# O-ring for piston ring



## Master (Jul 15, 2020)

Decided to try starting my Jerry Howell Farm Boy.  I used the recommended vinyl o-ring in place of a cast iron ring.  Almost no compression.  Has anyone had success with these o-rings?


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## Engine maker (Jul 15, 2020)

Master said:


> Decided to try starting my Jerry Howell Farm Boy.  I used the recommended vinyl o-ring in place of a cast iron ring.  Almost no compression.  Has anyone had success with these o-rings?


I built Sn # 003 (the Yellow one you see everywhere) and never had a problem. Has hundreds of hour's on it. I've seen some people have problems. First is the bore on spec. at 1.00" and smooth and shinny? Second did you make the piston groove to the spec. Most of the ones I've seen changed the specs to what they think is correct and they make it too tight. The groove needs to be 0.105" wide and 0.110" deep. The ring is NOT squashed in the groove, it kind of floats. Next try replacing the "O" ring with a new one with a little light oil on it. I've seen engines that sat for a few days loose compression. For some reason the ring shrank, (haven't figured that one out yet, but I'm working on it). Last are you sure you have good valve and spark plug sealing?

I have a Hired man engine that called for 2 cast iron rings. I put 2 "O" rings on it instead.  I couldn't pull it through the compression stroke. Had to remove one "O" ring and now I can pull it through but it still has too much compression. So I know that the "O" rings work great. I put them on all my Hit and Miss engines now. Even went back and changed cast iron rings out on the engines I had already done. They run much slower and cooler.

Jim G


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## Jasonb (Jul 16, 2020)

As said you can't just put a Viton ring in a groove sized for an iron ring you need to size it to suit the O ring and don't use the generally published data as that squashes it too much, Model Engineers Handbook and Reeves catalogue shave the right sizes for out use.

Last 5 IC engines that I have made all have Viton rings and make good compression, that's 2 stroke and 4 stroke, spark and hot tube igniton also use them in steam engines


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## Noitoen (Jul 16, 2020)

You could try a X ring


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## popnrattle (Jul 16, 2020)

In my engines bore is 1-3/8". My piston is .002-.004 slip. I use a Sunnen hone to get the bore straight after boring on lathe, then get a cross-hatch with the 500-grit stone(J-95). I get 1-3/8" "plumber's" O-ring with 3/32" cross-section(measures .100" at local hdwe store. Then, I make my ring groove .110 wide so the ring can rock slightly at TDC and BDC. I make the groove dia. 1.185( so that there is .005/side pressure). Also, be sure to slightly chamfer where you push piston and ring in and careful not to install such that the ring has to go past the hole for the oiler. The sharp edges of the oiler hole could nick the o-ring. I get a year(+) of abuse out of each ring and replace them then when I take off the head to remove carbon deposits anyway. Hope this helps. Later,rt





.https://www.youtube.com/user/popnstart/videos


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## dsage (Jul 16, 2020)

I  presume some of your sideways squish distorts the O-ring and removes some of your clearance in the ring groove? Do you think it can still "rock" at TDC and BDC as you state?
 I was concerned that without a bit of side pressure in the groove the rings might "roll up" as it slides in the cylinder.

I always wondered about the required side clearance in the groove. I looked at quite a few O-ring suppliers websites and all of them spec'd quite a bit of clearance in the groove for sliding seals. I guess they know what they're doing. Nice to see your confirmation of that.


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## popnrattle (Jul 16, 2020)

dsage said:


> I  presume some of your sideways squish distorts the O-ring and removes some of your clearance in the ring groove? Do you think it can still "rock" at TDC and BDC as you state?
> I was concerned that without a bit of side pressure in the groove the rings might "roll up" as it slides in the cylinder.
> 
> I always wondered about the required side clearance in the groove. I looked at quite a few O-ring suppliers websites and all of them spec'd quite a bit of clearance in the groove for sliding seals. I guess they know what they're doing. Nice to see your confirmation of that.


you are correct in assuming they would start "rolling" as I tried a 1/16" cross-section ring with those dimensions I used with the 3/32"(.100) and the engine never seemed to run right and seemed the O-ring never seemed to "break-in"(new O-rings have a little more resistance). The 1/16" O ring had been twisting in the groove so I abandoned that size O-ring. I'm thinkin' with the pressure against the cylinder and inner piston groove may distort the o-ring and close up some of the side clearance with the groove.


