How To Choose the Best Piston Ring for Your Application

By Mark Houlahan 6/12/ Share Add Article To List

Piston Rings Have a Tough Job, Choosing the Wrong Ones Will Make That Job More Difficult

There has been a lot of advancement in piston ring technology over the last few decades. Piston ring materials, coatings, edge profiles, and even ring thickness have all seen great improvements in oil control, sealing, and wear. Of course, these enhancements in ring technology only work when they are used in the proper manner. A basic street engine built for a cruiser will use a much different ring package than a 1,000 horsepower turbocharged engine. There are many decisions to be made when choosing the right set of piston rings for your engine build. While some piston kits include rings, often the higher you go up the performance ladder the rings become a separate purchase decision.

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There is no one 'best' ring package on the shelf. Determining the engine's use, power level, compression, type of fuel, and of course any power adder, are all factors in choosing the proper ring package. The decision should factor in proper sealing, wear, and durability so that your engine produces maximum power with minimal blowby and proper oil control, all with a ring package that will wear appropriately for the intended use with minimum friction loss. What follows is a breakdown of modern piston ring materials, ring types, coatings, and more that will help you determine what type of piston rings are indeed best for your build. If you're having the short block assembled by an engine builder, then obviously we suggest following the ring package guidelines that they have for your reciprocating assembly.

What Are Piston Rings Made Of?

When it comes to piston ring material types there are a few ring materials no longer used or only used in specialty applications now. Currently the most common piston ring material types for automotive engines are cast iron, ductile iron, and steel. While steel does have the highest tensile strength, don't count out cast iron or ductile iron rings for the right applications. For example, if you're performing a basic 'hone and ring' job to drop back into your daily driver there is no need for the added expense of ductile iron or steel rings.

What Is the Benefit of Different Piston Ring Materials?

• Cast Iron: Fragile piston ring material properties, best used for stock engine builds due to low tensile strength. Low cost, great option for a budget rebuild.
• Ductile Iron: Stronger piston ring material properties with double the tensile strength of gray iron rings. Better option for performance engine builds.
• Steel: Better still in tensile strength and fatigue strength over ductile iron rings. Better option for boosted and nitrous applications. Used in narrow ring width applications for better sealing and less blow-by.

What Are the Different Types of Piston Rings?

Now that we've discussed piston ring material composition it is important to explain how many types of piston ring are commonly used. Modern pistons feature three different types of piston rings. Starting from the top of the piston you have the top compression ring. This is the primary ring that seals the piston to the combustion chamber wall. Below this ring you have the second or intermediate compression ring. This ring backs up the top ring by sealing the combustion chamber while also aiding in heat transfer and scraping oil from the cylinder wall. Finally, you have the oil control ring at the bottom, which has the piston ring function of controlling the amount of oil delivered to the combustion chamber wall for lubrication and cooling. Know that you can have different top and intermediate compression piston ring material selection in various ring packages, such as a ductile iron top ring with a cast iron intermediate compression ring.

Top and Intermediate Compression Ring Types:

• Conventional Ring: Top and 2nd rings with gaps that can be set for various uses (N/A, nitrous, etc.). This style of ring is often file-fit by the engine builder to a specific final specification. See the section on ring gaps below for more details.
• Gapless Top Ring: Provides increased horsepower and crankcase vacuum, used mostly on N/A engine applications to help fill the cylinder due to better ring seal. You want the gapless ring as close to the intake valve as possible. We offer Total Seal Gapless rings for your engine project build.
• Gapless 2nd Ring: Preferred ring for turbo or supercharged applications as well as boxer engines. With a turbo or blower helping to fill the cylinder the gapless 2nd ring is utilized to keep heat and contaminants out of the oil pan. A gapless top ring can be used in boosted applications as well but is certainly more effective in N/A setups.
• Gas Ported Top Ring: Increases horsepower by improving ring seal. The gas ported top ring features lateral gas ports machined into the top of the ring, which adds the benefits of gas porting to any piston. Works for both street and competition engines.

