yet another ring design thread, hot vs cold forming

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peterl95124

Well-Known Member
The Trimble "hot" method for forming rings results in a ring with uniform contact and uniform wall pressure all the way around it when it is compressed into the cylinder, when the ring is not compressed into a cylinder it is not a perfect circle, if it were Trimble could have gotten away with just a perfectly round mandrel during heat treat and eliminated the dowel.

this makes me wonder about all the cold methods I've read about for making rings, and rather than trying to "do the math" here's how I think about them, what they're doing seems to me to be analogous to as if Trimble had used only a perfectly circular mandrel and not a dowel in the gap during heat treat.

the idea is you start with a ring who's circumference is oversize by about the same amount as the Trimble dowel, then cut out that much for the "when open" gap, then squeeze that ring into a cylinder (as part of centering the ring in a jig to hold it while turning the OD) so that the gap is closed. and there's the rub --- to squeeze the ring into a perfectly round cylinder it won't result in the "uniform wall pressure" that Trimble's method results in, in fact it most of it won't touch the centering cylinder, and those non touching portions are receiving zero inward force so after machining the OD they will have zero outward (installed cylinder wall) force.

to obtain Trimble's "uniform wall pressure" the ring would have to be contracted by *pulling* the gap closed (the opposite of Trimble's pushing the gap open) along the "zero stress line" of the ring (essentially its mid diameter for all practical purposes) without any other forces, in particular no inward forces on the sides of the ring by any jig centering cylinder, and then machining it. but I don't think even superglue could hold a ring closed at the gap. I think perhaps a tight string around the ring holding it shut would have the right effect or near enough so, but I don't see any practical way to do this either.

and yet lots of people have made "cold" rings and claim they have worked and I have no reason to doubt them.

perhaps the cold formed rings work because they seal good enough for the intake stroke, and I think everyone agrees that it doesn't matter for the compression and expansion strokes because then its the high pressure in the cylinder getting behind the ring that provides the vast majority of the wall sealing force. And perhaps because no one runs a model engine long enough to notice that they wear unevenly due to uneven wall pressure (or maybe they don't because most of the wall pressure is from high cylinder air pressure).

thoughts ?
opinions ?
here's the math ?

Last edited:
Hi Peter !
I made rings for my engine including heat and cold
The truth is I made cold rings before I read the topic about cold rings - When my diesel was running for the first time for about 15 seconds and with too small tolerance it resulted in piston-rings-cylinder jams. I made a new ring and I was too lazy so I made a cold ring similar to the cold ring topic
Up to today, I have made more than 12 cold rings (1 piston 2 rings x 6 pistons) and they are all good.
My personal opinion, Each ring making method has its own good and bad. Sometimes this method is more suitable for a certain diameter range
Cylinder - piston + "ring" : are the three basic elements for compression
For me, the cylinder is the key and when my cylinder compresses well with a ringless piston I know it will be better with a ring - no matter what method the ring is made of.
I don't know if my rings have more compression at compression stroke, but I usually make rings small in thickness and height and that increases the chance of pressure compressing the ring into the cylinder - but I'm not sure and I can't prove it .
With rings made too thick, too high, and there is an area of the ring that is not making good contact with the cylinder, I do not believe the pressure in the cylinder is capable of making that area better contact. Unless you run long enough with hand drill ...(a few hours ,a few dozen hours or ...- I don't know) for the rings to wear out
Again , Just personal opinion

I have followed the ring discussions for years, and made my own gray iron rings for steam engines, using both the cold and hot-sprung methods.

I prefer the cold method, which is to machine the ring from Class 40 gray iron round stock, oversized, cut an appropriately sized gap, compress the ring to close the gap, install the ring (closed) in a mandrel that clamps it closed, and then machine the ring to final size, assuming the typical allowance for heat expansion (ie: leave a bit of gap when the ring is in the cylinder, so that when it heats and expands, it does not seize in the cylinder).