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## Brian Rupnow (Jul 16, 2020)

I use a 1/16" cross section viton o-ring (which is actually 0.070"0. I cut the piston groove 0.094" wide, (which is the width of my cut off tool) and make it 0.058" deep. I have made over 10 i.c. engines this way and never had a problem with compression.---Brian Rupnow


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## Master (Jul 17, 2020)

I may have gotten myself stuck.  The cylinder of my Farm Boy is .980.  Not exactly 1 inch.  Removing the cylinder and making a new piston is not a major job.  Just don't want to have to remake a new head too. Making a cast iron piston ring is easy.  Did want to try the o-ring system tho.


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## Jasonb (Jul 17, 2020)

That's just about 25mm you could use a 2.4mm section x 20.6mm ID metric Viton 75 ring in that


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## BobsModels (Jul 17, 2020)

Here is another thread on the subject, I posted some clearance information in a PDF.






						Which O-Ring for piston ring?
					

There are so many out there, I have no clue which to use for a piston ring.  The one I got at the hardware store lasted about 10 minutes of run time.   McMaster-Carr has a huge selection of types? https://www.mcmaster.com/o-rings




					www.homemodelenginemachinist.com
				




Bob


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## dsage (Jul 18, 2020)

Brian Rupnow said:


> I use a 1/16" cross section viton o-ring (which is actually 0.070"0. I cut the piston groove 0.094" wide, (which is the width of my cut off tool) and make it 0.058" deep. I have made over 10 i.c. engines this way and never had a problem with compression.---Brian Rupnow



Your dimensions seem to be consistent with those from the calculations provided on the Parker Seals (and other) seal company websites for sliding seals. I would have thought the 20thou or so side clearance would lead to "rolling up" of the ring. But the squish in diameter probably removes some of the side clearance. Your experience confirms the theory. I like that approach. I'm a sort of "try it and see" sort of guy. 
Thanks


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## mrehmus (Jul 19, 2020)

Here is a PDF of Tom Stuart's article from _Model Engine Builder_ magazine on using O-Rings. He was very successful with them.


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## coulsea (Jul 19, 2020)

It is hard to go wrong with an O ring, take the head off and put your hand over the end to check compression. the problem is probably valves.


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## Misterg (Jul 19, 2020)

Master said:


> The cylinder of my Farm Boy is .980. Not exactly 1 inch



Just a thought - the smaller bore isn't causing the o-ring to kink as it's fed into the bore, is it? (As @Engine maker pointed out the ring groove clearances on the Farm Boy drawing are relatively large (and don't adhere to any of the recommendations above), so there's the opportunity for the o-ring to buckle away from the cylinder wall if the bore is under-sized for the ring.)

Could it just have machining / running in debris on it that is preventing it sealing?


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## popnrattle (Jul 20, 2020)

dsage said:


> Your dimensions seem to be consistent with those from the calculations provided on the Parker Seals (and other) seal company websites for sliding seals. I would have thought the 20thou or so side clearance would lead to "rolling up" of the ring. But the squish in diameter probably removes some of the side clearance. Your experience confirms the theory. I like that approach. I'm a sort of "try it and see" sort of guy.
> Thanks


Brian, what was the bore ranges of the engines you have had success with the the 1/16"(.070") O-rings? Thanks. Later, rt.


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## Vietti (Dec 12, 2020)

Brian,

I've been trying o rings for the first time.  I was told to fit them 'sloppy"

I was interested when you said you put a .070" ring in a groove only .058" deep, is this correct?

I've been trying quad rings with a few thou clearance in both the width and groove.  So far seeing modest compression improvement and less oiling.

John


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## Peter Twissell (Dec 13, 2020)

The general rule for o rings is to target 20% compression for static applications (where there is no motion against the o ring) and 10% compression for dynamic applications.
A 0.070" o ring in a 0.058" groove gives about 15% compression. Add a bit of clearance between piston and bore and the compression is probably closer to 10%.


