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Ask Away! with Jeff Smith: Piston Ring Thickness and Why Thin is In!

 

I’ve heard that late model engines now run much thinner rings than the older production engines. Are these thinner rings just there to improve fuel mileage or is there a power advantage with these new rings? I’m about to rebuild my small-block Ford and I’d be willing to try something newer if it’s better. Thanks

F.R.

Jeff Smith: According to the old Hollywood line – you can’t be too rich or too thin. In the case of piston rings – those two items are pretty closely tied together. Thinner is definitely better but that reduced girth will also cost you some coin. But that’s way too simplified an answer.

Let’s jump into this area with some interesting details.

In the muscle car days, the standard ring package was a 5/64-inch top and second ring with a 3/16 oil ring set. This dimension refers to the ring’s thickness as viewed from the side of the piston. This 5/64-inch thickness (0.078-inch) required a significant amount of load pushing outward to ensure an adequate seal. This load – called radial tension – also produced a significant amount of friction. The most friction in any three-ring piston package is created by the oil ring but the combination of all three rings in this width is significant.

Moving closer to the 21st Century, OE engineers revised these specs and realized that a thinner ring package would help reduce friction and improve better fuel economy. The GM LS engine family, for example, came with a 1.5mm/1.5mm/3.0mm ring package. This equates to 0.058/0.058/0.118 compared to the older thicknesses of 0.078/0.078/0.187. This is a 25-percent reduction in ring thickness. As the ring becomes thinner, the amount of radial (outward) tension required to seal it to the cylinder wall is reduced. This occurs because as we reduce the total surface area of the ring touching the cylinder wall, the radial tension can be reduced to produce the same amount of load on the ring. Think of it this way. A woman can easily walk across wet ground with normal, flat shoes with lots of surface area. But if she attempts to walk on soft ground in a pair of spike high heels, that tiny area under her heel will sink right into the soft ground. Her weight hasn’t changed, but that spike high heel concentrates that same weight (load) in a very small area. So if we think of a thin ring as that spiked high heel, it now requires far less outward load (radial tension) on the ring to present the same amount of sealing load against the cylinder wall.

By reducing the radial tension on a thin ring, this also reduces the friction created as that ring slides up and down the cylinder wall. This occurs with all three rings, including the oil control rings. So now we’ve just added a slight amount of horsepower just by reducing friction. But the news gets better. Thinner rings also tend to seal better to the cylinder wall, which means blow-by past the rings is reduced and more cylinder pressure is retained above the pistons to make more power.

So for performance engines, the industry has been slowly migrating toward thinner rings. Not all that long ago, the drag racing standard performance ring was a 0.043-inch wide Dykes ring with a step that improved ring loading. That equates to roughly to a 1.1mm ring. Until recently, Mahle’s performance ring package on most of its PowerPak performance forged pistons utilized a 1.5/1.5/3.0mm ring combination. The top two rings were equivalent to a 0.059-inch wide ring. But last year, Mahle introduced a new ring piston and ring package that now trims the rings to 1.0/1.0/2.0mm dimensions (0.039/0.039/0.078-inch). It’s clear that the movement is toward increasingly thinner ring packages.

Total Seal seems to be the company that is leading the way with an entire line of thin ring packages with top ring offerings in their Ultra-Thin Advanced Profile series with 0.9mm top and second rings with a 2mm oil ring. This 0.9mm equate to 0.035-inch thick top and second rings with the 2mm oil ring at 0.078-inch. According to Total Seal, replacing a typical 1/16/1/16/3/16-inch ring package with this Ultra-Thin 0.9/0.9/2mm combination would reduce ring friction by 90 percent!

You might also think that using these ultra-thin rings for example might demand a custom-built piston. But Total Seal’s Keith Jones says that they also offer ring spacers that allow the use of a thin ring package in pistons with wider ring grooves. As an example, you could go with a 0.043-inch wide top and second ring with spacers to fit within a typical 5/64-inch ring groove piston. The original reason for this was to accommodate NHRA Stock Eliminator engine builders were forced by the rules to use stock replacement pistons. The rules only governed the piston dimensions, not the rings. Enterprising racers began demanding spacers to allow them to run ultra-thin rings in these stock ring grooves.

If you take this idea a step further, long-stroke engines will especially benefit from this reduced friction since the longer the stroke, the father the piston must travel. This additional travel creates its own friction but you can mitigate that somewhat by using thinner rings.

As you might imagine, none of this new thin-ring technology is inexpensive. A typical Total Seal Classic 1/16-inch ring set from Summit Racing for a 4.030-inch bore for example runs around $112 for the complete set. Step up to a set of 0.043-inch rings for the same bore size with a Total Seal Gapless Second ring and the price jumps to around $380 for a set of rings. Spacers will bump that even further –potentially placing a set of rings near $500. Compare that to a stock replacement set of 5/64-inch iron rings for a small-block Chevy at $21 and you can see that technology comes with a steep price.

We also looked into a set of Mahle PowerPak pistons and rings using the 1.5/1.5/3.0mm ring package in a typical small-block Chevy 383 flat-top piston combination and that  would run just a touch over $700 for the pistons, rings, and wrist pins. That’s just one example of how you could take advantage of this new ring technology, but there are as many different avenues as there are creative engine builders. You might want to think about taking advantage of some of this technology – the savvy engine builders are already doing exactly that.

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6 Comments

  1. We all owe thanks to the efforts of Hank-The-Crank for his work in these areas many years ago.

  2. How much shorter is the service-life of these thin piston rings?
    I imagine they wear a little faster than the stock rings.
    Any weartests being done with these?

    • Great question – with thinner rings, the axial (outward) tension is reduced, that’s why friction is reduced and hp increases. So I would say that – assuming proper installation – that wear would be as good, if not better than a standard set of rings. Ring selection is critical that’s why a call to Total Seal for your application would be a good idea. But since the OE’s have been using 1.5mm rings for quite awhile, I would say that undue wear would not be a problem!

  3. Jeff Urbain says:

    Thinner rings, less material to make, more expensive. That makes sense.

  4. Here’s the only issue I see with this concept from the perspective of use on production engines:

    Not everyone runs their engines hard. In fact, some people don’t push their engines at all. Ever.

    For example; when GM designed the Northstar/Aurora V-8, it was originally intended for “high performance” applications and included thin, low-tension rings…Unfortunately, the applications in which that engine was used was anything BUT high-performance. The Northstar/Aurora V-8s were known for terrible oil consumption issues.

    The thin, low-tension rings and the oil consumption issues were DIRECTLY related.

    Why? The ACTUAL driving conditions seen by the bulk of the Northstar/Aurora engines never saw any performance use at all and because of that, saw a very high level of coking in the ring lands, thus the REASON for the oil consumption issues.

    According to the TSB from GM, their “fix”: (in a VERY paraphrased version)…flog the “ever-living-snot” out of the customer’s car for 15-20 minutes.

  5. Pingback: Piston Rings - Ford Truck Enthusiasts Forums

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