Piston FAQ

Piston Anatomy

A piston is a cylindrical engine component that slides back and forth in the cylinder bore by forces produced during the combustion process. The piston acts as a movable end of the combustion chamber. The stationary end of the combustion chamber is the cylinder head. Pistons are commonly made of a cast aluminum alloy for excellent and lightweight thermal conductivity. Thermal conductivity is the ability of a material to conduct and transfer heat. Aluminum expands when heated and proper clearance must be provided to maintain free piston movement in the cylinder bore. Insufficient clearance can cause the piston to seize in the cylinder. Excessive clearance can cause a loss of compression and an increase in piston noise.

Not a Subaru piston, pictures are for representation purposes only

Piston features include the piston head, piston pin bore, piston pin, skirt, ring grooves, ring lands, and piston rings. The piston head is the top surface (closest to the cylinder head) of the piston which is subjected to tremendous forces and heat during normal engine operation.

Not a Subaru piston, pictures are for representation purposes only

A piston pin bore is a through hole in the side of the piston perpendicular to piston travel that receives the piston pin. A piston pin is a hollow shaft that connects the small end of the connecting rod to the piston. The skirt of a piston is the portion of the piston closest to the crankshaft that helps align the piston as it moves in the cylinder bore. Some skirts have profiles cut into them to reduce piston mass and to provide clearance for the rotating crankshaft counterweights.

Not a Subaru piston, pictures are for representation purposes only

A ring groove is a recessed area located around the perimeter of the piston that is used to retain a piston ring. Ring lands are the two parallel surfaces of the ring groove which function as the sealing surface for the piston ring. A piston ring is an expandable split ring used to provide a seal between the piston and the cylinder wall. Piston rings are commonly made from cast iron. Cast iron retains the integrity of its original shape under heat, load, and other dynamic forces. Piston rings seal the combustion chamber, conduct heat from the piston to the cylinder wall, and return oil to the crankcase.

Not a Subaru piston, pictures are for representation purposes only

Piston rings commonly used include the compression ring, wiper ring, and oil ring. A compression ring is the piston ring located in the ring groove closest to the piston head. The compression ring seals the combustion chamber from any leakage during the combustion process. When the air-fuel mixture is ignited, pressure from combustion gases is applied to the piston head, forcing the piston toward the crankshaft. The pressurized gases travel through the gap between the cylinder wall and the piston and into the piston ring groove. Combustion gas pressure forces the piston ring against the cylinder wall to form a seal. Pressure applied to the piston ring is approximately proportional to the combustion gas pressure.

A wiper ring is the piston ring with a tapered face located in the ring groove between the compression ring and the oil ring. The wiper ring is used to further seal the combustion chamber and to wipe the cylinder wall clean of excess oil. Combustion gases that pass by the compression ring are stopped by the wiper ring.

An oil ring is the piston ring located in the ring groove closest to the crankcase. The oil ring is used to lubricate the cylinder wall during piston movement. Excess oil is returned through ring openings to the oil reservoir in the engine block.

Piston Manufacturing Techniques

Pistons are manufactured via a cast or forged technique. Some consider hypereutectic pistons to be the “third manufacturing technique”, but as they are actually a cast piston with physical properties that fall between cast and forged pistons due to their unique aluminum alloy.

Cast pistons are made by pouring melted aluminum into a mold that shapes the metal into a piston.

Forged pistons are formed using a giant press that takes a block of metal and pounds it into shape under thousands of tons of pressure. The tooling needed to do this is much more expensive than the tooling used to make a casting, and it wears out quicker. This makes forged pistons more costly. Forged pistons have inherent advantages in terms of density, ultimate strength, and durability. Forging eliminates metal porosity, improves ductility, and generally allows the piston to run cooler than a cast unit. Within reason, forgings can be lightened without adversely affecting structural integrity. However, forged pistons expand and contract more under changing temperatures, so they traditionally require greater piston-to-wall clearance than cast pistons. The manufacturing technique produces a metal slug, which is then CNC milled to produce the final piston shape.

Piston Alloys

With regard to cast piston, they generally use aluminum alloys doped with silicon. Aluminum silicon alloys used fall into three major categories: eutectic, hypoeutectic, and hypereutectic. Probably the easiest way to describe these categories is to use the analogy of sugar added to a glass of iced tea. When sugar is added and stirred into the iced tea it dissolves and becomes inseparable from the iced tea. If sugar is continuously added, the tea actually becomes saturated with sugar and no matter how much you stir, the excess sugar will not mix in and simply falls to the bottom of the glass in crystal form.

