Piston sealing devices, iron rings vs. everything else

[gbritnell]

Gentlemen,
I have been building engines for a number of years and I have read about and talked to many builders over these years about piston to cylinder sealing techniques. I started building steam engines with a graphite packing in a groove in the piston, Stuart 10V, 10H etc. I then purchased a Stuart Twin Launch engine. This came with a set of iron rings. I machined my pistons for them and mounted them but this engine only ever operated on air at shows so I couldn't judge the quality of cylinder sealing. If a steam engine leaks a little past the rings it's not the same as an IC engine. My first IC engine was the Paul Briesch Little Brother hit and miss engine. His drawings called for a tight fit of the piston with I believe a couple of oil grooves on the piston. I don't have my drawings any longer so I'm going from memory. I met Paul at a show one time and talked at length about his engines and engines in general as I was just learning about IC engines. He had his engine running at the show (I presume without rings as per his design) and it ran quite well albeit kind of messy as he dripped a lot of oil into it. My Little Brother would run but not great. As I learned more I changed a few things to his original design which helped the performance but it still never ran great. Feeling that compression was part of my problem and not having any way to exactly measure the pressure I decided to make a set of rings for it. Now I had heard of other modelers using 'O' rings but was highly skeptical to use them for several reasons. First and foremost is that although 'O' rings are used for cylinder sealing devices they aren't generally used in high speed movement applications such as an engine piston running back and forth but more for a hydraulic piston sliding inside a cylinder. Second is the drag factor. If you try to measure an 'O' ring it's darn near impossible. You can turn up a bar to what is supposed to be the I.D. and keep test fitting the ring until you get what you feel is a 'good fit' (whatever that might be), measure the 'O' ring thickness and that will give you the O.D. But now the question arises, how much difference between bore and cylinder wall do you allow for the 'O' ring?, too much and you have no seal, too little and the rings drags and creates more friction than the engine can overcome so now you have another problem. At this point I'm not saying that they won't work I'm just saying that it seems very trial and error to make them work effectively. Now back to my direction. I had started purchasing Strictly I.C. magazine from early on and an article was published by the author George Trimble on making iron piston rings. I followed what was outlined in the article and made rings for my Little Brother engine. Boy oh boy what a difference in compression! I have the original set in that engine and have probably 20 hours of running time on it and the compression is still great. My compression gauge is flipping the engine up on compression and seeing how well it bounces back. Kind of a seat of the pants gauge. I have since read about other methods for making iron rings, with each author expounding on his design. I'm sure they all have their merits. I can say that I have used one other method but it seemed quite fiddly to me so I still use Trimble's method. Now if someone doesn't want to go to the effort of making iron rings and would like an alternative to 'O' rings I have another method. In the process of making my miniature spark plugs with Teflon insulators I thought about making a Teflon piston ring. This would provide simplicity of manufacture, accuracy of machining, heat environment tolerance and the anti friction quality of Teflon. Just get a piece of Teflon large enough to make your ring, drill it, bore it, turn it and cut it off. Ah but you say, how can I get it onto my piston? Take a sharp utility knife and cut it open. Personally I recommend cutting it at an angle. Have I tried it? Yes I have. Does it work? Yes it does. How do they compare to iron rings I can't honestly say. I have used them in steam engines and one I.C. engine, which I don't run a lot, but they do work, and better than 'O' rings.
Thanks for your time,
gbritnell

 

[Maryak]

Just a thought but if O rings or a system other than conventional piston rings was worth while pursuing, I think engine manufacturers would be using it. ???

Best Regards
Bob

 

[shred]

FWIW, Kozo Hiraoka uses O-rings on the steam pistons and glands of his excellent locomotives and has a section on sizing them for sliding steam use in the back of his New Shay book. I used his #'s to O-ring my Paddleducks engine and it works well as best I can judge. I'm not thinking they'd work well for IC engines though-- the gas temperatures are likely to be far hotter and more abusive.

I like the teflon idea too... must try it sometime.

