How I made my piston rings. No heat

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Yes Paul - I had realised what I was missing in understanding what gap you were talking about!

Now my query on your method is reduced to what rational governs the size of the uncompressed ring gap you cut in the mill before the final OD and ID are machined.

Since I first read your first post I have caught up with another article on making piston rings without heat, written by Tom Schwartz, I think for Model Engine Builder, quite a while ago. His article goes into quite some detail on the theory behind his method and gives calculations/formula to calculate what sizes to make rings for different size bores and different ring materials and different fuel. Quite comprehensive. He also gives drawings of the fixtures to enable the rings to be made.

Amongst the formula is one for calculating the size of the initial gap to be milled before the ring is finished, which is the total of the compression gap and the final ring bore gap. The major difference in his method to yours, I think, is that he calculates the size of the ring in its expanded form based on bore diameter plus 10 thou and the compression gap plus final bore gap for the OD, and the ID is expanded OD minus the ring thickness . He then compresses the rings and machines 10 thou to give the OD to size as per your method, but not the ID which has been already set.

I hope I set that out clearly!!


No no, I too did calculate the fully expanded gap before milling it. There was a formula for calculating the wide open gap back in the SIC. days. But as I said earlier IMO, with in reason, the wide open gap is not all that important. What is much more important is that the ring's final OD is perfectly round when installed in the cylinder. The closed ring gap should be near zero once in the bore.

I agree that the ID of the ring is not anywhere near as important as the OD. But if using the method I described its not really much more work to turn the ID in the final ops also.

Note that this is the same method that industry use to make commercial piston rings. In other words the industry also machine the wide open gap prior to final OD and ID finishing. The do not split the ring, and they do not heat treat the ring to form the wide open gap.
I once had a 60s VW beetle that had a piston with a broken top piston ring that was in two pieces, probably from detonation. Anyway it ran like this for many years and I would never have known until the day I rebuilt the engine and found it like this.
So even though the ring was broken in two parts and had zero wall tension when static, it was still round, . It continued to function because the pressures from the rapidly expanding gasses during the combustion process cause the ring to snap out to the cylinder walls this is where most of the sealing comes from.
G'day guys, I thought I would share my method of making piston rings, I have not heard this technique discussed in any the forums, but then again I don't get out much, so it very well may have been.

In 97' I designed and built my own 7 cylinder radial, the heads were inspired by the Hodgson engine, but the rest was all mine.
The engine runs really well, still even today.
The piston rings were made from a good grade of close grain cast iron using a method a local Aussie model builder showed me in person, this before the internet! No heat was used. So no distortion and perfectly round rings using this method.
The method starts by parting your rings off to near width with good meat on the OD and ID. Then in the mill, the ring gap is machined to the desired size. Over to the lathe you make a simple turning fixture with a face clamp that holds the newly milled ring gap closed. I made a tapered sleeve to gently tap over the ring to close the gap, clamp the face then tap the sleeve off, then simply turn the OD of the ring to your desired bore size.
The ID is turned to final size in the same way only with an OD clamping fixture holding the gap closed. I'm sure you get the idea.
I used some fine sand paper on a flat plate with some WD-40 using a figure 8 pattern to give the rings a nice smooth surface on the faces.
These rings worked great.
Each to their own but after using this method I would never consider using any heat to make piston rings.

I've included some of the parts here, folks may find these of some interest.
Cheers Paul
Using this method , would it be possible to make ring from steel instead of cast iron? Could these be used in a cast iron bore? What type of steel would you use? How do you calculate the dia. of the blank before milling the gap? Nothing but questions!
Just to add some more confusion / interest to the topic. Hastings isn't the only company in the game. In this video Perfect Circle perhaps an even older company goes through a process of machining the ring to a precise ellipse so that when it's compressed it becomes round. (at about the 8 min mark). The commentary adds a lot to the understanding unlike the Hastings video with only music.

It would be interesting to do the mathematical FEA analysis of paulc's method of making rings in terms of the distribution of pressure around the cylinder bore of the compressed ring. I suspect there would be more pressure at the sides and closer to the gap than across from the gap.
I think Trimble took this into account in his method. There were a lot of calculations way over my head which lead to the choice of a particular diameter round pin used to open the gap before annealing. Apparently this round pin distributed the stresses in a particular fashion around the ring inside the material. Of course the annealing removed all the stresses but I assume those stresses were then returned again when the ring was compressed. There were a lot of details most people glossed over when reading his article. Probably because they were mired in mathematics. I believe (and may be confused with another method) Trimble also had a final procedure to put the rings on a mandrel to finish turn the rings to ensure the rings were round. Even though they should be round without that step.