Oil Control Ring Types:

• One-Piece Oil Control Rings: Rarely used today, they are like a compression ring where the tension against the cylinder wall is taken from the ring's cross section. A U-shaped design, the groove in the center moves excess oil back to the crankcase. Available with various ring profiles.
• Two-Piece Oil Control Rings: A coil spring is placed into the oil ring groove of the piston first and a special oil control ring is then placed over the coil spring. The spring provides the tension of the oil ring to the cylinder wall. Available with various ring profiles.
• Three-Piece Oil Control Rings: A pair of support rails with an expander between them for rail tension. The expander pushes the two rails, which act as scrapers, against the cylinder wall to remove engine oil and return it to the crankcase. This is the most used oil control ring design today.

Oil Ring Tension: When ordering piston rings, you often have the option of choosing the type of oil ring tension you desire for your engine build specs. You can choose from standard tension, low tension, and high tension oil ring offerings.

• Standard Tension: Varies by oil ring thickness, so a standard tension 3/16 oil ring is not the same tension as a standard tension 3.0mm oil ring. The thicker the oil ring, the higher the standard tension for that size.
• Low Tension: Also varies by oil ring thickness but does not always drop below the next size down in oil ring. For example, a low tension 3/16 oil ring is 15 lb/ft while a standard tension 3.0mm oil ring is 12 lb/ft. Utilized correctly, a lower tension oil ring increases horsepower and extends cylinder bore life.
• High Tension: Also varies by oil ring thickness, but inversely. For example, a high tension 3.0mm oil ring is 15 lb/ft and a standard tension 3/16 oil ring is 23 lb/ft. High tension oil rings are recommended for boosted and nitrous applications to reduce motor oil related detonation.

What Are the Different Types of Piston Ring Coatings?

Piston ring coatings are applied to the face of the ring (the side of the ring where it contacts the cylinder wall) to improve durability and lower friction. These coatings also provide faster break in. No longer do you have to drive 500 careful miles to break in your piston rings. With modern coatings they can break in quickly and provide a long service life. Ring coatings do affect piston ring price a bit, but we feel the added expense is well worth it for a modern performance engine build.

• Uncoated Cast Iron: Very soft for an easy break-in but doesn't offer good durability.
• Hard Chrome Coating: Very hard for good durability but is very difficult for break-in with lower scuff resistance.
• Plasma Moly Coating: Rings bed in faster with higher scuff resistance, normally use a ductile iron base ring. Not for use with nitrous applications as the moly coating can fracture and break off the face of the ring.
• PVD Coating: Physical Vapor Deposition coatings provide a lower coefficient of friction, better adhesion and increased hardness compared to other coatings. Ideal for boosted and nitrous applications.

Are There Different Types of Piston Ring Profiles and How Are They Installed?

When we talk about piston ring profiles, we are referring to the outer edge of the ring that seals to the combustion chamber wall. Different profiles, or faces, are used for varying reasons, including increased sealing, greater oil control, and more. These ring profiles are often hard to see clearly, which is why all manufacturers mark their rings with a dot or the word 'top' on the ring face so that the ring profile can be installed in the proper direction. This does not mean it is the top ring on the piston, but the orientation of the ring itself. Always install rings with the dot or 'TOP' facing up.

• Square Face: Seals well but has higher wear, eventually wearing to a barrel shape, used on top ring.
• Barrel: Best sealing properties with longer life/lower wear, used on top ring.
• Taper Face: Used on 2nd compression ring, usually 2-4 degree taper of ring face to help scrape oil off cylinder wall.
• Napier: Groove machined under 2nd compression ring to improve oil removal from the cylinder wall.

The top compression ring will usually be a barrel face, while the second ring will often be a taper face or Napier face ring. The reason for the different profiles is to optimize the performance of the ring for the job it must perform.

How Do I Know What Size of Piston Ring I Need?

A piston ring's diameter is directly proportional to the cylinder bore. If an overbore of the cylinder has occurred, then the proper piston ring size (and piston) must be ordered to properly fit. For example, a standard 4.00-inch bore that has been machined .030-inch to remove wear or wall damage will now require both 4.030-inch pistons and rings. A file to fit ring is +.005 over the bore size to allow the fitting of a tighter end gap in performance engines.

