Piston Ring

Hello, I am fitting cast iron piston rings to a 1 1/4" piston. One ring fits in its groove fine, the other ring is slightly too big for the groove.

My question is this - should i reduce the size of the piston ring, say with emery, or increase the size of the groove? Should the rigs be tight or slightly loose in the groove?

Andy

On car engines I allow at least a couple of thou clearance. It depends on what the piston is made out of and if is a steam engine or IC engine. But the should never be tight in the groove.

Clearance, but minimum when cold. Clearance will increase as the assembly heats up.

There should be clearance in the groove, because otherwise the gas cannot get behind the ring and push it out to make the seal. However if the groove is too wide, then the forces as it swaps sides at top and bottom dead centre can break the ring. So high speed engines have narrow rings with not too much side clearance in the groove, while lower speed engines can use wider rings with more clearance. The couple of thou mentioned above sounds about right for a car engine. The piston rings in my little Honda 100 (49.5 mm bore) used to be specified for 1 thou side clearance.

The clearance behind the ring is of less importance, provided the ring never bottoms in the groove.

John

If it is just a small adjustment that is needed I lay some wet and dry on a flat surface and rub the ring around on that. If it is a lot then probably quicker to put the piston back in the lathe and run a parting tool down the edge of teh tight groove.

For next time I would suggest you make/buy the rings first and then you can try them into the groove when making the piston.

I think the idea of gas pressure sealing the ring against the pistons is largely flawed. Rings need to be mechanically pressed against the cylinder walls. Any amount of clearance behind the ring results in no better compression at a few hundred revs per minute, in my experience.

Try it for yourself. Fit relaxed rings and see how poorly they work. They are not made with a 'set' for no good reason. Side clearance was what I assumed the OP was referring too. End clearance is also necessary as the piston rings will expand more than the cylinder diameter. Depth of the ring groove must always exceed the ring dimension, unless the piston has excessive clearance in the bore (which will inevitably lead to excessive wear due to 'piston slap'.

phew, thanks all. Looking carefully the parting tool i was using for the grooves had a rounded off corner so the ring wasn't able to fully seat. I reground the tool and very carefully machined out the corner.

The ring now fits and the piston fits in the cylinder - its a fairly tight fit though - I have to push it to make it move.

Thanks again, its great to get such quick advice here - Andy

If you decide to modify a too-tight ring, remember that in service the ring is pushed out by pressure behind it (as already mentioned above). On a single-acting engine (eg petrol or diesel) this means that it is better to re-machine the top edge of the piston groove, or to rub down the top of the ring, to give you the clearance you need. On a steam engine - usually double acting - you can do either way, but be very careful to achieve a good smooth finish.

And 'a fairly tight fit' may not be good enough. If the ring is cast iron and the piston is aluminium, all might be well, as the light alloy expands more with heat.

And during the running-in process, the relevant surfaces should rub lightly against each other, rubbing off any high spots, and resulting in a smoother, gas-tight assembly.

Regards, Tim

Thanks Tim - when you say a fairly tight fit may not be good enough, do you mean it should be tighter or looser? Andy

If a ring has to be replaced at any time it would be nice to think a replacement would fit in which case the piston error needs correcting. That's what I would do. Colin

My comment about a fairly tight fit not being OK was based on the idea that the ring must 'float' - move easily in its groove under all conditions, with minimum up-and-down clearance. So, in a word, Andy - it should be looser. But not by very much.

And I concur with Colin Hawes comment - if the ring is off-the-shelf, a commercial item, then correct the groove so that spares will fit.

Cheers, Tim

The idea of "pressure" behind the ring expands it, is totally flawed and can easily be shown to be untrue. I had to learn the hard way.

Dykes rings on the other hand ARE designed to expand under pressure, They are L shaped and have been used from time to time in model aircraft engines. They show almost zero compression when flicked by hand. I have obtained several Dykes ring engines for next to nothing, because the seller thought they were "clapped out"!

Rings should have plenty of spring in them and they should push against the cylinder wall. I will not go into the proper way to make rings as this is getting off topic.

The ring should be a relatively loose fit in the groove, otherwise they are likely to stick and then compression is compromised.

Andrew.