Silicon additions to aluminum are very similar to the sugar addition to the iced tea. Silicon can be added and dissolved into aluminum so it, too, becomes inseparable from the aluminum. If these additions continue, the aluminum will eventually become saturated with silicon. Silicon added above this saturation point will precipitate out in the form of hard, primary silicon particles similar to the excess sugar in the iced tea.

This point of saturation in aluminum is known as the eutectic and occurs when the silicon level reaches 12%. Aluminum with silicon levels below 12% are known as hypoeutectic (the silicon is dissolved into the aluminum matrix). Aluminum with silicon levels above 12% are known as hypereutectic (aluminum with 16% silicon has 12% dissolved silicon and 4% shows up as primary silicon crystals).

Pistons produced from these alloy categories each have their own characteristics. Hypoeutectic pistons usually have about 9% silicon. This alloy has been the industry standard for many years but is being phased out in favor of eutectic and hypereutectic versions. Most eutectic pistons range from 11% to 12% silicon.

Eutectic alloys exhibit good strength and are economical to produce. Hypereutectic pistons have silicon content above 12%. In addition to greater strength, scuff, and seizure resistance, the hypereutectic will improve groove wear and resist cracking in the crown area where operating temperatures are severe.

It is the primary silicon that gives the hypereutectic it’s thermal and wear characteristics. The primary silicon acts as small insulators keeping the heat in the combustion chamber and prevents heat transfer, thus allowing the rest of the piston to run cooler. Hypereutectic aluminum has 15% less thermal expansion than conventional piston alloys.

A. Graham Bell, in his book “Forced Induction Performance Tuning” (published in 2002 by Haynes), says hypereutectic pistons are a poor choice for turbocharged engines. Hypereutectic cast pistons have twice as much silicon in the aluminum alloy as regular cast pistons (15-20% instead of only 7-8%). According to Bell, the added silicon leaves them “quite brittle and, as such, prone to breaking when subjected to detonation.”

Forged pistons, barring unique manufacturer’s specifications, generally use two aluminum alloys, which are 4032 and 2618. Typical recommended applications are as follows: 4032 is a durable and lighter material usually used in naturally aspirated engines. 2618 Alloy is designed for the rigors of blown, marine, and nitrous applications.

4032 pistons will have quieter cold start operations due to their tighter piston to wall clearances compared to 2618 pistons. This is due to the 15% greater thermal expansion seen in the 2618 alloy. 15% may seem like a lot, but do the math. Considering a piston to bore clearance of 2/1000’s of an inch, 15% is only .0003″. Once the pistons have reached their operating temperature, the noise (piston slap) differences should be nearly identical in volume between the two alloys. 4032 pistons will have reduced oil consumption and longer ring life compared to their 2618 cousins due to their better cold start tolerances. While to many these physical comparisons point towards 4032, you must understand that 2618 pistons, for their slight “defects”, are clearly superior in terms of tensile strength and fatigue endurance to 4032. This is why most piston manufacturers specify the 2618 alloy for use in Subaru (turbocharged) pistons.

A wonderfully informative thread about piston expansion by be read via this link.

Piston Manufacturers


For the record, Cobb Tuning’s pistons start life as JE Piston forgings that are wholly CNC machined by Cobb Tuning. This is a common practice though, as many “manufacturers” use other manufacturers’ forgings as a 2000 ton forge costs millions whereas a CNC machine may be within their capital reach. In fact, JE Piston themselves are rumored to use TRW forgings. Additionally, you may find XXX’s pistons are actually pistons from one of the above manufacturers made to XXX’s specifications. This list represents true manufacturers and not resellers. Also realize these are the companies that stock Subaru specific pistons. As long as you have the correct measurements, almost any aftermarket piston manufacturer can custom make pistons to your specifications.

Piston Coatings

Dry film lubricants, also known as solid film lubricants, provide a lubricating film that reduces friction, inhibits galling and seizing, reduces piston scuffing, extends cylinder bore life, and in some instances can aid in dispersing heat. One of the obvious reasons for using a lubricating coating is to reduce friction, which improves wear, extends part life, and frees up power normally lost due to friction. A second major benefit is a reduction in part temperature. As well, no machining is 100% perfect, so the coating will wear and make up for very slight differences decreasing blow by.