Perhaps this is why Perfect Circle (video above) grinds their rings to be an ellipse so that the material is radially thicker in one area so the cylinder wall pressure becomes even around the ring when compressed.
All food for thought. I don't have the answers.
I will add that I have a set of full sized rings from a Ford 302 engine here and I don't measure any radial difference at the gap or across from the gap. Maybe someone that has Perfect Circle rings on hand can figure out where this ellipse shows up. (Or maybe I don't understand the ellipse concept they speak of).
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Hi dsage: You mention the "elliptical" machining process: New to me! Working on engine design in the late 1980s and visiting the Hepolite ring manufacturing process, with Japanese experts on rings, etc, I learned that Perfect Circle were seen by the industry as being a benchmark company, who led the way in steel ring manufacture when it commenced. The rings at more than about 15 degrees away from the gap act like beam loaded and supported at the ends, but as the gap is approached they act as cantilevers. For Cast Iron rings, the finishing (in the factory) was a "shift" (or thereabouts) or running in a slave cylinder (called "Barrelling"), with a stack of a hundred or so rings reciprocated in a bore at ~50rpm from a crank, and rotated by the machine and this removed a tiny amount of excess material (fraction of a thou!) where the tangential load was highest. (- at the free ends of the rings?). Thus the rings were already "run-in" ready for the "perfectly round" bores they would be fitted into. I GUESS, the "elliptical" machining actually reduces the section a little as it approaches the gap, but leaves the diametrically opposite a fraction of a thou thicker, to reduce deformation (and TAN loading?) opposite the ring gap? "I'm sorry I haven't a clue"!
Conversely, steel rings were not so treated. They were formed from wire (Constant cross-section): with a strange custom cross section (Flat on 3 sides and curved on the cylinder-bore contacting outer face pre-formed on the wire before making circles). When wound into rings - first by winding a coil, splitting, laser marked "top" (relating to the shape of the curve on the outer face) then finished (individually) on the final "bender", they were given a non-round shape, so that when compressed into the bore the Tangential load would be constant all the way around the ring. I.E. I think the ends were bent inwards very slightly over a specific curve, so the transformation of load from "Beam" to "Cantilever" gave a constant tangential load. - The rings were finally Nitrided to harden the surface for added durability. Steel rings were less than half the thickness of cast iron rings. I think it was a high tensile steel, almost like piano wire? The curved shape on the outer surface gave a high contact pressure (almost a line contact), yet a low tangential load, compared to cast iron rings. = Lower frictional losses.
But my memory is >30 years old on that one, so please excuse any errors I have made?
My fellow Aussie, bluejets, You have stumbled into my thread, where I discuss making rings cold, using no heat treatment.
It's not a poll!

Seeing as you mentioned it, I wonder how much more beer I can buy from what I saved not having to purchase an oven?
But at least I fixed your ignition design, she sparks now right? ...hehe
I buy rings from Hastings Piston Ring company $1 per set. They have all bore diameters, all thicknesses & all designs. I can't make a set of rings for $1. Rings are sold by, bore diameter, thickness, design, not by engine model number or engine serial number. I bought 8 sets of rings to rebuild an engine for $9 plus $6 postage. At age 72 I don't have time to make my own rings I might be dead tomorrow.
Like the way you roll!! Down wit that.......
It would be interesting to do the mathematical FEA analysis of paulc's method of making rings in terms of the distribution of pressure around the cylinder bore of the compressed ring. I suspect there would be more pressure at the sides and closer to the gap than across from the gap.
Dave, I asked a friend of mine, who's career was mechanical design with lots of familiarity with FEA software, to do exactly this, we set up a perfectly circular gapped ring with uniform outward stress around its circumference against a solid cylinder, then removed the cylinder to see what shape the unconstrained ring took, but we were in a rush and kept running into software issues that we didn't have time to resolve before he had to leave town (we don't live near each other any more), so never got the results, but someone should definitely re-do and finish this experiment, to see what circular or elliptical or other mathematical shape best approximates the result. Pete.
I have made rings from cast iron, and just cracked the joint and installed them. As the bore size was under an inch, when they wore in they probably had enough of a gap. SOme of this is all sweating over details that really will make no difference in the real world... And I have seen many hobbiest posts where tey are sweating over tenths, making 5 pistons to get the "perfect" one... Relax, if it works, and lasts and performs well, then it is probably "perfect" for the job.
I have made rings from cast iron, and just cracked the joint and installed them.
So you would have to machine them to just under the bore size to install them. Can't machine them larger than the bore size since you have no gap to compress. When cracked and installed there would be no spring tension from the ring to the cylinder wall. What am I missing?

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