 

[Engine maker]

Jerry Howell's new IC engine "Farm Boy" uses an "O" ring for the piston ring. Having built one I can say that it works excellent! Perfect compression immediately, no break-in required. Mine now has about 35 hours of running time on it and shows wear or loss of loss compression. If it ever dose, "O" rings are cheap. Another thing that won't often happen with a cast iron ring, I can put the piston on Top Dead Center and come back to it tomorrow and still have no loss of compression. It impressed me so much that I went back and changed a few other engines I have to the "O" rings and each was an improvement! Another thing is that the bore doesn't have to be perfectly parallel. The finish only needs to be as smooth as you can get it and you have to keep it lubricated, which is the same as you would do with a regular ring. If you want to be even more adventurous you can use an "X" ring.

As for size, Jerry uses a 1' OD x .094" Dia. "O" ring in a .110" deep x .105" wide piston groove. The piston OD for the 1.00" bore is .995" ahead of the ring and .998" behind the ring (for the piston skirt.)It seems like as the engine runs the "O" ring swells an makes even a better seal.

 

[Jadecy]

Interesting thought George and thanks for the test data. I may have to try that.

As to the comment about "wouldn't manufactures have changed if something else was better?" - We as model builders don't often built engines that see the type of use of a production engine and some of us don't want to go through the process of making our own cast iron rings. Depending on the size there are a couple vendors that sell pre-made rings. DeBolt sells some cast iron rings and if you want pre-made teflon rings PM Research sells them. They come in some of the steam engine kits and can be purchased seperately if you want some.

 

[bentprop]

Just a thought but if O rings or a system other than conventional piston rings was worth while pursuing, I think engine manufacturers would be using it. Huh?

I think that boils down to cost,Bob.Every cent they save on manufacture is more profit.Cast iron is cheap,Teflon isn't.I assume you,re talking of car engines,etc.O-rings in such sizes would possibly cost more than ci as well.Or perhaps it's a speed issue.I don't know of any model steam engines that get up to 6000 or more revs.
It's an intriguing thought all he same.

 

[Lew Hartswick]

They do make PTFE "O" rings too.
...lew...

 

[robcas631]

Where can you find the correct cast iron for rings? I've read it must be a certainty type. Another site indicated that a soft pack might be another good alternative. Also, being that I have two different types of cylinder walls, one steel for the main and the other brass for the steam chest. I believe brass is giving me a lot of issues having ports is a factor! I've thought of steel lining it as well.
Rings are extremely important. I thank Gbitnell for the information provided.

 

[robcas631]

What is PTFE?

 

[barnesrickw]

PTFE is interesting. The Cox .049 diesel conversion have a small PTFE gasket between the contra piston and combustion chamber, so rings may work too. I don't know the cylinder temperature of the combustion chamber in these engines. I know diesel itself combusts below the melting point of Teflon, but other things happen in combustion chambers with compressed O2, and evaporative heat loss cooling, etc.

 

[cncjunior]

PTFE is teflon. The term PTFE is more common usage in Europe, whereas teflon is a North American name.

Daniel

 

[stevehuckss396]

Where can you find the correct cast iron for rings? I've read it must be a certainty type. Another site indicated that a soft pack might be another good alternative. Also, being that I have two different types of cylinder walls, one steel for the main and the other brass for the steam chest. I believe brass is giving me a lot of issues having ports is a factor! I've thought of steel lining it as well.
Rings are extremely important. I thank Gbitnell for the information provided.

McMaster Carr has cast iron. Also many of the metal suppliers have it. Just look for "gray" or "fine grain" and it should work for you.

http://www.mcmaster.com/#cast-iron/=q3h70o

 

[Walltoddj]

Reading the post I see a couple of things that might help.
1 in non combustion engines orings, packings, and PTFE will probably work well for rings or for gasket, but will harden and become brittle in a combustion engine.
2 if you don't want to make your own cast iron rings I found hyd cyl piston rings in various sizes and they over lap each other to help seal they can be found at Motion Industries, Kaman Bearing, or companies of that types.
Todd

 

[gbritnell]