Most dry film lubricants are Molybdenum Disulfide based. Why not Teflon? PTFE, also known as Teflon, is listed as having the lowest coefficient of friction (COE). However, under high speed and load, the COE of PTFE degrades while that of MOS2 (Molybdenum Disulfide) improves, until it is significantly better than PTFE. Moly also attracts oil, keeping an adequate film on the part unlike PTFE, which sheds oil.

Dry film lubricants are primarily applied to piston skirts.

Thermal barrier coatings are designed to reduce the transfer of heat. Thermal barrier coatings generally consist of a ceramic material. They are primarily applied to piston crowns. Coating the piston’s crown will cause heat reflectivity, driving a percentage of any detonation energy back into the fuel burn zone, to increase fuel burn efficiency. It will also lower carbon buildup, which reduces detonation quality, as it builds up on the piston’s crown and increases the risk of detonation damage to the piston crown surface. By retaining minimal heat on the surface of the piston, less heat is transferred to the incoming fuel mixture, leading to a reduction in pre-ignition which leads to detonation. This type of coating also reduces oil temperature. It also provides a good safety margin in case you get a sudden rise in EGTs from a bad tune or a bad tank of gas.

Thermal barrier coatings are primarily applied to piston crowns.

Thermal dispersants are capable of transferring heat faster than the bare metal surface alone. This is particularly important to pistons to prevent hot spots on the piston face. Depending on the coating manufacturer, thermal dispersant properties are combined into a thermal barrier coating or a dry film lubricant. The coatings can also allow heat at the surface to move more evenly over the surface reducing hot spots. They also reflect heat into the combustion chamber for more even distribution of heat, allowing more efficient combustion of the fuel. This allows more of the fuel molecules to be oxidized, which in turn, means less fuel is needed for optimum power.

Thermal dispersants are primarily applied to piston crowns.

Oil shedding coatings are designed to increase cooling efficiency by not allowing oil to coat certain surfaces where it may heat up. By continually shedding and replenishing the oil supply to treated surfaces, this will ensure maximum thermal conductivity. Heat transfers most rapidly when there is a large difference in temperature. The longer oil clings to a hot surface the hotter the oil becomes. By shedding the cooling oil more rapidly, cooler oil is splashed over the surface more frequently.

Oil shedding coatings are primarily applied to piston bottoms.

While there are numerous coatings manufacturers, the three largest are Swain Tech Coatings, High Performance Coatings (HPC), and Polymer Dynamics, also known as Poly Dyn.

The other method of piston treatment is cryogenic treatment. This is a method where material is slowly brought down to well below zero and then slowly brought back up. There are a few trains of thought on this subject and visiting this link will provide loads of technical information on the pros and cons of this treatment option.

The various treatments of finished pistons have been discussed here singly. With respect to coatings, they are widely accepted as being well worth the additional time and expense. In fact, many piston manufacturers will automatically treat their pistons with various coatings prior to shipping. For untreated pistons or personnel ordering custom pistons, these options are listed to build up your knowledge base so that you might speak intelligently with your piston manufacturer about coatings. Generally speaking though, each manufacturer has their own coating formula and their advice/experience should always be trusted. With respect to cryogenic treatment, this is almost always an aftermarket treatment option for those with interest in the benefits and additional lead time and cost of this treatment method.

When ordering custom or replacement pistons with aftermarket coatings on the piston skirt area, keep in mind that the bore size may need to be opened to accommodate the extra thickness. Generally, the piston manufacturer will automatically compensate for coating thickness when using their in-house coatings, but it never hurts to double check. As well, some aftermarket coating manufacturers use such a thin coating that this is not a consideration, but again, verify coating thickness and discuss this with your piston manufacturer and engine builder.

What are the anti-detonation grooves on the top ring land for?

The Anti-detonation grooves prevent carbon-buildup from locking up the top ring. They also help keep the air and fuel in suspension. These grooves knock the peaks off shock waves within the cylinder, reducing the propensity to detonate. They also temper pressure spikes and enhance piston ring life. The presence of this feature depends on your piston manufacturer as some have them, some don’t.

What are the pressure grooves or equalization channel on the 2nd ring land for?

