piston ring position 1

[dexion7]

In an engine, what is the purpose of offsetting rings 120 degrees when installing the pistons in the bores? Out of several engines I’ve pulled apart and looked specifically at the ring positions, not a single piston has had the rings at 120 degrees.

No bores, pistons, rings, blocks etc are perfect and considering expansion/contraction & that the rings have been up and down millions of times during the engine’s working life, it seems hardly surprising that the rings want to move around. Don’t they just wriggle around after first start up and assume a position which most accurately aligns the new (and not perfectly round) ring with the not perfectly round bore and then bed in at that point, staying put until strip down?

Wouldn't manufacturers install some locating method in the ring groove if they wanted the ring to stay in a particular position?

 

[franzh]

I too have torn down hundreds of engines of all shapes and sizes. Rings do move around to circumference of the piston. This puzzled me for quite some time, and one old-timer (I've got to watch that term, I'm one of them now) told me that rings are twisted around the piston during installation and thats what makes them move around. Kinda-sorta-maybe makes sense. But, I have an expanding type piston ring installer and when I tore down some racing engines I built, the rings were different from installation. I lined up the gaps straight on one engine I knew I would be tearing down after a race and they were in different locations then and it had no significant performance issues, outside of dropping a valve.

On 2-cycle engines where the ports are in the cylinder bores, some manufacturers have a pin in the ring groove that meets with a notch in the ring end to prevent it from moving to the cylinder port. All in all, I guess it would be good practice to stagger the ring ends, if nothing but to prevent the cylinder bore from developing a vertical ridge, but certainly to minimize blowby.

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[ivymike]

unless they're pinned in place, piston rings generally rotate when the engine is running. (the other situation where they'll stop rotating as you mentioned is when you have a significant ovality to the bore which allows the ring to find a low energy orientation and stay there)

There have been a number of studies done with running engines & various methods of seeing inside, either by sensors or windows, and all that I'm aware of have shown all the rings rotating in fits & starts or even quite rapidly.

One thing which would tend to drive them around is the rocking of the piston back and forth in the bore (in the thrust plane) combined with any transient offset of the piston to one side or the other in the pin plane. You'll also have uneven distribution of friction at the ring-groove interface (which works w/rocking to move the ring), and you may have some amount of (aerodynamic) thrust from the blow-by gas depending on the details of the ring groove geometry.

Some people will tell you that they always find the rings in certain orientations when they tear down engines - I have never seen a scientific study of whether this occurs - I suspect it's bias on the part of the observer which skews their recollections. I have definitely seen more than one warranty claim where the servicing mechanic reported that the rings were not "clocked" correctly when the engine was built (ahem, BS).

 

[EdDanzer]

The honing marks are part of what makes the rings rotate. In hydraulic cylinders the twisting torque from the hone marks can loosen the piston or piston nut.

Ed Danzer

http://www.danzcoinc.com

http://www.dehyds.com

 

[Majik]

The honing marks and their angle in relation to the bore determine the rotation and speed. You can always tell a rookie because they will believe things like "the rings should be at 120* intervals" and they will break out a protractor and start marking the gaps or some manuals like mitsu/honda give actual degree'd increments in relation to the front of the motor that people will measure to. The only facts are, the rings rotate. The rotation speed is determined by oil on the walls, load on the pistons/ring, and ring tension. The gaps do line up at some point and the engine still runs. Put them in the engine at whatever way that makes you comfortable.

Many pistons now contain a phosphate coating to prevent micro welding of the ring to the ring land to promote longer ring land life.

 

[BrianPetersen]

Offsetting the ring gaps is supposed to help on the first start-up when the rings are not yet seated, by giving the gases going through the end gaps something a labyrinth to get through, and also, when the piston is pushed to one side during the compression stroke it'll cover up one ring gap more than the other one, and then on the power stroke when the thrust side is the other direction, switch around, but either way, there is not a condition with two ring gaps in line.

After the engine starts, and you have at least a bit of ring seal, it doesn't matter any more. The gaps will be in random positions - but generally not aligned.

 

[superman777]

So if i were to fit gapless rings to first and second ring positions what benefits could i see;

 

[Tmoose]

lower leakdown numbers on the gage in a static leakdown test.