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## TSutrina (Dec 13, 2020)

Peter Twissell said:


> The general rule for o rings is to target 20% compression for static applications (where there is no motion against the o ring) and 10% compression for dynamic applications.
> A 0.070" o ring in a 0.058" groove gives about 15% compression. Add a bit of clearance between piston and bore and the compression is probably closer to 10%.


The manufactures of o-ring have tables Parker is the best guide that can be down loaded.  Other companies have better information about calculating the dimensions and can be down loaded.   Didn't find a calculator the last time I looked.


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## Vietti (Dec 31, 2020)

Been trying O rings, quad rings really.  Having some trouble getting reliable compression.

Seems there is a difference of opinion on how much pressure the o-ring should exert on the cylinder wall.

Tom Stewart calls for .005" clearance between the back wall of the piston and the ring.  This is about what others, having success have told me.

Others (Pete Twissell) call for 15% compression of the o-ring.  Can both be correct or is there an ideal?

I'm now going to try a regular o-ring as I suspect the quad rings are twisting in the oversize groove and loosing compression.

What a great hobby!!!

John in Wyo


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## sniffipn (Jan 1, 2021)

Vietti said:


> Been trying O rings, quad rings really.  Having some trouble getting reliable compression.
> 
> Seems there is a difference of opinion on how much pressure the o-ring should exert on the cylinder wall.
> 
> ...









axial (along length of piston) clearance. for a 0.070" seal, the axial length of groove is typically 0.089/0.099"
Pete refers to radial (across section of seal) squeeze. for a 0.070" seal, typically 12% to 15% 0.060/0.062" housing radial width. different manufacturers quote slightly different numbers. 

dont worry about minor differences, its helpful to understand the principles - as extract from a leaflet above.

both radial squeeze and axial clearance are required to optimise seal performance

Does Tom refer to the diametral clearance between piston and bore? for a 0.070" seal, typically 0.005"

quad ring less likely to roll and twist, than an o-ring - of the same section width
David


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## Vietti (Jan 1, 2021)

David,
Thanks for the information.  Did I misread or misinterpret Tom's statement when he said:

The piston ring groove is cut to provide 0.005" clearance top and bottom and 0.005" clearance on the back wall of the groove.

I'm about to remake the pistons and after 4 sets of them I'd like to get it right.  The engine is Bob Shores Silver Bullet.  3/4" bore and stroke.  I wonder if other builders might comment on their experience with this engine?

I seem to remember when a good friend, now deceased, Randall Cox, designed the Hoglet he used o-rings.  I will have to dig out his build article and see what he says.  I've seen many Hoglets at the various shows here in the US and most all ran really well.

I"m not trying to be argumentative in any way, I"d just like to get it right and move on!

John in Wyo


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## sniffipn (Jan 1, 2021)

Vietti said:


> David,
> Thanks for the information.  Did I misread or misinterpret Tom's statement when he said:
> 
> The piston ring groove is cut to provide 0.005" clearance top and bottom and 0.005" clearance on the back wall of the groove.
> ...


0.005" clearance on what ...

1. the piston outside diameter - in comparison with the cylinder bore, but Toms explanation is 0.002" clearance, between piston and bore
or
2. axial length of groove, in comparison with the O-ring diameter section, for example?

top?
bottom?
back?

to avoid misunderstandings, Petes suggestion of downloading/looking online at a catalogue, is sensible ...

as would providing a simple sketch, showing dimensions/geometry that you intend machining.

looking at the catalogue will also help you to select an o-ring, with the appropriate inside diameter. the o-ring will typically have an inside diameter, that is a little smaller, than the root diameter of the groove (in the piston).


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## Peter Twissell (Jan 1, 2021)

I would somewhat qualify my suggestion to use standard / catalogue O ring compression.
The standard compressions are intended for hydraulic and pneumatic applications.
I could well imagine that the application in an IC engine piston may be different.
My approach would be to start with 15% compression and reduce progressively as required to achieve the balance between adequate sealing and acceptable friction and wear.


