piston rings

[Mark69]

 

I have fitted new rings to my pistons , I have followed the manufacturers instructions but find the bottom ring lines up with the hole to the right of the gudgeon pin hole in the piston.

Should the gap in the ring be located somewhere else due to oil being forced into the expander ring.

Mark.

[Motorsport Mickey]

Mark,

 

Life's too short, if it bugs you move it 12mm away from the hole.

 

Mick Richards

[Peter Cobbold]

They rotate in use, at roughly 1/1000th of crank rpm.

p464:

http://www.amazon.co.uk/Four-stroke-Performance-Tuning-Graham-Bell/dp/0857331256/ref=sr_1_10?ie=UTF8&qid=1411241574&sr=8-10&keywords=a+graham+bell

[RogerH]

Hi Mark,

your photo also shows the compressions rings - do not let the gaps align as you have shown.

Staggering them around also.

 

Roger

[Mark69]

Thanks for the input wise ones

I know life's to short that's why I only want to fit them once, and hopefully the right way .

The rings will be in the right order just wanted to be sure as all literature I have show solid pistons

Mark

[Guest ntc]

They do rotate but not as far as you think. Never put the gap on any ring on the thrust side.

[Mark69]

Thanks Neil

Have seen that in the brown book.

Mark.

[Motorsport Mickey]

Duplicate

Edited September 22, 2014 by Motorsport Mickey

[Graze]

They do rotate but not as far as you think. Never put the gap on any ring on the thrust side.

On the basis that there is no such thing as a silly question

 

Can you explain which is the thrust side.

 

Thanks

Graze

[Peter Cobbold]

The thrust side is the side of the cylinder to which the top of the conrod points on the power stoke. As the gas expands it pushes the piston with a sideways component as well as downwards, because the conrod is off-vertical. On TR engines its the same side of the engine as the inlet manifolds.

In this gif its the side of the piston under the inlet valve:

http://www.autonetnewengland.com/images/stories/auto/engine-animation.gif

 

Rings rotate and aligned gaps soon become misaligned:

http://www.eng-tips.com/viewthread.cfm?qid=338363

 

Peter

Edited September 22, 2014 by Peter Cobbold

[Motorsport Mickey]

Just harking back to Piston ring wear, gaps and how to measure them, this article from the Institution of Diagnostic Engineers (first published 1995) may give you food for thought, and it's how I've approached piston ring fitment through the years.

 

Ring Gaps vs Knowledge Gaps

 

Frequently I hear in court that when an engine is dismantled it is discovered that the ring gaps were not staggered when they were installed. Frequently I read workshop manuals that go into great detail on the necessity to stagger ring gaps. Frequently these manuals specify ring gap limits which is yet another myth we can bin before we are finished.

 

Piston rings are free agents and can rotate or not rotate as they see fit. They are not in touch with the base of the groove and neither are they trapped between the upper and lower faces of the groove. The rings are entirely free to rotate except where a stop peg is fitted so what's the point in staggering the gaps on installation?

 

We used to work on a minimum of 0.020" back clearance on radius or, to put it another way, the inside diameter of the ring when installed in the cylinder must be at least 0.040" bigger than the groove root diameter. Minimum side clearance was generally held to be 0.0015" and if you could fit a 0.006" feeler gauge in the groove along with the new ring then the groove was "goosed" so the piston was replaced.

 

The piston ring was manufactured with a tangential load the force with which the ring presses against the cylinder wall but apart from that it is completely uncontrolled. There is no way, under these circumstances, that you could prevent the ring from turning so, to repeat the question, why stagger the ring gaps on installation?

 

Equally mislead are those who expect ring gaps to stay staggered when the engine is in operation. When there is clearly nothing to prevent the ring from rotating, why should the gaps stay staggered?

 

More than one county court judge has fallen for the hocus pocus that because the gaps were in line when the engine was dismantled, they must have been in line when installed. Absolute poppycock! Yet the inclusion of such rubbish in workshop manuals does admittedly give it an air of authority.

 

When you think about it, you don't need me to tell you but I will anyhow that rings do rotate in operation. Every now and again the gaps do line up and once lined up there is a tendency for them to stay lined up at least until the vehicle hits the next pothole in the road when one or other will rotate and break the line. Staggering ring gaps when installing rings is a myth that we can bin forthwith.

 

Even worse is the preoccupation with the size of the ring gap. Yes, there is a minimum but this varies considerably depending on the material used. Normally 0.003"/0.004" per inch of bore size is given but where, for example, low expansion SG (spheroidal graphite) iron is used, it can be considerably less, so what about rings with gaps that are too big.

 

Well, the answer to that was that you melted them down and started afresh until AE research asked the question, "What is too big?" and set out to quantify that. The results were interesting very interesting and what you are about to read was kept quiet because it bestowed an enormous commercial advantage on AE. This is probably the first time the information has been published although the research was undertaken in the late 1970's almost 25 years ago.

 

A Ford Kent engine was stripped and fitted with compression rings which had end gaps of 0.015" when fitted in the bores. The engine was wired up with the usual telemetry to measure blow-by and oil consumption and then run in one of the test cells. After making due note of the blow-by and oil consumption, the engine was stripped and fitted with new compression rings with gaps of 0.025" and the test cycle repeated.

 

These rings were subsequently replaced by ones having end gaps of 0.035" and the test cycle repeated again. It had been planned to stop at 0.035" gaps ----------> but the results were so interesting that it was agreed to proceed to 0.045 and then not to 0.0055 but to 0.0625 1/16"!