I have heard of machining rings in an assymetrical section, but I am not aware of anyone such as auto engine manufacturers, or small engine manufacturers using such a ring. Is anyone aware of an example of this in common use in an engine?

So if the typical ring that is made worldwide for most combustion engines has a constant section, and it works very well (my car never uses oil), then why change from that?

I understand why some use the expand and heat method, and my only objection to that is that I got some warpage that I had to straighten out, and I would prefer not to deal with any warpage at all.

Some use o-rings as piston rings, and it that works for your purpose, then more power to you.

My personal preference is a Class 40 gray iron piston ring, that is turned oversized, gap cut, then turned to final size in a mandrel.
This type of ring works very well, and with a bit of honing of the cylinder walls, will last a very long time, even under full load.

Theoretical discussions are good, and interesting, and I like to understand all the posibilities of a given design, but in the end, just as I do in the foundry world, I look for a straightforward solution that works well every time with any engine type.

There are some good articles in MEB about turning rings oversize, and then compressing them and turning them to final size.
Someone else will have to point out what the numbers on those articles are, but I do recall reading them.

.

Tom Schwartz's MEB #27 article was "Free Gap without Annealing" . William Shaefter had an o-ring article in #15.
There are other ring related items in various editions.

Philip Duclos wrote a build article for his "Odds 'n Ends Hit 'n Miss Engine" published in Home Shop Machinist. He described
shop made gears and piston rings. Seemed to me his approach was more about practical machining than mathematically derived
dimensions (using the same fly cutter to cut both gears).

Theoretical discussions are good, and interesting, and I like to understand all the posibilities of a given design, but in the end, just as I do in the foundry world, I look for a straightforward solution that works well every time with any engine type.
That's how I think and do my engines.
When I make the cold ring, my thought is the ring won't have a large expansion force to press against the cylinder wall compared to the hot ring but it won't be a problem, why?, because I used an o-ring a long time ago I have to make the cylinder very smooth with the o-ring (if it's not smooth I can't run the engine because the friction is too great and if the engine can run the o-ring will wear out quickly or it will be damaged. ) - which proves that the o-ring presses very little against the cylinder wall - it's like a trap of air
So, with CI rings it is enough to press lightly against the cylinder wall, and with 2 rings they will be even better.
And currently, my diesel engine uses 8 cold rings and they have good compression.
But ......rings are made by cold method...it seems to break more easily when assembled - I'm not sure, because maybe the ring size...maybe it's a little different after a few ring making - I think it would be better if it was heat annealed to relieve stress.
I don't know which is better hot ring or cold ring - and I can't prove it. Choose a method that feels best to you and works for you is enough

I recall Kozo Hiraoka illustrating a nice gray iron overlapping end piston ring design in Live Steam Magazine.

I have never tried the overlapping ends, but after reading Kozo's method, I believe I could do it successfully if I tried it.

I aways have admired Kozo's magnificent hand-drawn graphic illustrations, and I have tried to mimic that fabulous style a bit in my engine illustrations.

Years ago, I started making engineering drawings using pencil and ink on vellum, and so I appreciate what an artform the work that Kozo does is, and also appreciate how difficult it is to get that look and feel to a hand-drawn illustration.

It is very difficult to not smudge an ink drawing, and making mistakes can be rather unforgiving.

I have seen a few photos of Kozo at his drawing board at his home in Japan.
He is an impressive and inspirational illustrator and engine builder for sure.

.

And perhaps because no one runs a model engine long enough to notice that they wear unevenly due to uneven wall pressure (or maybe they don't because most of the wall pressure is from high cylinder air pressure).

thoughts ?
opinions ?
here's the math ?
With the hot and cold rings I made, I never tried to put a lot of pressure on the cylinder wall
With the hot ring, I only widened the gap by about 1->1.2 mm. This keeps the outer diameter of the ring from changing much - especially the opposite side of the gap - as well as ensures it makes better contact with the cylinder wall - but it has another problem: difficult to assemble rings and and the ring breaks easily when assembled
Again . Rings homemade - each method of making rings has its own good and bad
And I just focus on cylinders rather than rings. That's my way and it's a bit different from everyone .

