IC Engine Piston Rings

[ChrisHParticipant @chrish]

I have been reading ways to make piston rings for IC engines and am getting confused – easily done I know.

Some advocate heat treating the rings with the rings held apart a certain gap to relieve stress, but not too much heat so as to take out all the 'spring', some have a different construction method to get a more round ring giving equal pressure all round the cylinder wall but without heat treatment; some advocate a ring thickness (outside to inside diameter) greater than the width (top to bottom of ring in direction of the bore) and some a greater width than thickness with a minimum width of 45 thou (inch) and a maximum of 60 thou.

Re the latter, in full size experience I have always known rings to have a greater thickness than width, and would lean towards that plus a narrower width would reduce friction, but am prepared to accept it may not apply in model IC engines.

Anyone have any views on all this and have a fool-proof, idiot-proof method of construction of IC Engine Piston Rings to suggest and to be followed by someone new to making piston rings?

Chris

[ChrisHParticipant @chrish]

[mickParticipant @mick65121]

If your following a set of plans, then the designer should have taken care of the sizes, I'm assuming your using continuously drawn cast iron bar. Bore and turn the diameters in a continuous bar form, then support with a pipe centre, or pressure pad mounted in the tail stock and part to finished width. A handy tip here is to rig up a digital caliper to the saddle and the depth probe attached to the bed to ensure they are all identical width. Split the ring by using a sharp pair of side cutters with one confident movement. Make a simple fixture to hold a stack of ten or so rings and hold the gap open with a piece of metal approx. 3mm wide, this of course depends on the rings outside diameter. heat evenly until cherry red and allow to cool. This is a tried and tested method, but always make more that you actually need!

[JasonBModerator @jasonb]

I don't think I have done any where the depth into the groove is greater than the width most are either square or wider some by quite a bit, I thinks these were about 1/8 wide by 1/16" deep

[John OlsenParticipant @johnolsen79199]

The radial depth into the groove determines the stiffness of the ring, so if it is too deep they will be too stiff and put too much pressure on the cylinder wall, and also may be difficult to fit. I seem to recall that Professor Chaddock suggested something like 1/40 of the bore, but don;t have the article to hand. He also gave a ratio for the block to form the gap when heat treating, as per the method described by Mick above.This does vary with the ring diameter.

In the other direction, the main consideration is the maximum speed that the engine runs at. It seems that for high speed engines, it is better to use thinner rings. It is also critical that the groove clearance should not be too great, but there has to be some. I believe the thinking is that the rings swaps from one side to the other due to inertia loads, so if the clearance is too great it gets a bit of a smack and will break up. On the other hand if the ring is too tight in the groove, there is no room for gas to get behind it, improving the seal. For low speed engines this is all moot

John

[ChrisHParticipant @chrish]

I suppose whether you have the thickness bigger than the width is a bit down to personal inclination and the size of rings you are making. Too narrow a ring and it becomes a bit fraught to handle. The engine I am building specified 0.032" width; I chickened out and up'd that to 0.040" and that still seemed too narrow to me. The pressure of the ring against the cylinder wall is a function of the ring thickness, the thicker it is the greater the pressure in very simplistic terms. You pays yer money and takes yer choice as they say.

What I want to know is why are the rings heat treated during manufacture? I have looked at two methods of making rings, one using heat treatment and one without. Both are very similar methods in certain respects, both make the rings oversize, squeeze them onto a mandrel and then turn to size. The difference is that the heat treated version is made to bore diameter plus a few thou, the rings are stretched apart by about say 0.100 and then heat treated before being squeezed onto the mandrel. In the non heat treat method, the rings are turned a diameter that equates to a circumference made up from the bore diameter circumference plus a 'compression gap'. The compression gap is then cut off and the rings squeezed onto the mandrel for turning to size. The argument for this method is that the rings are turned perfectly round before squeezing; the heat treatment leaves the rings out of round where the gap ends are. Although the rings are then turned true, the argument says, the pressure is now not uniform around the ring unlike the non heat treated rings. I don't know if this is true but it sounds good!