Tubal Cain did some experiments on piston rings for models. He found many had radial pressures that were far too high, especially for steam engines. ME 3971, 3,5 & 7.

Bear in mind he was an authority on diesel engines at Loughborough.

Neil

My experience is with engines that have been seriously overheated, to the point where the rings have relaxed. No amount of starting compression available for a diesel engine. But replace the rings with ones with a set and away they go again. A quick fix, and cheaper than replacing liners and pistons, in those instances where one could 'get away with it'.

Hello Neil,

I would not disagree with Tubal Cain, I have not seen his articles that you mention. But too much radial spring, increases friction and promotes wear. My final step is to anneal the ring and then to use diamond paste to lap the ring. Amazing what you can see in the way of high spots as the lapping process continues. The finished product shows excellent fitting of the ring. However it SHOULD exert some positive pressure on the cylinder. Don't forget we are talking of compression ratios of 1 : 10 or so for these engines. Diesels are a different kettle of fish having far higher compression ratios. I do not have any experience of rings for such engines, so I am hardly in a position to give advice.

Andrew.

My main interest these days is in vintage spark ignition engines intended for speed.

The ring must have some pressure against the wall to start with, since otherwise gas will be able to get between it and the cylinder wall. However it does not need excessive pressure, which will just cause friction and wear.

As for gas not getting behind them to improve the sealing pressure...try measuring up a well worn IC engine cylinder. You will find more wear at the top, where the pressure is highest. At the bottom, where gas pressure is minimal, the wear will be less. You will quite often find that a lip has been left at the top, at the highest point that the rings reach. You do not usually see a corresponding lip at the bottom. The extra wear at the top is due to the gas pressure getting behind the ring and increasing the pressure, just where it is needed. You don't need a Dykes ring for this to work.

John

Andrew Tinsley believes that piston rings do not rely on pressure behind them. Well all I can say is that Sir Harry Ricardo, a chap whose work improved many aspects of the IC engine, formed his own view (which is not the same as Andrew's) when trying successfully to get the earliest Tank engines to work properly.

Can I suggest a simple experiment for any other doubters? Take the piston rings from a known good engine, and carefully file a series of grooves across the bottom face of the compression rings. Then try to get the engine to work properly. All that makes the difference is the grooves, which prevent the pressure behind the rings (which Andrew says is not required) from having any effect.

There is an excellent exposition of Ricardo's work and his discoveries in this area in the book 'Memories and Machines' - Harry Ricardo FRS, pub Constable.

Regards, Tim

I stick by what I have said about piston rings. I am not going to get involved in discussions that usually lead nowhere. I have made enough piston rings t learn the hard way what works and what doesn't.

If the rings are not pressing against the cylinder wall then, you will have blow by and the pressure on the outer face of the ring will be marginally higher than behind the ring (dynamic set up, not static as many people seem to get confused with).

I know nothing about the work of Sir Harry Riccardo, so I am unable to comment. It doesn't seem to be appropriate to quote 1915/16 work, as I am sure things have developed since then!

Best regards,

Andrew.

Surely the wear at the combustion end of cylinder is going to be greater,in early engines the oil was poor and tended to get diluted due to incomplete combustion of the fuel and the combustion of the fuel tends to burn the cylinder,the crank end of the cylinder is well lubricated by splashed oil and as the rings do not get to that end of the bore the oil is not scraped off and it only has to lubricate the piston. The later use of oil control rings was to stop oil being burnt in the cylinder so what do you expect, the hot end of the cylinder has minimum oil the bottom end lots of oil ,so the hot end is going to wear more. I have always doubted that with conventional rings the compression forces the rings out,its more likely to find its way out through the gap where the rings meet before it has time to get behind the ring,Also in the early days piston rings were really thick and wide and I doubt if there was enough energy force out these very stiff rings. I had the L section Dykes top ring in my Greeves trials bike with the famous square finned alloy barrels ,many years ago.The info at the time was that it helped seal the piston,and the very light ring stopped ring flutter in the top groove, they worked well and were reliable,unlike the conventional rings on the iron barrelled cylinder (Villiers) which if the bore or piston got a bit too worn the rings found their way out of the exhaust port.