The accumulation of gases that get by the top ring can unseat it. A pressure groove delays this action. This is why today’s recommendation is to keep the 2nd ring end-gap as large as or larger than the top ring end-gap. It is a shaped channel that works by equalizing the pressure seen between the top and second ring with the pressure in the combustion chamber. This feature enhances ring seal, improving power, and engine life and fuel economy. The presence of this feature depends on your piston manufacturer as some have them, some don’t.

What do piston manufacturers say about the anti-detonation grooves and pressure groves/equalization channel?

Every aftermarket Subaru piston manufacturer was emailed about these two items and here are the only two replies:

The equalization channel in the second groove, acts as a reservoir for bypassing gasses of the top ring. It helps to keep the top ring seated.

As far as anti deto grooves. We offer them on custom pistons, don’t necessarily find them essential. They seem to collect carbon easily.

Ian Akiyama
Ross Racing Pistons

The pressure groove on the 2nd ring land helps to equalize the cylinder pressure between the top and the 2nd rings, allowing both to seal better.

The anti-detonation grooves on the top ring land are used to help reduce the friction between the top ring land and the cylinder wall; they don’t help preventing detonation if that is what you were thinking.

Sales 4
Arias Pistons

How important is the boring process?

This depends on the condition of the original bore. No aftermarket shop does as good a job on boring the cylinders as Subaru. That being said, if the block is new, this will be the best bore you can get. If you think there is a need to size the pistons to the bore, do it as Subaru does and make the pistons to fit the bore size. This will be necessary when using a cast OEM style piston. The clearances on a forged piston are so large that the difference between the “A” and “B” bore size is a very small percentage of the total. You can do the math on the difference between an “A” bore and a “B” bore. Take that number and do the math on the percentage of the difference, it will have on the piston to cylinder clearance on a give piston manufacturer. You will find that a forged piston has such a large clearance; the percentage of change will be nil from an “A” to a “B” bore.

On the other hand, if your block is used, you should have it bored to create a round and non-tapered bore for the new pistons to live in. At this point, you are at the mercy of the machine shop’s equipment and skill to supply you with a bore job that will give you the results you are after.

How many hours/miles break in period?

The break-in period is for the rings. The generally recommended break-in period is 1000 miles.

Synthetic or regular oil?

Regular for the duration of the break-in period, then the owner is free to use any type they desire.

First oil change should be performed after how many hours/miles?

After 1000 miles and before the dyno tune.

Oil weight for before and after break in period?


More frequent oil changes with new pistons?

Other than the break-in period, no.

Is there a benefit to using forged pistons on a stock or mildly modded engine?

Forged pistons are an insurance policy on a mildly modded engine. On a stock motor, aside from the insurance, there would be no performance (HP) gain by switching, and in fact, you might see some minor power losses.

Ring gap, what should it be for each ring? Types of rings? Gapless rings?

The correct ring gap varies with ring material, piston design, cylinder wall material used, overall engine use/goal, etc. It is ultimately down to each engine builder to understand their combinations and then to listen to how the customer will be using the motor and build accordingly. As with most things, too much and too little are both very bad.

What is piston slap?

The piston does not go “up and down” in the cylinder in a “nice and controlled manner” as some people envision. It will “bounce” against the thrust areas of the cylinder walls. The piston skirts ride on an oil film to keep them from actually coming into full contact with those sections of the cylinder wall. The noise you hear referred to as piston slap is when this contact is more harsh than normal. This can occur for many reasons, some as simple as piston clearance to piston skirt design, piston pin offsets, oil control, improper connecting rod angles, etc.

For some engine applications the design and clearances are required to be such that some degree of piston slapping will occur during cold engine temperatures. This again will vary with engine design and application.

What is the role of piston dome shape? Pros and cons of flat/dome/inverted dome?

Not Subaru pistons, pictures are for representation purposes only

Flat top pistons are often the most ideal as they offer the tightest quench area (squish).
The problem with flat top pistons:

a. They give too much compression for our forced induction Subarus.
b. They don’t give enough compression for all out High Compression N/A Cars.

Look at it as a sliding scale, and you are trying to find that sweet happy medium.

Dished Piston: Forced Induction
Flat Top Piston: Street N/A, depending on engine design, some forced induction VERY high octane builds
Domed Piston: All out N/A Monster

One unique aspect that Cobb Tuning pistons utilize is they take advantage of both spectrums. Their in-house designed pistons are designed to match the shape of the cylinder head chamber (reversed chamber piston). As a result, they get their exact target compression, and get a tight quench (squish) area which gives us the best burn properties versus a standard dished piston.