So why are the rings heat treated? Is it to relieve stress, or is it to put a 'set' in the ring to make it spring against the cylinder wall?

Chris

[JasonBModerator @jasonb]

The heat treated rings are machined as a ring that will fit in the bore and you then split them. If left like that they would have no spring to push them against the bore. So you wedge them open and heat treat them which makes them take on the "Set" of the expanded shape and that is what gives them the spring. I don't turn my rings after heating.

Rings turned oversize will not need treatment as they will need to be compressed down to fit the bore.

[ChrisHParticipant @chrish]

Jason. I understand what you are saying, a bit like a third way! You confirm the reason for heat treat is to give the 'set' and 'spring'. Making them oversize, cutting the gap and squeezing them down and machining them I guess does the same job.

Do you check the fit of the rings inside the bore against a back light to see if they are fitting 'tight' against the bore all the way round, and if so, do you have to fettle them to fit tight at all? Just wondering how tight a fit this method gives!

Chris

[Andrew TinsleyParticipant @andrewtinsley63637]

I always make my rings the same diameter as the bore. I then split them, emery the gap, to give the correct ring gap I then wedge open the ring and heat to 500 degrees Centigrade. This gives the rings the necessary spring to push against the cylinder bore. Otherwise you don't get compression and one needs an electric finger to start the blighter. The performance is very poor as well, as the ring is effectively worn out.

Now I am talking about racing .60 cubic inch engines. I have NO experience of any other engines! There are some weird ideas that the compressed gas gets to the back of the ring and expands them. There are other complaints that the rings made as I explained above will have serious friction problems and thus give a reduced power output. NONE of these gripes are true, in the size of engine that I am talking of.

Other styles of engine and sizes may give these statements some credence. But in these 10 cc racing engines, they don't hold. So it depends on what engines you are talking about. As far as the ring width to depth is concerned. I simply use the original manufacturers sizes. Unless I have to make new pistons and then I am not too fussy about dimensions.

There is a lot more to getting rings to work than stated above. I usually hone cylinders to ensure circularity. I also use fine diamond paste to bed down the assembly. The latter improves some aspects of the engine and it can be run at full power straight away without "running in" (I use cast iron pistons for this and then swap to aluminium pistons, for obvious reasons.

Andrew.

[JasonBModerator @jasonb]

Chris I don't check with a backlight but if you don't heat the ring evenly there is a chance that it will take all the set in one place and this can be seen by the colour of the outer edge of the ring when the engine has been run – bright where it is making contact – dull where it is not.

I seem to get the compression I need with this method.

[Anonymous]

For cast iron rings at least the heat treatment of itself does not create the "spring". What it does do is change the natural set of the ring, allowing the elasticity of the material to provide a spring effect when the ring is compressed.

When I've made piston rings I grind to size on the OD and part off slightly over thickness. The rings are then split and heat treated in an electric furnace. The rings are sprung apart and also lightly clamped so that they don't twist during heating. Finally they are reduced to the correct width using wet 'n' dry paper.

Andrew

[Howard LewisParticipant @howardlewis46836]

On "full size" four stroke engines, to minimise blowby, the compression ring gap is usually 1/1000 of the bore daimeter. Do not make the gap so small that the ends butt together when the engine is hot. That is a shortcut to broken rings and ruined bores.

If you wish to have the ring gas backed, (This reduces friction on non firing strokes because of the lower wall pressure used.) make the ring slightly narrower than the groove, but produce both the ring and the groove with a fine finish, to aid sealing. Sealing should improve as the engine runs, as the ring and groove lap together.

A second ring with an upward facing internal chamfer or groove (About 1/3 of the ring width, certainly no more than half) will twist and serve as an additional oil control scraper ring. Again make the groove slightly wider than the ring to allow for the twisting movement.

Howard

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