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## Engine maker (Jan 1, 2021)

I have many engines running with just off the shelf black Bunan "O" rings. From 3/4 inch up to a 2 " bore FarmBoy I just built. No problem with the "O" rings. Last forever. Most people try to make it too tight and it binds and doesn't float. I use the sizing that Jerry Howell uses on his regular size Farmbow with a 1"bore. Piston .998" dia with a .996'  dia above the ring towards the piston face. Them for a 3/32"(.094") "O" ring I plunge cut the groove .110" deep by .105" wide. Should work fine on a 3/4" piston.

Remember the actual OD of a 1" "0" ring is 1.005" and for a 3/4 "O" ring it's .755".  Keep the bore as smooth as you can get, keep it cool, and keep it lubed and you won't have any trouble.

Jim G


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## Vietti (Jan 1, 2021)

Thanks for all the reply's.  They open up some options.  

I'll remake pistons with 15% compression and then back off if need be as suggested by Pete.    Also I've been using 1/16" o-rings, perhaps try the bigger 3/32 ring.  Also will forgo quad rings till more comfortable with the  technology.  By the way, reading up on the Hoglet Randall specifies a relaxed, no compression fit for the o-ring.



sniffipn said:


> axial length of groove, in comparison with the O-ring diameter section, for example?



is axial length of groove same as groove depth?

Welcome additional comments and will up date with findings in the future

John in Wyo


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## MikeG (Jan 2, 2021)

Hi John
Here's my 2 cents (probably what it's worth)  I'm currently making a .375 bore 4cyl engine.  I'm using  one 1/16" quad O ring per cylinder.  I went out to the shop and measured the piston ring groove.  The groove allows .005 total clearance for both ring thickness and ring width  With my thumb closing the top of the cylinder, I can pull the piston almost out of the cylinder and then let go and it will snap back.  I assume that when the time comes that will give me enough compression.  By the way putting a quad 
O ring into the piston groove is VERY frustrating because it ALWAYS wants to twist!  

Hope to see you again at 2021 Rapid City show.

Regards
Mike


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## PoppyAnn (Jan 3, 2021)

Are you not bothered about the o ring melting and sticking too the cylinder walls?


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## MikeG (Jan 3, 2021)

No, viton O rings can take the heat of ignition and the presence of petroleum without "sticking". At least that's how understand it

Mike


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## Vietti (Feb 10, 2021)

By way of an update I have now installed o rings on two of my recent engines.  In both cases the compression improved as did the oiling problems.  So I'm sold.  
I just searched o rings on the forum and read most of the threads and found interesting, contrasting ideas on how they should be fitted.  Most everyone agrees Viton rings are the best.

There seems to be two schools of thought concerning fitting the o ring.  One school recommends fitting the ring loosely while the other school suggests some compression of the ring.  I suspect both will work.  I subscribe to the loose fitting theory.  I notice that some prominent builders/designers  like Tom Stewart, Randall Cox and Jerry Howell used the loose fit approach with a .002" piston clearance below the ring and .005" of clearance above the ring.  I think what happens is when the compression pressure hits the ring as the piston moves up for compression, the ring expands as the pressure forces the ring against the cylinder wall and down in the groove. Once the compression pressure is relieved the ring shrinks back to original size and there is then minimal drag.  Drag seems to be a problem with tight fitting o rings resulting in premature o ring wear and shorter coast time on hit and miss engines.

In the past race engine builder drilled a series of small holes down thru the piston top that intersected the back of the top ring groove to help force a better ring seal.  Interestingly Total Seal has introduced a new compression ring that has shallow scallops ground on the top surface of the ring with the intention of getting more compression pressure behind the ring and creating a better seal.  Would it work on a small engine?

For me the jury is still out on Quad rings and X rings.  I tried them and my problem was they swelled up after a couple days to where they fit so tight they rolled in the groove.  These rings started out about .071" in cross section and after swelling were .078".  They were not viton.  I believe everyone is correct and viton rings are the way to go.  I did notice one builder who reports his o rings shrank.

