OEM, and performance aftermarket, skirt coated, etc. The engine you’re rebuilding and the features you desire in a piston dictate what options are available and the respective price points. And regardless of what you choose, you typically get what you pay for.
Original Equipment Manufacturers (OEMs) design pistons for specific applications and design parameters. For example: pistons for certain utility applications are built to handle high loads, stress, heat, and wear—not necessarily performance or with weight in mind. On the other hand, high-RPM, high-output race engines are built with lightweight, high-performance pistons, designed to last a specific service life.
OEMs can get away with using cast pistons, because they can be made with detailed internal shapes and high silicon content, and are inexpensive to manufacture in high quantities. The down side is they are purpose built for specific motors, sizes, and applications, and when modifications are made to an engine, the OEM piston may no longer be suitable or combatable with the application, or the output created with other aftermarket parts. As soon as you bolt on a pipe or cams, the original design parameters on the piston have changed, and the OEM piston could quickly become a weak link.
While OEM pistons are usually the least expensive option as a replacement, they also offer the fewest options and are lowest in terms of quality and performance. If you run a stock motor, a stock replacement piston is an economical option for rebuilding the top end.
Aftermarket piston manufactures offer more options for replacement pistons, many of which come with performance advantages. For starters, high-end aftermarket pistons are made from forgings, not castings. Forged pistons are forged into shape with 2000 tons of force, which compresses the grain in the aluminum, aligns the grain to the features of the part, and adds strength, toughness, and resistance to impact and fatigue. Wiseco Piston in Mentor, Ohio forges all their own pistons in-house and has a massive selection of raw forgings for each new application. Furthermore, they build all their own forge tooling and regularly release brand new dedicated forging designs to optimize each new piston application. This allows Wiseco to offer the best forging design to minimize weight, maximize strength, and produce the highest quality forged pistons in the aftermarket.
Conversely, cast pistons start as liquid metal that is poured into molds. The aluminum solidifies, taking the shape of the mold. However, the internal grain pattern is random and without direction. Certain casting alloys contain high levels of silicon, which is good for wear resistance, but they can also be brittle and porous, and contain inclusions. To a certain degree, castings are strong; but like a piece of fine china, they shatter catastrophically when they break.
Forged piston’s strength and toughness make them stronger than castings, and far superior in ductility and durability. Top racers and engine-builders choose forged pistons, because they know cast pistons cannot handle the abuse of high-output engines.
Another option to consider is stock versus high-compression pistons. The compression ratio is the ratio of volume above the piston at bottom dead center (piston all they way at the bottom of the stroke) to the volume above the piston at top dead center (piston all the way at the top of the stroke in the cylinder). If there is 10 times the volume of air above the piston at bottom dead center versus above the piston at top dead center, the compression ration is 10:1. Compression ratio is important because the more the air/fuel mixture is compressed prior to ignition, the bigger the “bang” during ignition. High-compression pistons produce more torque and power at the low-mid range of the power band. This makes high-compression pistons advantageous since many of us spend most of our time riding in that portion of the power band. Engines built specifically to rev-to-the-moon are not generally high-compression, because the extra compression is more of an advantage at low-mid RPM ranges.
There are two questions that seem to surround high-compression pistons: Will the high-compression hurt my engine? and Do I have to run race gas? Generally speaking, a .5 or 1-point higher compression ratio than stock will not shave years off the life of your engine. If the rod and crank are in good shape and you maintain the overall condition of your bike with frequent oil and air filter changes, the extra compression will not have any adverse affects on the life your engine. That said, high-compression pistons usually go along with riders/engine-builders who take a devoted interest in the condition of their engine and who are very aware of the recommended service intervals of their engines.
Race gas is a generic term that refers to various fuels available, but it always comes up when discussing compression ratios. The question you should ask regarding race gas is “Do I need to run high-octane fuel with my high-compression piston?” High-compression engines can lead to pre-ignition or detonation if run too lean on the jetting or when using too low of a fuel octane rating (like 87 or 89 Pump Octane Number). Commonly available high-compression pistons, such as a 12.5:1 or 13.5:1 piston, run fine on pump 93-octane fuel when jetted properly. The problem is oftentimes, riders/engine-builders make other modifications to an engine in conjunction with a high-compression piston, such as race cams or a pipe. Subsequently, the combination of all the high-performance modifications can make the engine too high strung for pump 93-octane fuel. However, if you simply replace a stock compression piston with a 12:5 or 13.5:1 high-compression piston, they run very well on pump 93-octane fuel.
Some final features to consider when piston shopping are oversizes and skirt coatings. Modern-day engines are nickel-silicon-carbide (Nikasil) plated, so they don’t wear nearly as fast as the cast iron bores of years past. In your old cast iron bore from the 80s or 90s, you could freshen up the top end by boring and honing to a .25mm or .5mm oversize. This is not possible with Nikasil-plated bores, as going to any oversize requires the bore to be replated. The beauty of Nikasil is its wear resistance, so you can replace a standard bore piston several times without having any significant wear in the bore. Generally speaking, so long as the bore size and shape stays within the service limits, you can simply replace a standard bore piston with another standard bore piston. Oversizes or big bores are more often used to increase the displacement of your cylinder. We’ve all heard the cliché: “There is no replacement for displacement!” Increasing your 250F to a 290 or your 450 to a 480 is a great way to grab a lot of extra torque and power. Some cylinders can be bored and replated to accommodate the large oversizes, but others must be replaced with a larger aftermarket cylinder to have sufficient strength at the larger bore size.
Skirt coatings are a nice feature, usually available only on aftermarket pistons. Some pistons come with spray-on coatings to assist with break-in and defend against cold seizures. Wiseco offers a permanent skirt coating that is screenprinted on the skirt for exact fit and to maintain a precise coating thickness. The coating assists with break-in and protects against cold seizures, as well as reduces friction and quiets the piston. Reducing friction decreases inefficiencies and improves the power output from the engine.
Changing a piston and freshening up your top end for a few hundred dollars is cheap insurance. Many argue the recommended service life of a piston, despite what the factory service manuals recommend, but no one can dispute that a fresh top end will extend the life of your engine and save you money in the long run. With so many options for which piston to buy, it comes down to a few simple questions to ask yourself: What sort of rider are you?; How much do you want to modify your engine?; How much money do you have to spend?; What features are important to you in a piston?
Piston quality and the number of features—such as oversizes, coatings or compression ratios—are proportional to the price of a piston. Even in a society full of Internet specials, you still get what you pay for. Decide what is right for you—OEM or aftermarket—then invest in cheap insurance to keep your engine running superbly!