Oval Pistons

[mjthewlis]

Yes, that's what I said.

Measured nos. 3 & 4 and they are both oval by more than 10 thou. They are both smaller in the non-thrust direction.

See picture of piston showing discolouration at side of piston.

20170226_142153.jpg

Is this discoluration consistant with gases escaping down the side of the piston?

Bores in the block are round within a couple of thou. Very little end float on the crank shaft.

Any ideas? Any looks like new pistons required.

John

[Motorsport Micky]

Err...you do know that the pistons are manufactured oval...right ?


The pistons only purpose is to carry the piston rings and block up the hole in the middle between one edge and another ! On these size pistons you can quite happily run at up to maybe 5 thou difference at the top of the clearance range in size between piston and cylinder bore dimensions, ( the 5 thou difference likely on race or competition cars, they use the increased clearance to reduce internal drag). Forgive me if I'm teaching you to suck eggs but I'd suggest you carry out the following.

You need to measure and plot the cylinder bores defining where and what the dimensions are in 2 planes across the centre ie like a hot cross bun + , and carry out the same measurements on the pistons in the same planes and then compare the figures. Then when you have the figures you'll find it easier to come to a decision as to what condition the components are in, depending upon measurements it may prove that piston ring wear has reduced the radial loading onto the cylinder walls and is causing blowbye which can be cured with a new set of piston rings only.
Of course it could be that somebodies bodged a set off mixed piston sizes together just to last enough miles for them to unload the car, I don't know any of your or the cars history, how long you've had it ? does it smoke ? Only just started or always ? etc etc.

Hope this start may get you going anyway.

Micky

[mjthewlis]

Micky,

Thanks for that. I didn't know the pistons were made oval but I do now. More measurement suggest the pistons & bores are very close to specification but, following another lead, I inserted the piston rings in to the cylinder bore & checked the gap - more than double the allowance, over 40 thou.

So, right now I am thinking new rings.

Looked at the bearings. All look fine.

Any other comments welcome.

John

[022Dave]

• John, Motorsport Micky wrote a very comprehensive piece about ring gaps last September; search for Oil Burning Engine Stripdown #13. Have a look at it before buying new ones. Dave

[Motorsport Micky]

John,

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 didn’t 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.

Micky

[022Dave]

Thanks Micky; that was the article I was referring to. I couldn't work out how to do a link....
Dave

[Guest]

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 –

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 !
Micky

I deal with a major Swiss (ring manufacturer) OEM
The CEO is a friend.

They supply the rings that powered Audi / Peugeot to several Le Mans wins with turbo diesel cars (immense cylinder pressures).

The piston rings rotate in their lands up to several times per minute, and at totally different speeds.
If they don't rotate, they will wear out, which is why old engines with stuck piston rings will always die.

It's therefore pointless to align ring gaps.

Also most piston rings are NOT flat, they are angled or even convex at the surface.
As the >60bar cylinder pressure arrives at a few degrees around TDC the shape of the ring compared to the wall & the micro pressure via the oil film becomes very important.

The top ring is under particularly high stress as it must resist very high temperatures and pressures all its life. This one is often chrome plated.

The 2nd ring must be able to back up the pressure seal as well as act as a semi oil control system.

The bottom 3rd ring must control the oil supply to the top 2. Once worn out, the oil consumption will rise substantially.
The 3rd ring uses a lot more power than the other 2 because it is up to 4x wider.
Because of the large width, to get adequate wall pressure it is often reinforced with an extra spring tensioner,("expander") to get the wall pressure optimal.

All engines have an optimum wall pressure for the oil control ring, but is often not optimal or even correct when using replacement rings in worn ovalised bores.

This is why it is recommended to rebore an engine rather than try to reseal worn bores.
Not all people that rebore and hone do a good job.
Poor quality honing finish and misaligned bore walls are often common after reconditioning, with incorrect clearance thrown in for good measure.

An engine with excess clearance will never seize or scuff pistons, but an engine with too TIGHT clearances can cause significant extra heat and poor performance.

Absence of contaminants & ring/piston land wear are essential to good sealing.
Proper alignment is the name of the game to get good sealing.

Bore ovality and misalignment of the rings in worn lands cause blow by, and loss of performance in most engines.
Blow-by causes further contamination of oil, as well as the danger of leakage and oil loss from the crankcase through excess crankcase pressure.

Generally speaking older wider designs of ring align badly, which is why modern car engines with pent roof or symmetrical designed chambers take great care to ensure the pressure arrives evenly on the crown of a lightweight piston and use very nice narrow rings with low frictional losses getting low oil consumption and long life with rings of only 1>1.2mm
(which used to be used in competition engines).

[mjheathcote]

What happened to 'gap less rings' that were advertised once? can you still buy them?

[Motorsport Micky]

Never used or seen them advertised.

Micky

[flying farmer]

Once found some on a Dolomite Sprint engine I rebuilt, about 30 years ago. I had no idea what they were at the time, I thought it was some sort of bodge job when I found two very thin rings in the top ring groove.

Unfortunately I broke one half of one of the thin rings removing them from the pistons, so ended up buying a standard ring set to replace them. It was only a year or two later that I read about them in some performance magazine.
Neil

[Guest]

The Le Mans rings I referred to, were gapless oil control, which had a french patent "U flex", which they then redeveloped.
The latest news I had is the company went into liquidation in 2014.