Lastly I've heard and read that two o rings on a piston will either not work or the compression is excessive.  I have no experience with this but am going to try it at some point.  What got me thinking about this was there is a YouTube video of some one overhauling or at least disassembling one of those inexpensive Chinese hit and miss engines.  There in all its glory were two rather thin o rings on the piston that looked to be tightly fitted but hard to say.  He called them fluorocarbon or some such but I think it is another version of viton.

It is interesting that rings are such a vital part of our small gas engines and yet there is so much divergence of opinion re how to make, and fit cast iron rings,  how to fit o rings and which is superior.  I suppose that this divergence of opinion is what helps keep the hobby interesting.


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## TSutrina (Feb 10, 2021)

The O-ring in the space shuttle booster rockets were Viton with packing materials trying to prevent the hot gas from touching the O-ring.  Leakage around the O-ring resulted in burning of the O-ring and failure.  Internal combustion temperature and solid propellent temperatures are similar.   
The only reason that the O-ring doesn't burn is that the gas is cooled in the gap between the piston and the cylinder.  An oil film on the cylinder wall adds to the cooling by evaporating.  So long as the amount of oxygen is low the evaporation cooling is significantly greater then the burning of the oil.   This didn't happen in the case of the shuttle boosters.  
O-ring is hydroplanes on the oil on the cylinder wall.  So the pressure of the O-ring against the wall determines, the pressure plus the pre squeeze, the thickness of the film available to be evaporated.  
One thing to consider is backup rings instead of  X  or square profile which has two a longer face against the cylinder wall thus the final oil fill for the same pressure will be thinner.  The back up ring will help prevent high pressure from extruding the O-ring into the gap and will add more resistance to decrease gas leakage.


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## Brian Rupnow (Feb 10, 2021)

The figures I gave for piston ring groove width and depth are correct. I have never had any issue with compression on my engines running a Viton ring. The largest bore size I have used Viton rings in is 1" bore, the smallest is 1/2" bore.


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## Kasey (Feb 11, 2021)

MikeG said:


> No, viton O rings can take the heat of ignition and the presence of petroleum without "sticking". At least that's how understand it
> 
> Mike


Would "tight" o rings work ok on a 2 stroke engine, or just chop up?


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## Brian Rupnow (Feb 11, 2021)

In a 2 cycle engine the viton style rings get sliced and diced as the piston passes the ports in the cylinder wall.


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## wespete66 (Feb 11, 2021)

Some have mentioned using two "soft" seals on a piston. The problem with doing this is pressure trapping. Trapped air/oil in the space between the seals will begin to create pressure in that space, due to thermal expansion or from the seal scraping the oil film from the cylinder wall and trapping it in that space. As the piston cycles that pressure increases, causing increased seal force on the cylinder wall. It is possible that this can cause forces great enough to actually stop piston movement. It can also cause premature seal wear or failure.
It is also possible for this to happen between a dynamic o-ring and its backup washer (if one were to be used). There are special backup washers made to remedy this situation.
(edit: soft seals only, doesn't apply to cast iron rings)


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## coulsea (Feb 11, 2021)

wespete66 said:


> Some have mentioned using two seals on a piston. The problem with doing this is pressure trapping. Trapped air/oil in the space between the seals will begin to create pressure in that space, due to thermal expansion or from the seal scraping the oil film from the cylinder wall and trapping it in that space. As the piston cycles that pressure increases, causing increased seal force on the cylinder wall. It is possible that this can cause forces great enough to actually stop piston movement. It can also cause premature seal wear or failure.
> It is also possible for this to happen between a dynamic o-ring and its backup washer (if one were to be used). There are special backup washers made to remedy this situation.


The first time I used O rings I thought that two must be better than 1, it would lock solid after a few turns.


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## almega (Feb 11, 2021)

Should the wall of the cylinder be honed smooth or should there be a cross-hatch pattern if using Viton o-rings?

I have read that when using two o-rings, you need to provide a small vent hole which vents the space between the rings out the bottom of the piston to prevent pressure build up between the rings. the lower ring only provides for alignment in the cylinder and does nothing to enhance compression.