 

Whoever heard of rings with 1/16 gaps a ridiculous figure but the interesting thing was that the increase in blow-by and oil consumption at 0.0625 was only marginally above the figures obtained with 0.015 gaps.

 

Practical tests established that the gap was not the villain of the peace. To all practical purposes the size of the gap didnt matter. It is important to stress at this point that we were dealing with compression rings that were brand new when fitted to the test engine.

 

The gap was specially manufactured for the tests. So how come all oil burners and heavy breathers have ring gaps you can back a bus through? Well, the tangential load that the ring exerts onto the cylinder wall is a direct function of its radial thickness.

 

As the periphery wears in contact with the bore, the radial thickness obviously decreases, as does the tangential load. Peripheral wear means a smaller ring o/d and this manifests itself as an increase in the ring gap, it's not the gap but the reduced tangential load that is detrimental to the performance of the engine. The ring gap is a complete red herring.

 

Imagine four top compression rings all with 1/16 gaps. The total gap for all four would be 1/4. Now imagine the seal provided in an 80 mm diameter bore. Pi x Diameter = Circumference, so we have 3.14 x 3.15 = 9.891. Multiply that by four cylinders and we have 39.564 over a yard of contact seal between piston rings and bore. Now visualise the many litres of blow-by and consider whether all the gas is squeezing through 1/4 of total gaps or passing through 39½ of reduced pressure contact seal!

 

But even this ignores one important facet of the argument because there is not just the one compression ring on a piston there are usually at least two and that is because rings work as a team to form a labyrinth seal.

 

For gaps to be the villain of the peace, the gas would have to find the gap in the top compression ring and pass through. It would then have to circulate to find the gap in the second ring and pass through that and so on. Now this may be possible if the power stroke lasts for 10 minutes but it doesn't, does it?

 

At 3,000 rpm the power stroke duration is a mere 1/100second. Quite simply, the power stroke does not last long enough for the combustion gas to find its way around the maze or labyrinth seal, so the villain of the peace has to be the reduced tangential load of the ring on the bore caused by peripheral wear or reduced radial thickness of the ring.

 

This was our hypothesis based on the results obtained in the engine test cell but it took a very clever American to prove it. This genius invented telemetry that measured gas pressure between the piston rings in a working engine.

 

Use of his brainchild revealed that some gas did get through the top ring gap sufficient to generate a hell of a pressure between the top and second rings so it clearly was not finding the gap in the second ring. The labyrinth was working well. Caterpillar and IHC must have thought it was working too well because they increased some second ring gaps to 0.050 and 0.070 thought to be beneficial.

 

Come to think of it, the exception proves the rule as usual. Two stroke engines would not need stop pegs to prevent the end gap from crossing a port if the ring didn't rotate. The people who allege that ring gaps were not staggered when installed just because they are in line when the engine is dismantled don't need stop pegs.

 

The very presence of a stop peg also proves my point about the size of ring gaps. Where a peg is fitted, the end gap has got to be 1/8 to accommodate the peg. There would be one hell of a draught through that if the 0.015/0.018 boys were correct.

 

This knowledge was commercial dynamite because, instead of the '0.015/0.018 spec., it meant that new rings with gaps over 0.018 could be used without any detriment to the engine's performance. The gap was only detrimental when it was the result of peripheral wear. Customer acceptance was the only problem.

 

To re-educate the customer would let the cat out of the bag, thereby losing the distinct commercial advantage. It was decided, therefore, to accept rings with gaps of up to 0.030 in a nominal bore but even then there were arguments. 0.030" in a nominal bore is 0.045 in a bore worn by 0.005 and it's normally engines with this sort of wear that get new rings.

 

It made diagnostics a lot harder too because when all rings set off in the 0.015/0.018 area gaps of .040 meant something but now, when you don't know what they set off at, what they measure is meaningless.

Well, now you know. All rings are free agents to rotate as they like, making staggering of gaps on installation a joke and ring gaps are not a problem provided that the gap is not the manifestation of reduced ring radial thickness caused by peripheral wear.

 

M H Booth F.I.Diag.E

 

 

Interesting eh... for what it's worth I always fit my Piston rings staggered, it just makes me feel better, having confirmation that it doesn't matter a damn is just insurance...I can't possibly get it wrong !

 

As for Piston ring wear as stated in these posts previously, it could only be estimated by confirmation that the bores were in a good condition in the first place and the piston rings had been accurately gapped. Where this information is not known measuring the piston (remember they are not round) and the cylinder dimensions will show how worn the cylinder is in comparison, remembering that on Pistons up to these sizes wear up to as much as .005 or .006 is allowable. (Complete Automotive Engine Rebuilding by Robert Scharff ) If you get up to this wear then resizing of the pistons and bores are the way forward.

 

Mick Richards

Edited September 24, 2014 by Motorsport Mickey

[TR24178]

Superb stuff Mick and Peter, all that time wasted in the past staggering rings!!

[RogerH]

I'm staggered - but doe sit matter

 

 

Roger

[Stanpartmanpartwolf]

At last!! Very well put chaps, an excellent discourse if I may say so.

Returning to the "ring blow-by vs. time" section- I am led to believe that the recent 18000rpm NA F1 engines used one piston ring only.

The explanation for AE's gigantic stock ring gaps is most interesting; indeed, recently I have fitted a set of USA-made 6-pot rings that arrived with a 15 thou overlap for a 77mm bore size. Hmm.

But in the end, as Mick says, it always feels better when you've gapped all the rings properly & pointlessly staggered them!

[Guest ntc]

And remember some top flight piston's have a pin to stop rotation