What Is the Standard Piston Ring End Gap?

End gap is usually specified by the ring manufacturer, but most fall back on the general rule of thumb of .-inch of ring gap per inch of bore diameter (for example, a 4.00-inch bore naturally aspirated engine would take a .018-inch top ring gap). Second rings are usually gapped at .006-inch per inch of bore. Again, for a naturally aspirated engine. The goal here is to have enough gap that as the rings are exposed to the combustion chamber's heat that the ring end gap provides enough room for ring expansion without the ring ends butting up against each other, which will cause ring scuffing and even breakage. A piston ring end gap filing tool is the proper way to file both ends of the piston ring equally.

Boosted applications require larger ring gaps due to the increased combustion chamber temperatures these engine combinations see. Finally, some ring manufacturers spec the second ring to be gapped between .005-.010 more than the top ring to aid in preventing gas buildup between the top and second rings. Ultimately, we suggest going with the ring manufacturer's specifications, for the ring material you're using and the application. Be sure to watch our video on piston ring gap placement (clocking) for more details on proper ring installation.

Are Thicker Piston Rings Better?

Traditional piston ring sizing has been in fractional inch measurements. You'll typically find top and 2nd rings in 5/64-inch, 1/16-inch, or .043-inch sizes, with oil rings typically in the 3/16-inch size. Modern engines moved to metric ring measurements of 1.5mm to 1.0mm for top and second rings with 3.0 to 2.0mm oil rings. These ring thicknesses have been the norm for decades, but moving to a thinner ring package has shown several advantages. With custom pistons, you'll find types of piston rings as thin as .5mm (.020 inch). The thinner rings provide some great benefits, including increased horsepower and torque while reducing weight and compression height. Significant power gains can be had from utilizing thinner, modern rings and piston designs. While it has been more critical to use the proper piston ring installation pliers on thicker rings, we highly recommend that you use the same tool on thinner rings as well. The only types of piston rings that are OK to be 'spiraled' onto the piston are the oil ring's top and bottom rails. Never spiral the compression rings onto a piston.

As you can see, piston ring materials and piston ring function are just as critical to a successful engine build as the camshaft specs, cylinder head flow, and other major engine building decisions that you must make. We hope this guide has helped you understand what your piston ring options are and what is best for your build. If you have any questions on the types of piston rings your engine build should use, simply give our techs a call for expert assistance or reach out to your engine builder.

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How to choose piston rings

• Quote

Post by frnkeore » Sat Jun 01, 3:17 am

So not to high jack the 5/64 vs 1.5mm ring thread, I thought I'd open this one, to maybe get more details on how and what works, for different applications.

This ring thing, is very confusing.

I think I understand that a thinner ring with have less friction but, there are so dam many thin rings now. This is a list of the ones I know about:
.9mm = .
1mm = .
.043
1.2mm = .
1.5mm = .059
.
.
., still used in some old OEM applications

There are also different materials:
gray iron
ductile
steel
SS

And there are many face contours, as well as plating's, you can add to that gas ported rings, too. That for the upper two rings and then you have lots of options for the oil control rings, in both width and tension.

The first two things I would like info/opinions on is gas ported rings and low tension oil rings. When is gas porting needed and how low can you go with low tension oil rings w/o a vacuum pump?

Re: How to choose piston rings

• Quote

Post by mag » Sat Jun 01, 7:22 am

In terms of gas porting I think the preference is now to use half moon grooved in the top ring land.
There far less likely to load up and clog off, but I guess you need more of them then the round hole type due to the reduction in area. You can cut a man's tongue from his mouth, but that does not mean he's a liar, it just shows that you fear the truth he might speak about you!

Re: How to choose piston rings

• Quote

Post by Lizardracing » Sat Jun 01, 2:07 pm

I choose the piston first, then the rings for the block material, hose to ring manufactures spec. That's per Total Seal instructions to me.

Re: How to choose piston rings

• Quote

Post by frnkeore » Sun Jun 02, 12:19 pm

I was hoping for more input on this subject so that I could have, at least, good, better, best options to choose from but, more than that, I think it's info that most high end engine builders would have strong opinions on and before laying out money for pistons, I have to spec a ring for them.

Also, I was hoping to get some general info on directions to go, that could have a good cost/benefit ratio, if not for me but, for others, in selecting rings for their applications.

In hopes of getting some specific info, for my build and what rings would be best, here is what it is:

4.280 bore x 4.100 stroke
1.550 CH, 18cc dish
aluminum
0.000 deck clearance x .040 head gasket

This engine has 350 cfm heads and is cammed for to rpm.

At the moment this is the ring set I'm considering, unless I can find something better.

Hastings SM 4.28 - .043 Steel, .043 Duc/Nap, 3mm 11 lb Oil

What are your thoughts?

Re: How to choose piston rings

• Quote

Post by Walter R. Malik » Sun Jun 02, 1:46 pm

What and how rings are chosen and used has everything to do with the end usage of the engine to be built.
Using something normally used in a high horsepower, high RPM engine would be totally wrong in a grocery getter rebuild ... and visa-versa.
AND, then there is all those engines built "in between". http://www.rmcompetition.com
Specialty engine building at its finest.

Re: How to choose piston rings

• Quote

Post by frnkeore » Sun Jun 02, 6:30 pm

Walter R. Malik wrote: 'Sun Jun 02, 1:46 pm What and how rings are chosen and used has everything to do with the end usage of the engine to be built.
Using something normally used in a high horsepower, high RPM engine would be totally wrong in a grocery getter rebuild ... and visa-versa.
AND, then there is all those engines built "in between".

So, at what point would you use gas ported pistons, in a NA engine?

Also, in what kind of build would .9 x .9 x 2mm be an advantage over what I'm considering?

How low of oil ring tension, can you use with exhaust evacuated crank case?

Re: How to choose piston rings

• Quote

Post by Walter R. Malik » Sun Jun 02, 6:57 pm

frnkeore wrote: 'Sun Jun 02, 6:30 pm

Walter R. Malik wrote: 'Sun Jun 02, 1:46 pm What and how rings are chosen and used has everything to do with the end usage of the engine to be built.
Using something normally used in a high horsepower, high RPM engine would be totally wrong in a grocery getter rebuild ... and visa-versa.
AND, then there is all those engines built "in between".

So, at what point would you use gas ported pistons, in a NA engine?

Also, in what kind of build would .9 x .9 x 2mm be an advantage over what I'm considering?

How low of oil ring tension, can you use with exhaust evacuated crank case?

I will tell you what I have found but, it is certainly not to reflect the "be all, end all".

Gas ports tend to make the rings wear faster, (the thinner the ring, the faster they wear), but, do give slightly more power to the engine; your choice as to what is most important in THAT engine.
They are almost necessary with .043" thick radial dimension rings; unless you are seeking longevity.
Otherwise, they are usually most power effective for me when using fatter, back-cut rings in shallower grooves.
The best quality of thin rings is their conformability to the bore.

I have run 3/16" oil rings as low as 7 pounds fish scale drag with ATF, leaving a whisper of oil residue in the chamber without a crankcase air pump.

Anything else, I usually do whatever the ring manufacturer wants. http://www.rmcompetition.com
Specialty engine building at its finest.

Re: How to choose piston rings

• Quote

Post by Warp Speed » Mon Jun 03, 10:10 am

Walter R. Malik wrote: 'Sun Jun 02, 6:57 pm

frnkeore wrote: 'Sun Jun 02, 6:30 pm

Walter R. Malik wrote: 'Sun Jun 02, 1:46 pm What and how rings are chosen and used has everything to do with the end usage of the engine to be built.
Using something normally used in a high horsepower, high RPM engine would be totally wrong in a grocery getter rebuild ... and visa-versa.
AND, then there is all those engines built "in between".

So, at what point would you use gas ported pistons, in a NA engine?

Also, in what kind of build would .9 x .9 x 2mm be an advantage over what I'm considering?

How low of oil ring tension, can you use with exhaust evacuated crank case?

I will tell you what I have found but, it is certainly not to reflect the "be all, end all".

Gas ports tend to make the rings wear faster, (the thinner the ring, the faster they wear), but, do give slightly more power to the engine; your choice as to what is most important in THAT engine.
They are almost necessary with .043" thick radial dimension rings; unless you are seeking longevity.
Otherwise, they are usually most power effective for me when using fatter, back-cut rings in shallower grooves.
The best quality of thin rings is their conformability to the bore.

I have run 3/16" oil rings as low as 7 pounds fish scale drag with ATF, leaving a whisper of oil residue in the chamber without a crankcase air pump.

Anything else, I usually do whatever the ring manufacturer wants.

What portion of the ring is wearing faster due to gas ports?

Re: How to choose piston rings

• Quote

Post by Walter R. Malik » Mon Jun 03, 11:00 am

Warp Speed wrote: 'Mon Jun 03, 10:10 am

Walter R. Malik wrote: 'Sun Jun 02, 6:57 pm

frnkeore wrote: 'Sun Jun 02, 6:30 pm
So, at what point would you use gas ported pistons, in a NA engine?

Also, in what kind of build would .9 x .9 x 2mm be an advantage over what I'm considering?

How low of oil ring tension, can you use with exhaust evacuated crank case?

I will tell you what I have found but, it is certainly not to reflect the "be all, end all".

Gas ports tend to make the rings wear faster, (the thinner the ring, the faster they wear), but, do give slightly more power to the engine; your choice as to what is most important in THAT engine.
They are almost necessary with .043" thick radial dimension rings; unless you are seeking longevity.
Otherwise, they are usually most power effective for me when using fatter, back-cut rings in shallower grooves.
The best quality of thin rings is their conformability to the bore.

I have run 3/16" oil rings as low as 7 pounds fish scale drag with ATF, leaving a whisper of oil residue in the chamber without a crankcase air pump.

Anything else, I usually do whatever the ring manufacturer wants.

What portion of the ring is wearing faster due to gas ports?

The face and the bore itself is what I have witnessed.

I am talking about 10,000 street miles instead of 25, 000 or more. http://www.rmcompetition.com
Specialty engine building at its finest.

Re: How to choose piston rings

• Quote

Post by jsgarage » Mon Jun 03, 2:58 pm

For poor boys that want to freshen up an existing combination of still usable pistons with wider grooves instead of buying all-new pistons for some size of new thin rings, there are several companies that now make flat steel spacers to resize wider piston grooves for thin rings, ported or not.

They likely will not work quite as well as pistons specifically cut for a thin ring and it introduces more complication in engine assembly, but they're a cheaper way of trying the concept out, maybe for several different thickness of ring in your engine combo and ascertaining any extra face wear.

Re: How to choose piston rings

• Quote

Post by Walter R. Malik » Mon Jun 03, 4:37 pm

Clicked on the wrong subject. http://www.rmcompetition.com
Specialty engine building at its finest.

Re: How to choose piston rings

• Quote

Post by frnkeore » Mon Jun 03, 5:23 pm

jsgarage wrote: 'Mon Jun 03, 2:58 pm For poor boys that want to freshen up an existing combination of still usable pistons with wider grooves instead of buying all-new pistons for some size of new thin rings, there are several companies that now make flat steel spacers to resize wider piston grooves for thin rings, ported or not.

They likely will not work quite as well as pistons specifically cut for a thin ring and it introduces more complication in engine assembly, but they're a cheaper way of trying the concept out, maybe for several different thickness of ring in your engine combo and ascertaining any extra face wear.

Can you list the company's and/or web sites?

Re: How to choose piston rings

• Quote

Post by Roundybout » Mon Jun 03, 7:31 pm

Want more information on motorcycle piston ring? Feel free to contact us.

I know Total Seal makes a spacer to be able to use thinner rings in pistons with grooves meant for thicker rings. 1.2mm" or .8mm" rings in 5/64 groove or .8mm" rings in 1/16 groove for example.

Not sure how I like that idea long term or as a solution though. Ring flutter and not a true representation of what a thin ring can and can't do for you. Not to mention just more crevices for things to hide and carbon up. Don't know how that would react to piston ports either. I think they are mainly used for classes where standard width piston grooves are required or some other silly rule.