N/A engines need more compression to gain power, that is why they typically use a domed piston. Forced Induction engines need lower compression pistons so that when you add the pressure the turbo/supercharger produces your cylinder pressures are not too high.

Great thread on piston shape with pics.

What are the horsepower limits of the WRX and STi stock aluminum alloy pistons with a good tune?

Look at the limiting factor as being a heat/cylinder pressure limit, not power. Stock pistons fail not due solely to tuning but simply because the factory piston design was not capable of handling the thermal loads placed on it. You don’t NEED detonation or even a lean A/F mixture to create enough cylinder pressure (and thus temperature) to cause an edge/crown failure.

Is it true forged pistons need extra clearance with cylinder wall due to their expansion during heavy use?

Yes, it can be about double that of the OEM clearances.

Ring wear/oil consumption of 2618 vs. 4032 alloy pistons?

Due to the increased cold start piston slap, rings on a 2618 alloy piston will wear faster than those on a 4032 alloy piston. The amount of proportional wear is relatively small though and should not be used as a basis for determining suitability of one piston alloy vs. another though. In the same line of reasoning, 2618 alloy pistons also have slightly more oil blow-by while cold vs. a 4032 alloy piston. Once up to operating temperature, oil use is nearly identical between the two piston alloys.

Noise/piston slap of 2618 vs. 4032 alloy pistons?

The noise usually does not come from the material but from excessive piston to sidewall clearance. The alloy will dictate your required piston clearances which can affect piston slap noises. So, it not necessarily the alloy but how the alloy effects your build specifications. Generally speaking though, during cold starts, 2618 alloy pistons will create more initial noise. Once warmed up though, both alloys will sound about the same if doing a side by side comparison.

Compression ratio: Higher or lower? Benefits of each?

When deciding on a compression ratio, people often base their judgments versus the stock OEM pistons’ compression. The most common mistake seen in the industry is going too far in one direction. This can be good and can be bad. Remember, engines of today are designed around emission laws, fuel economy, and the OEM fuel system. Generally, when you utilize a built engine, you increase injector size and go with a larger fuel pump, so this puts your engine on a different level. You are now looking for performance, and not so much at fuel economy.

Remember the Subaru market is lucky in that so many ranges of turbos have been tested and we have a base to work from to get a closely sized turbo for the application.
Generally, if you are building an engine, you would put into mind what it is used for, the amount of air it flows, and what cams to use (which aid in airflow and power band). Put all of this together and come up with a design that will suite your torque and horsepower goals; then you would choose a turbo to match (bore size, compression, piston top design/shape, valve size, etc…the whole build).

The reason this is brought up is if you are building a project that is far from ordinary, this would be the plan of attack to yield the best results.

The “system” approach is very important. People are often too hung up on a specific (Static) Compression Ratio without looking at how the system as a whole will interact.

The other issue, often missed in the ’street tuner market’, is the concept of Dynamic Compression Ratio. This is your “real” Compression Ratio where you take things like camshaft profile, bore and stroke, rod lengths/ratios, altitude, inlet boost pressures, and exhaust gas backpressures into consideration. The Static Compression Ratio that works on your buddy’s EVO motor, or even someone’s EJ20 is all but meaningless unless your entire SYSTEM is the same.

Will oversize pistons make my 2.5L engine turn into 2.7L (or whatever?)?

Using a stock 79mm stroke crankshaft for every .5mm you enlarge the bore you gain approximately .03 liters.

What about piston and pin weight information?

NASIOC user modaddict has compiled a list that can be found on this thread.

Editors Note

My thanks to Quirt Crawford of Crawford Performance, and Trey Cobb, CEO and Jeremy Anderson, Lead Engine Builder, both of COBB Tuning all of whom provided critical information that was essential in the formulation of this FAQ. While I sometimes rely on professionals to “tidy up” my FAQs, I really leaned on these gentlemen this time, and the amount of support I received from each of them borders on awe inspiring.

This post was created because I wasn’t able to find a good piston FAQ. I came up with the text based on LOTS of searching here. Upon reading this you should have an idea of which pistons best suit your needs. The manufacturer is up to you.

If you find an error in this FAQ, please PM me with factual details and I will update this post. Responses such as, “I have XXX’s pistons and they are great!” or “XXX’s pistons broke after 1 month” are not appreciated here, that is what the Car Parts Review Forum is for.

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