How to choose the right piston ring for your application

When it comes to choosing the right piston ring, the devil is in the details. Slot design, fixing groove depth and width, and material all play an important role in the performance and lifetime of the component. In this blog post, we'll fill you in on what you should consider when selecting the right piston ring for your application.

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Mechanical requirements for piston rings: pressure, speed and lubrication

The most important factor in the selection of pistons rings is the pressure (force per area), which acts on the rings in the application. But frequency and speed of movement also play a major role. The type of lubrication and the coefficient of friction should be taken into account too. But why are these values so important?

Piston rings that are exposed to high pressure should be made of the most resistant material possible. Self-lubricating fiber-reinforced plastics or steels are commonly used here; however, they have the disadvantage of being heavier and requiring constant lubrication.

Speed of movement also plays a role. Depending on the material used, if the product of speed and pressure is too great, the resulting frictional heat cannot be sufficiently dissipated without additional lubrication.

The type of lubrication depends on application requirements and the technical characteristics of the piston rings in question. Piston rings made of plastic or plastic fabric are often self-lubricating. Solid lubricants such as PTFE or molybdenum sulphide (MoS2) minimize coefficients of friction and protect the piston rod or cylinder from wear.

Piston ring shape and dimensions

Piston rings can be mounted in two ways. They can either be fitted on shafts or rods or installed as guides in cylinders. This is why guide rings are slotted -- to provide the necessary clamping force and flexibility.

Piston rings also serve as a sealing function or are designed to let lubricating or hydraulic oil through (depending on the shape of the slot). There are also angled and straight slots or form-fitting variants with a Z or tooth profile. Piston rings made of plastic offer the advantage of less complex shaping since plastics can be processed and machined relatively easily and cost-effectively.

Material variety for piston rings

The world of plastics has an especially wide range of materials. Not all of them are suitable for piston rings though. Additives are also required to specifically optimize the material properties for use in piston rings. In addition to improved properties such as elasticity, coefficients of friction, and mechanical stability, the materials differ primarily in their temperature resistance, thermal expansion, and chemical resistance.

The properties mentioned above, mechanical strength and elasticity, maximum PV values, and thermal properties can be compared especially well by means of data sheets. But what do they say about component durability and service life? Here the comparison is often difficult. Design programs and formulas, like our plastic bearing service life calculator, can help here. 

For assistance choosing the right high-performance plastic piston ring for your application, contact us here or call us at (800) 521-. You can also learn more about our maintenance-free iglide bearing materials here. 

I use Total Seal in Arizona. They can make any ring you need. Just call them with the dimensions.

800-874-

I&#;ll look through my piles of supplies, I&#;m 99% sure I have a ring that will work for your application. If you want a used ring.. I call them &#;Tried and True&#;..

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What you need to look at is the ring gap with your old rings: This is a good indication of how worn they are.

For a 3.50&#; bore, the ring gap should be no less than .014&#; and not over .025&#; . These are very general specs.

Some engines are a bit more specific with the range of gap preferred.. A good average is .004&#;-.005&#;/inch of bore.

Check the gap with a ring up near the top of the cylinder bore, this is where the greatest wear usually is, Then use one of your pistons without rings or pin to push the ring down to the middle of the cylinder bore, the ring gap will close up.

If the rings are worn, or the cylinder bore worn or has a lot of taper, [wide at the top, correct at bottom] then the ring gap will clearly show it.

Reassembling an engine with used rings has a high likelihood of the rings not sealing or seating very well. The bores do not wear perfectly round, and a ring usually will find it&#;s &#;spot&#; and then the ring will wear the same as the bore.. so if you don&#;t get the rings back on the same piston, in the same groove, in the same position on the piston, the oval wear patterns may not match, and you will have to endure some oil burning and compression loss, until either the rings rotate around in their grooves and get back to their &#;spot&#;.. or wear to fit tighter against the cylinder wall.

All that said, usually our engines run ok with reusing old rings,, but don&#;t expect low oil consumption.

Greg Long

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