Over the years I have read many threads about modelers having trouble getting good compression with their I.C. engines. Usually the biggest problem is valve/seat machining and sealing. In my personal opinion if a model engine builder can machine to tolerances good enough to solve these problems then making piston rings should be a piece-of-cake.
I have used several different methods and have settled on the Trimble method of making them.
All that's required is some fine grained iron, (Durabar, Schedule 40, continuous cast to name but a few.)
I fixture needs to be made to expand the ring and hold it while it's brought up to annealing temperature.
The machining is straight forward. Turn the I.D. to size. Then turn the O.D. to size or +.0005. Never undersize.
Part off up to .002 wider than nominal then hone it down to the proper thickness. It's too hard to part it off dead nuts.
Each of these steps are much easier than turning valves and seats and making them seal.
Give it a try! You'll never know until you do.
By the way, here's a link to a great article on making rings.
http://modelenginenews.org/techniques/piston_rings.html
gbritnell

 

[barnesrickw]

Over the years I have read many threads about modelers having trouble getting good compression with their I.C. engines. Usually the biggest problem is valve/seat machining and sealing. In my personal opinion if a model engine builder can machine to tolerances good enough to solve these problems then making piston rings should be a piece-of-cake.
I have used several different methods and have settled on the Trimble method of making them.
All that's required is some fine grained iron, (Durabar, Schedule 40, continuous cast to name but a few.)
I fixture needs to be made to expand the ring and hold it while it's brought up to annealing temperature.
The machining is straight forward. Turn the I.D. to size. Then turn the O.D. to size or +.0005. Never undersize.
Part off up to .002 wider than nominal then hone it down to the proper thickness. It's too hard to part it off dead nuts.
Each of these steps are much easier than turning valves and seats and making them seal.
Give it a try! You'll never know until you do.
By the way, here's a link to a great article on making rings.
http://modelenginenews.org/techniques/piston_rings.html
gbritnell

Great reply. Thanks.

 

[Walltoddj]

Has anybody thought of using Dyke rings or some may call them L rings. They are L shaped and fit at the very top of the piston the compression helps to push them out to seal better. They worked great in my go-kart engine k88, k77, B bomb, and BM they were single ring Dykes chrome moly coated faces. The pistons were from 48-55 mm so they are not a lot bigger than the model engine pistons.

Todd

 

[lohring]

My piston sealing experience is on model glow and ignition two stroke engines from .049 cubic inches to 36 cc. By far the lowest friction system for the moderate temperatures of small glow engines is a high silicon aluminum piston in a carefully tapered chromed brass or aluminum cylinder. Pistons are fitted to have up to .007" (.18mm) clearance at the bottom of the stroke to 0 clearance at the top. Most engines that use this system have less than a 1" (25 mm) bore. See Jim Allen's gallery (sign in required) for a pictorial description of what it takes. A look through the rest of the gallery shows the result of years of engine development.

This has been tried on a 1.38" (35 mm) bore gasoline burning ignition engine and does not work. There, the best system is still a single, thin, very hard ring running in a hard chromed or Nikasil coated bore with a brass or aluminum liner. A lower cost alternative has been a nitrided steel liner. Piston clearance is still low and a lot of oil in the fuel helps seal the bore. I don't know the ring material and commercial rings are so inexpensive I haven't considered making them. Jim Allen has made cast Iron rings that run in a chromed liner on a 26 cc gasoline engine. There doesn't seem to be a clear advantage, but an accurate and round bore is essential.

Lohring Miller

 

[robcas631]

Thank You cnc!

 

[robcas631]

Gb thank you. Also, I think a reference video should be made showing the various ways pistons and rings are made.

 

[stevehuckss396]

Over the years I have read many threads about modelers having trouble getting good compression with their I.C. engines. Usually the biggest problem is valve/seat machining and sealing.
gbritnell

I would agree with this one George. I also think that some might not properly fit there piston thinking the ring will make up for it. I have actually seen posts on forums to that effect. Just to spotlight the importance of piston fit, Bob Shutt built the Shores Peewee about 5 years ago. He assembled it without rings figuring he would put them in later. To this day they are still not in there and it still runs very good. As far as good piston fit, he nailed it. If you have a problem not running because the rings are bad, you might want to make new pistons while you are at it. Should run without them