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## Ken I (Feb 12, 2021)

With regard to using more than one "O" ring.
Firstly an "O" ring behaves like a liquid - it is displaced by pressure and the internal pressure of the "O" ring is always the applied pressure plus the preload.
It simply cannot leak - no matter how much pressure you apply - other than due to poor surface finish or excessive clearance.
This image from the Parker-Hannifin manual :-




I have used "O" rings to 16000 psi in hydraulic burst testing of tubing - by filling the test piece with oil and using two "O" ringed plungers, squeeze in a tensile test machine (applyling tonnes) until the tube swells & splits - great fun. Used zero clearance, very slow speed - but never lost an "O" ring.

However it will try and "leak" through the clearance (behaving like a liquid itself) and the combination of sufficient pressure and too much clearance will allow it to "extrude" this leads to seal failure or "nibbling". Nibbling also occurs because of a sharp edge on the "O" ring groove - hence the need for radii.
A bad case of extrusion / nibbling / melting :-




When you use two "O" rings the pressure between them builds and deflects the rings outwards - called a "pressure trap" due to dynamic forces (boundary layer and film strength etc.) this pressure can be incredibly large.
The pressure building in the "O" rings causes them to bind with the cylinder walls (as pointed out by Coulsea - above) Since the active internal "O" ring pressure keeps on building so does the pressure of any material (oil, air etc.) leaking past via surface finish or film strength into the "trapped" volume - its a vicious circle and absolutely can only end in failure - typically very quickly.
If you want to use two "O" rings for guidance or stability, then Parker-Hannifin recommend splitting one of the "O" rings (now referred to as a wiper) or by  venting the gap between the "O" rings.




For an IC engine use piston rings for guidance and one Viton "O" ring to seal.
What should also be apparent from the fluid dynamic nature of an "O" ring, the higher the pressure, the lower the tolerable velocity and importance of lubrication / cooling if burning / melting is to be avoided.
Surface finish should be a very fine cross hatch (_A'La_ honing) in order to carry oil through the seal on film strength. Mirror finished surfaces seal well - which is fine for static applications - but will "wipe" clean and cause lack of lubrication to the "O" ring and consequent "melting" type failures in dynamic applications.

Regards, Ken


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## Vietti (Feb 12, 2021)

Interesting discussion.  The diagrams shown by Ken show the o ring constrained by both the piston and the cylinder wall.  I kind of think they work best when there is some clearance behind the o ring so the pressure of compression forces the ring out to seal and relaxes on the other 3 strokes.  

I suspect the factory literature is focused on the sealing of a piston/rod in a cylinder and the pressures are from above and maybe below and the 0 ring is expected to do its job by always providing a nearly perfect seal, speeds are relatively low so drag is not an issue.  We desire an o ring in our application to seal on one stroke and then relax,  speeds can be quite high.   
I am not certain of any of this and only started using 0 rings in the last few months so no expert.  I can say that I now have real world experience with loosely fitting the o ring which works well.  Next time out I'll try the tight fitting scheme.

I understand the logic behind the notion that two 0 rings will not work without modification.  I will attempt to attach a link to the you tube video of two rings on a small Chinese hit and miss.  If anyone has one of those engines maybe they can chime in and describe how they are fitted.https://www.youtube.com/watch?v=0oYgB3FBqjc&t=167s


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## Ken I (Feb 12, 2021)

Vietti - the link you showed (Tip when inserting a link use the link icon  - 9th icon from the left - as I have done below).



Is for two "O" rings on the water jacket which are static - that you can get away with - but not for a dynamically loaded "O" rings. (Looks to me like he replaced two Nitrile Rubber rings with Viton replacements.)

The dual rings on the piston are a bad idea - in fact exactly what you shouldn't do. It's possible there is a vent hole between the rings but it doesn't show in the video.

You are also correct in that a lightly loaded "O" ring is best - running an "O" ring "tight" to overcome poor surface finish or fit up problems may solve the problem at the expense of running life. If it works with little preload, there is no reason to go for more.

Stick to the manufacturers recommendations for interference, grooves, radii, surface finishes etc.