I have always used the hot method with a final skim to diameter having aligned the rings on the mandrel with a sizing ring as described in 'Model Engineer's Handbook' by Tubal Cain. This book also has several pages on the design calculations and wall pressures for steam and IC rings.
I agree with Minh that the cylinder quality is most important, mine are finished by lapping with 40 micron diamond paste using an Acrolap. This seems to work as my diesel has a measured compression pressure of over 30 bar (450psi) with two rings.

I've made my first batch of cold-process rings, and when installed into a cylinder they definitely don't touch the cylinder wall all around, near the gap there's "light" clearly visible between the ring and the cylinder.

I've no idea if they would "wear in", or if they would work anyway from gas pressure,

but I'm probably going to make a second batch, first sanding a couple thou off the periphery at the gap before inserting in the sleeve and on the mandrel for final turning and polishing, this I hope will result in a ring with better shape and spring, and less "light" when installed

Oddly I had the same problem with Trimble's hot-process rings which in theory at least don't have this problem, which I solved by lapping them with a dummy cylinder, I might do that with the first set of cold-process rings too.

so many variables, so little time...

I have to guess that cast iron rings would wear into a cast iron cylinder, but I am not sure just how much wear-in you can expect.

I recall my dad rebuilding a John Deere Model "H" tractor, and it was very difficult to start (had to start it by spinning the flywheel by hand), and it did not idle well.

Then my truck got stuck in the mud a few miles from the house (don't ask why I was in the middle of a mud field in the middle of the night, but we use to hang out anywhere to get away from the parents).
I walked home, got the John Deere, and drove it full speed to my truck, pulled out my truck, and then drove the tractor back to my dad's house.

My dad was furious that I was dumb enough to drive a tractor around on the streets at night with no lights (LOL, that was one of the more safe things I did as a teenager).
The next time my dad went to start the John Deere, much to his suprise the compression was way up, the engine turned over very easily, it started easily, and idled easily at a low rpm.

So the rings seated in my late night jaunt.

Dad was much happier after discovering how well his tractor started and ran.

.

The Trimble "hot" method for forming rings results in a ring with uniform contact and uniform wall pressure all the way around it when it is compressed into the cylinder, when the ring is not compressed into a cylinder it is not a perfect circle, if it were Trimble could have gotten away with just a perfectly round mandrel during heat treat and eliminated the dowel.

this makes me wonder about all the cold methods I've read about for making rings, and rather than trying to "do the math" here's how I think about them, what they're doing seems to me to be analogous to as if Trimble had used only a perfectly circular mandrel and not a dowel in the gap during heat treat.
Here's a video of ring making, a bit different size but no heat treating!

I recall Kozo Hiraoka illustrating a nice gray iron overlapping end piston ring design in Live Steam Magazine.

I have never tried the overlapping ends, but after reading Kozo's method, I believe I could do it successfully if I tried it.

I aways have admired Kozo's magnificent hand-drawn graphic illustrations, and I have tried to mimic that fabulous style a bit in my engine illustrations.

Years ago, I started making engineering drawings using pencil and ink on vellum, and so I appreciate what an artform the work that Kozo does is, and also appreciate how difficult it is to get that look and feel to a hand-drawn illustration.

It is very difficult to not smudge an ink drawing, and making mistakes can be rather unforgiving.

I have seen a few photos of Kozo at his drawing board at his home in Japan.
He is an impressive and inspirational illustrator and engine builder for sure.