Although >>some<< swear by gapless rings in various configurations.
(Last time I looked a few weeks back total seal said go gapless in either 1 or 2, my choice)

I question if there would be any functional superiority over a well done conventional ring installation. (well done = about 30 well controlled parameters. some examples - cylinder roundness and taper with head installed, cylinder finish, and pre-assembly cleanlinesss, piston ring groove finish, flatness, back clearance, break-in procedure, crankcase ventilation and pressure management)
One of the mechanisms behind shaft pressure sealing devices like o-rings, lip seals and piston rings is a pressure differential forcing the seal radially outward. The greater the differential the better the seal. No differential and the pressure can puff right past the sealing faces. Piston rings have inertia, so when the piston is accelerating fast down the bore from TDC the ring would prefer to migrate to the top of the groove. If the pressure (differential) is insufficient to keep the ring against the bottom of the groove and the ring lifts off, the pressure in the back clearance will bleed off and the main seal against the cylinder wall will be lost. that one reason why high rpm requires narrow (light) rings.

Speed Pro recently has been speaking publicly about adding a little extra gap clearance to the second ring gap, to intentionally release pressure that build up between the 1st and 2nd rings during the power stroke to bleed down, ensuring the greatest possible differential across the first ring and best sealing under extreme rpm.
Ring gap for the top ring remains the smallest that will still not butt during operation.

A total seal ring in the 2nd groove would seem to invite capturing pressure between 1 an 2, ruining ring 1's stability at higher rpms.

 

[evelrod]

To start, I always carefully 'clocked' my piston rings but that was well over fifty years ago and, from my point of view today, in another galaxy. My current practice is to just put the top two rings opposite and not worry about the oil ring(s) on the Mini's JE pistons 1/16th rings and we use a gapless second ring on the Lotus 1600 using CP and one mm rings. Upon teardown (rare in the twincam but fairly common in the Mini's little 1300) top ring is never where it started. Leak down with standard rings starts around 5 to 10% and the one mm, gapless second around "set" and 3% (even after several race weekends in vintage racing). The Mini's rev limiter is 7800 which limits it's max to ~8000 and the Lotus is 9400 limit. Ring gaps tend to the tight side of the specs in all cases, never a problem in a race engine. Different strokes for different folks is the cliche...true enough...I've known several other pro engine builders over the years that don't subscribe to my methods. However, my win/loss record speaks for itself. I don't think it matters much in the long term on a street engine. The rings will move but if they are initially 'clocked' it will ease the first startup, perhaps. I have discovered that getting old does not go hand in hand with getting smarter, for what that is worth.

Rod :-(

 

[BrianBT]

The ring rotation also evens out wear on the ring and prevents hot spots.

FYI, on large slow speed engines you can have fitted ring wear measuring equipment. using this you can actually see the rings wandering around.

On one medium speed engine design not only to the rings rotate but the piston does as well (Sulzer ZA)

the angle isn't critical so long as you make the 'torturous path' commented above.

On Slow Speed engines its nearly alwys the 2nd ring that breaks initially

 

[anyoldname]

BrianBT,

I'm very interested in your comment about ring wear measuring equipment on large, slow speed engines (I'll assume you mean very large ship engines). I'm about to start working on the design of my first engine of this type (all my previous experience is with automotive and off-highway diesels), this engine will be for testing lubrication systems so ring wear is one of the key parameters.

What sort of system can be used to measure the ring wear as the engine operates? We're going to have to investigate what's possible in this area but any pointers you could give that would help me short-cut to real world systems would be appreciated.

Thanks.

 

[Lou Scannon]

There is a proven method of using specially processed mildly radioactive components (e.g. piston rings) for non-intrusive, real-time wear measurement.
When I was involved with this technology, there was only one commercial proponent, TMT (Tribologiemesstechnik) in Karlsruhe, Germany. They may have competitors by now, but I don't know.
Here is a publication of a project I worked on using this technology.
Correlation of Radiotracer Wear Measurements with conventional Methods in the Navy 1000-hour High-speed Diesel Durability Test.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[140Airpower]

hemi, I suppose the extreme sensitivity of a method that uses radiologicals in the production of fine particle wear products excuses the hazards. I can imagine the comparisons of the cancer risks from radioactive particles in the lungs to working in coal mines, etc. With diesels, that do not run under intake vacuum, virtually all particles end up in the crank case and oil and can be trapped. Anyway, non-radioactive wear analysis also works.

 

[Lou Scannon]

Well, even back when I was involved with it, extreme care was taken to avoid exposing people to appreciable radiation. All of the personnel wore dosimeters and according to the dosimeter results, nobody received more than background radiation during the course of the project. Not to say there wasn't a potential hazard, just that it was managed such that people were kept safe. There are permissible dosimeter levels above background for short-term and lifetime exposure. If an individual's periodic or lifetime dosimeter results were to approach the corresponding limit, that individual would have been reassigned off the project so as to either pause or end their exposure accordingly. BTW, this was a diesel engine test.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz