The Science of the Second Ring

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Danno said:
I've often wondered why more gapless (Dykes?) piston rings are not in common use. The advantages seem obvious. What are the disadvantages other than increased cost?
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Most Norton owners haven't tried them so they don't know the difference. They are used to a little oil burning and don't want to spend the extra $50 for a better sealing ring.

See the gapless 2nd ring test by Hot rod at:
https://www.hotrod.com/articles/childs-alberts-zero-gap-second-ring-test/

a quote from the article:
"produced a significant improvement in the entire torque curve, which surprised everyone"

Again - it depends on who you listen to. But I'm seeing less oil contamination on an air cooled motor with a gapless 2nd ring.
 
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Danno said:
I've often wondered why more gapless (Dykes?) piston rings are not in common use. The advantages seem obvious. What are the disadvantages other than increased cost?
Click to expand...
Dykes not gapless
The Science of the Second Ring
 
jseng1 said:
Most Norton owners haven't tried them so they don't know the difference. They are used to a little oil burning and don't want to spend the extra $50 for a better sealing ring.

See the gapless 2nd ring test by Hot rod at:
https://www.hotrod.com/articles/childs-alberts-zero-gap-second-ring-test/

a quote from the article:
"produced a significant improvement in the entire torque curve, which surprised everyone"
Click to expand...

They say the apparent improvement was due to better sealing yet most of what I have read indicates that it is done by the compression ring. You think the increased torque is due to reduced friction. Not sure if the author of the article knows for sure as it came across as a bit of an infomercial. Would be neat if they could measure crankcase gas flow and constituents to make a more informed statement.

Interesting topic.
 
It's friction vs sealing.

Late model hard coated aluminum cylinders allow tighter piston/cylinder clearances which in turn limits rocking, holds the ring's plane more constantly perpendicular to the bore axis, and that allows a thinner ring cross section/less surface area presented to the cylinder wall, and that means less friction so lighter ring pressures still seal.
 
I can't see any advantage in a wide gap in the top ring - the more gap the more blow by. The tighter the gap the better until the ends butt. When I was racing I heard from tuners who tried to close the top ring gap as much as possible (and it worked for me). A gapless top ring should work very well for racing. But you better have good valve seals or the increased vacuum will suck oil through the guides.

The 2nd ring is what total seal suggests for the street and I think the all around performance is better. But many point at ring flutter problems near redline which is very hard to prove and it depends on who's talking - the powerful established companies who don't make gapless rings and want to shoot them down or the upstarts that make them such as Childs Albert and total seal.

I've found less oil contamination with the gapless 2nd ring and methanol racers like it because it helps keep the alky from diluting the oil (and they rev the shit out of their race engines). You would think that less oil contamination means less blow by and so a gapless ring would be a no brainer.

But experts disagree and the argument goes on.

Top or bottom gapless, street or racing, fluttering or not. Etc and lots of contradictions. But I like the 2nd gapless rings in my cafe Norton.
 
I think it might be pretty rare that people who work on their own bikes, only change one thing at a time. So cause and effect can be pretty obscure. If you get a performance improvement, unless you have a very smart dyno which measures torque accurately, there is probably no way of knowing it until either your lap times on a circuit improve - or you find that by raising the overall gearing, the bike goes faster between two familiar points on a road.
 
Dances with Shrapnel said:
Then there are the pistons with a compression ring and oil ring and no second ring.
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There were some air cooler Ducati twins running that configuration back in the BOTT days. The idea is to reduce friction. A gapless top ring would be tempting.

But

Way back when I was racing with lightened Hepolites I was revving them beyond reason. The top ring lands would loosen up and the top rings would break leaving only the 2nd ring (which I think was identical to the top ring). I could tell when this happened because the motor felt wimpy with an obvious loss of torque and pull. So I lost interest in the single ring idea.
 
I have spent a considerable amount of time on the dyno looking at crankcase pressure from ring blowby and the results of ring flutter.

A tight second ring gap [or a Total Seal] second ring definitely makes ring flutter happen at a lower RPM. It is hard to miss. It happens suddenly with a large increase in blowby and a flattening of the torque output.

Generally the stock ring package with stock end gaps starts to flutter below 6500 rpm. The more worn the ring gap -the lower the rpm where flutter begins.

Modern thin [1.5mm] rings will not flutter in the RPM range of a Norton motor.

When I raced using standard wide piston rings I would install two thin chrome plated rings in the top groove which prevented flutter pretty well. They were modified rings from a 2 stroke appllication.
Then I ran a wide second ring gap or no second ring at all.
 
comnoz said:
Modern thin [1.5mm] rings will not flutter in the RPM range of a Norton motor.
Click to expand...

If modern thin 1.5mm rings won't flutter in a Norton then forged pistons with thinner 1mm top rings certainly won't.
 
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Danno said:
I've often wondered why more gapless (Dykes?) piston rings are not in common use. The advantages seem obvious. What are the disadvantages other than increased cost?
Click to expand...

Amazingly Norton have used Dykes piston rings in the past, when Doug Hele used them in their 500 Domiracer way back in 1961
 
jseng1 said:
If modern thin 1.5mm rings won't flutter in a Norton then forged pistons with thinner 1mm top rings certainly won't.
Click to expand...

To me, this raises a question about the technology used in Commando pistons and rings, both original and after-market. Many things which have been developed for high performance two-strokes have been a major step forward.
 
comnoz said:
I have spent a considerable amount of time on the dyno looking at crankcase pressure from ring blowby and the results of ring flutter.

A tight second ring gap [or a Total Seal] second ring definitely makes ring flutter happen at a lower RPM. It is hard to miss. It happens suddenly with a large increase in blowby and a flattening of the torque output.

Generally the stock ring package with stock end gaps starts to flutter below 6500 rpm. The more worn the ring gap -the lower the rpm where flutter begins.

Modern thin [1.5mm] rings will not flutter in the RPM range of a Norton motor.

When I raced using standard wide piston rings I would install two thin chrome plated rings in the top groove which prevented flutter pretty well. They were modified rings from a 2 stroke appllication.
Then I ran a wide second ring gap or no second ring at all.
Click to expand...

Thanks for this. It answers a question, for me. While my bike is quick enough to be competitive, I don't change much. If I ever need to get it going quicker, I would consider Jim Schmidt's long rods and light 12 to 1 comp pistons. However I had never thought about the types of rings until now. If you can get ring flutter at 6,500 RPM and lower, that has meaning. The tendency to flutter might be a function of the rings' mass ? When I was playing with a T250 Suzuki, it had fat rings, however the replacement pistons from a TM125 Suzuki MX bike had skinny rings.
 
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xbacksideslider said:
It's friction vs sealing.

Late model hard coated aluminum cylinders allow tighter piston/cylinder clearances which in turn limits rocking, holds the ring's plane more constantly perpendicular to the bore axis, and that allows a thinner ring cross section/less surface area presented to the cylinder wall, and that means less friction so lighter ring pressures still seal.
Click to expand...

True but so far no one offers a Nikasil coated alum Nort cylinder. But you can still have zero piston skirt clearance with a warmed up Norton. The Graphite coated pistons can go in at .0025 to .003" installation clearance and they won't scuff. When warm during breakin they can actually have a slight interference fit and they still won't scuff. The interference graphite will wear away during breakin, then after breakin they end up with zero clearance when warmed up and no rocking.

Below is a photo of the graphite pistons after breakin.
The Science of the Second Ring
 
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Yes, and that system requires a process of running, take down, reading wear patterns, and re-profiling the piston’s shape, it’s barrel and roll , to match the conflicting expansion rates of the cylinder (at different points up and down and radially) and the piston (again at different points up down and radially), followed yreassembly for another round of takedown and reprofiling
 
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