.
My projects in my drafting classes (1965 to 1970) always ended with ink on vellum, including the hand lettering (anyone else remember TOM Q VAXZY? ). I’m not sure I could still ink with a lining pen, or even think of a reason to try! Nothing more fun that watching the ink run under your triangle on the vertical lines…

If you’re doing that kind of work GreenTwin, I’m definitely impressed!!!

John w

I have always used the hot method with a final skim to diameter having aligned the rings on the mandrel with a sizing ring as described in 'Model Engineer's Handbook' by Tubal Cain. This book also has several pages on the design calculations and wall pressures for steam and IC rings.
I agree with Minh that the cylinder quality is most important, mine are finished by lapping with 40 micron diamond paste using an Acrolap. This seems to work as my diesel has a measured compression pressure of over 30 bar (450psi) with two rings.

Roger, have you done the math, a 20:1 compression ratio is the "volume ratio", which translates to a "pressure ratio" of 66:1 (65 bar), or about 950 psi, and a "temperature ratio" of 3.3:1, or about 1280F. measurement technique at these high CR is critical, also in my Hansen Diesel it looks like 20:1 vs 21:1 is .004" head clearance difference so not sure how I'll machine that accurately. Peter.

For a 2-stroke .60 ci engine that I have posted on here, my ring was an L-shaped ring (my interpretation of a Dikes Ring) and it was turned to bore size, scored and cracked. I polished the crack slightly and ran the engine. That ring, under no compression barely drags the cylinder wal, but because of the L-shape, under compression it seals nicely. And after a bit of run it was polished uniformly all around.

I think that your figures refer to pure adiabatic compression. Small engines will have significant heat losses, especially when cold starting which will reduce those values.

I started out using the information that a ’full size’ direct injection diesel will cold start with around 14-1 compression. As the heat loss in a small cylinder will obviously be significantly more I decided to aim for 20-1. It then starts to get difficult. The cylinder filling will not be 100%, at higher speeds the losses in the inlet tract will reduce the pressure pushing the air into the cylinder and at low speeds the closing point of the inlet valve will be the start of the actual compression reducing the theoretical value. I achieved 5 bar more compression pressure by changing the camshaft.

Experiments with the two stroke variant showed I needed ~30 bar for consistent ignition. Making different volume cylinder heads was quick and easy. The actual compression ratio on a two stroke is even more difficult to determine, measuring the pressure in the only sensible way to know what is going on.

Calculating the volume of the compression space is difficult. I have a flat head with protruding valves and piston with a central bowl and cut outs to clear the valves. Luckily I got quite close at the first attempt. I actually think that the compression is a bit too high so I will make some 0.2 or 0.3 mm copper shims to increase the head gasket thickness (currently 1mm). This will also increase the negligible clearance between the valves and the piston at overlap when the valve are both around 1mm open (full lift 2mm). The valve clearance is set to just avoid contact.

I added a 0.5mm shim to the 1mm head gasket on this engine. It reduced the measured compression pressure by 5 - 6 bar. This is the combustion chamber layout:

I added a 0.5mm shim to the 1mm head gasket on this engine. It reduced the measured compression pressure by 5 - 6 bar. This is the combustion chamber layout:

looking good !, eager to see more details on the injection pump and injector, because...

my gasoline w/ spark ignition Hansen engine just hit the "first pop" milestone, still lots of
cosmetic work left to do, but now I can turn my attention to the diesel version

and I'm using cold-process rings for both, only a minor amount of lapping in a dummy
cylinder got them looking pretty good

I have been working on fuel injection systems for a few years. Every sordid detail is documented on MEM if you are on there. I can post some stuff on here but maybe I should start a new thread.

This is one of the last runs with a needle injector.

I have been working on fuel injection systems for a few years. Every sordid detail is documented on MEM if you are on there. I can post some stuff on here but maybe I should start a new thread.

This is one of the last runs with a needle injector.

I am not on MEM, and cannot access photos and such that are posted there, so that site is a blank to me, but would like to see your fuel injection work.

Pat J

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