However if you run minimum preload then you would also run minimum groove width - you want the width to be just wider than the "squeezed" "O" ring - more lateral play results in the "O" ring rolling or twisting unnecessarily - which in rapid cycling can fatigue the "O" ring or cause it to rotate (or twist) to a different position on forward and return strokes leading to a dual flat and point wear pattern which ultimately fails to seal.

As regards the "pressure trap" problem lets just imagine a small quantity of lubricating oil (hydraulic fluid, compressed gas - take your pick) is squeezed out from between the running surface and the "O" ring - it is ejected at the same pressure as the "O" rings' internal pressure plus the film or boundary layer pressure - into the "trap" raising the pressure of the trap which raises the internal pressure of the "O" ring - obviously a vicious circle which ratchets up the pressure in the trap with each cycle - ultimately extruding the "O" ring.

Regards, Ken


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## Vietti (Feb 12, 2021)

Thanks Ken- very rational reply. Will be interesting if someone comments on how the dual ring Chinese engines work, they do seem to coast OK

John


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## Steamchick (Feb 13, 2021)

Ken. Thanks for clear and sensible explanations. I now understand why my water pump wears o-rings at a horrible rate. Too tight! I made it based on static o-rings in a carburettor. So not understanding the sliding o-ring dynamics dropped a boo-boo. I'll sort it now, and change from silicon to viton.
Thanks K2


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## guy (Feb 13, 2021)

looks like you need a Teflon backup ring. I worked in a hydraulic repair shop. You need to widened you o-ring groove the thickness of the backup ring. if you can't find the right size you can make one. the backup ring goes down stream from the pressure. this should keep your o-ring from extruding out. Back-up ring - Wikipedia


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## ninefinger (Feb 13, 2021)

I've seen alot of discussion on o-rings for pistons, but usually the discussion is around "slow running" engines - hit'n'miss types.  What are peoples experiences with faster running engines?  My thoughts are to try o-rings on my 9 cylinder build by making a set of custom pistoms for it, but its expected to rev to about 6000 rpm (max).  (22.2mm / 0.875" bore x 24mm stroke)


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## MIKE4444 (Feb 14, 2021)

Hi  what advantage does one have by using 4 mm   or  a 3 mm  copper pipe for a small  steam engine, Tank size,  38  mm  dia ,  by  90  mm  long,  copper  tube ,  with two side  pistons , around Piston size  10 mm  dia ,   refer to my Dribbler that i am constructing, refer to ATT .
thank you MIKE


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## Ken I (Feb 14, 2021)

Ninefinger - Download the Parker-Hannifin "Bible" here :-

https://www.parker.com/literature/Section V.pdf

More information there than you can shake a stick at.

One of the more curious things pertinent to your question is that low reciprocating velocities are bad for "O" rings - like less than one foot per minute - whereas higher velocities produces less friction - due to the dynamic "wedging" effect of the oil film.
Obviously if lube failure occurs at high speeds (you are talking peak velocity of 6.8m/sec.) then the "O" ring will burn.

Personally never tried them at such velocities. It would be nice to hear from others that have.

As to Vietti's comment on dual ringed Chinese engines - If the gap is unvented, then I suspect they are running them with almost zero interference and relying on the ring to seal dynamically. So in between cycle transitions it effectively "leaks". This would explain why they "coast".
If that is the case ring life might be quite short.

Regards, Ken


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## Vietti (Feb 15, 2021)

Ken,

Just curious but why do you predict short life for a loosely fitted 0 ring?

I wish someone with one of these dual ringed Chinese engines would tell us more about how the 0  rings are fitted and is the gap vented.


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## guy (Feb 15, 2021)

you should look at this pdf. it is from parker seal and o-rings. look at paragraph 3.2orings. this should need to know about orings


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## Ken I (Feb 16, 2021)

Vietti, Using no preload is referred to as floating / dynamic in the Parker-Hannifin bible. As pointed out by Guy above.
That's fine for hydraulic where some small leakage during the stroke in either direction is permissible - particularly at slow or stationary speed / low pressure transitions.
The problem with using that approach on an IC engine, is that hot blowby may eventually damage the seals and once it starts will fail rapidly.
I have no personal experience of this so it would be nice to hear from those who have.
Like the "Challenger" as long as the "O" rings seal - then the hot gasses do not get to them - but once they start to leak........
Regards, Ken


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## Steamchick (Feb 16, 2021)

Re: Boiler query, post#45 on the dribbler boiler:- 3mm copper pipe for your little boiler I reckon. 90 mm long: bore needs to be >1/80th of the length: = >1.13mm. So 3mm is fine. - And more surface area for heat exchange.
K2


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## mrehmus (Feb 16, 2021)

Here is the Tom Stuart article on O-Rings as found in Issue # 15 of _Model Engine Builder_ magazine. He used them in his big model Fairbanks-Morse engine for 7 years with no problems. Please use this information for your use only and do not spread it around.

I  have been using O-Rings as piston rings in my Hit and Miss engines for the past 8 years with few problems. An interesting feature of O-Ring use is the very low friction when the engine is coasting and yet the immediate seal-ing of the O-Ring to the cylinder wall during a firing cycle. O-Rings do a good job on slow–speed engines. They will probably not survive in a high–speed engine and would quickly convert themselves into a melted plastic ring.
You can modify your engine to use O-Rings with minimal effort (new piston and polishing the cylinder walls) and will probably see an immediate improvement in performance. The improvement I mention is more revolutions between a 'hit' and very little exhaust smoke. Of course O-Rings are easier to 'make' than traditional cast iron rings and you only have to use one.
I've had good success as long as I followed a few simple rules about their application.
Unlike 'normal' O-Ring applications, the dimensions of the piston ring groove are such that the O-Ring is not under compression in the static state and does not effect a aero-pressure seal. It takes pressure above the ring to cause it to press against the sides of the cylinder and the bottom of the ring groove.
For the example shown on the Drawing Page 15, a typical 1.5" bore cylinder in this hit and miss engine example will have 0.002" clearance between the piston and the cylinder. The O-Ring with a 1.5" OD and a 5/32" circular cross-sec-tion is made of Buna-N material. The piston ring groove is cut to provide 0.005" clearance top and bottom and 0.005" clearance on the back wall of the groove. 
If we need a math formula for the groove it is:
Groove inner diameter = Cylinder I.D. – ((2  x  O-Ring circular cross section) + 0.010).  
With the values we have, that would be 1.5–((2x0.15625)+0.010) or 1.5-((0.3125) + 0.010) or 1.5" –0.3225" = 1.1775".
Cylinder wall finish should be as smooth as you can make it. Somewhere between 2 (mirror) and 32 (smooth shaft) micro inches is a good value. Some people actually recycle old hydraulic cylinders since they have chrome-plated inner walls. That isn't necessary for this application but a good smooth surface will help the life of the O-Ring.
During use, you want to have a light coat of motor oil on the cylinder walls. During a full day of running, I oil the engine about every hour.
_Editor's Note:
Because we cannot adequately show surface finish in print, I use a BAR S-22 Microfinish Comparator which is a cali-brated assortment of different finishes all cast into a single rectangle about the size of a candy bar. The finishes range from mirror to very rough (in machining terms). In use, you visually compare your surface finish to the samples and se-lect the closest finish on the Comparator. 
These are available from machine tool stores_.


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## Vietti (Feb 16, 2021)

Just a thought, you might be able to cut the 0 ring groove in an existing ringed piston.  Some think the lower the better to get the 0 ring further from the heat of combustion.  Another plus is that an engine that has been running with ci rings will generally have a polished bore.  

The Hoglet was designed for 0 rings and builders report long life and the 0 rings seem to survive the relatively high rpms the Hoglet can run at.  My last two engines rev up pretty good and so far no problems with the 0 rings.  One is a Silver Bullet and the other is a Kittiwake.

I have been building model engines for 40 years and have made numerous successful rings.  I only tried 0 rings in the last two months to see if they would be superior to what I had in my last two conventionally ringed engines.  I think they run better with the O ring.  I plan to use them in a hit and miss engine I'm beginning to build.

Try them, you might like them!


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