I.C. Piston Rings

[Brian Rupnow]

Lets talk i.c. piston rings. I have built two i.c. engines now, using Viton o-rings for piston rings. They work, and they work very well. On the Webster engine, I used one viton o-ring, and it worked marvelous. On the Kerzel, I used two Viton o-rings (I'm really not sure why---maybe I thought "If one was good, two would be better!!!!" Problem is, that on the Kerzel, they create a bit too much drag to let the engine slip into full "hit and miss" mode, which require almost zero drag in the engine. Since I plan on dismantling the Kerzel to address a crankshaft bearing issue, now would be a great time to make a new piston and a set of metal rings for the engine. My cylinder sleeve is a water jacketed peice of 316 stainless steel. My current piston is aluminum, and unless there is some really good reason to change it, I would build a second piston from aluminum and put a metal ring or two on it. I have an old set of plans for an Upshur engine, and Hamilton Upshur recommends using an aluminum piston and metal rings for his engine, which has the same 3/4" bore as the Kerzel. He shows 3 piston options on the set of drawings I have, and the option using metal rings is an aluminum piston with two grooves .062" wide x 0.046" deep for the rings. Unfortunately, he gives no info about the rings on my set of plans. I have been studying MetalButchers excellent thread about building Upshur engines, and he has metioned that HIS plans call for the ring blanks to be 0.003" greater in diameter than the cylinder bore. He also mentions that the ring gap is 0.010" sawed into the ring with an Exacto saw. His ring material is grey cast iron.---Okay, I guess I am mostly good with that. BUT-----Then MB goes on with info about heat treating his rings in a heat treat furnace at 1000 degrees for an hour with a special fixture to maintain the gap while heat treating. I don't have a heat treat furnace, and I don't fully understand exactly what his special fixture is about. Nor do I know how wide the finished ring should be for a .062" wide piston groove. Can I make rings without a heat treat furnace? Do they have to be grey cast iron? PatJ is suggesting a brass ring, but his recomendation is based on steam engine rings---I don't know if brass would work for an i.c. engine or not.-You can't heat treat brass!!! Educate me folks---Can I build an adequate set of metal rings without a heat treat furnace, what special fixture do I need (Or do I even need one)---Do I need to use grey cast iron------Or should I just forget metal rings and remove one of the viton o-rings?----Brian

 

[mu38&Bg#]

There is more than one way to make rings. What you describe is the George(?) Trimble method. The rings are machined to installed size, split, and gapped. Obviously if machined to size they have no spring or outward pressure on the cylinder. To add spring they are put in a fixture holding the square, spreading the gap, and insulating them from oxidation. You don't need a furnace it's been done countless times with a torch. Another method not requiring heat treatment was described well by Bill Lindsey on or about this page http://www.homemodelenginemachinist.com/index.php?topic=6014.225

316 is rather soft for rings, but running without load it might last a while? I'm not sure.

 

[Brian Rupnow]

I should have known enough to use the search function first!!! That article by Mr Lindsay on making rings which don't require heat treating may be just what I need. I REALLY don't want to bugger up my cylinder liner---maybe I shouldn't use metal rings??????

 

[Maryak]

Brian,

My method of cold forming cast iron piston rings.

http://www.homemodelenginemachinist.com/index.php?topic=5060.msg86453#msg86453

Hope this helps.

Best Regards
Bob

 

[NickG]

For a hit & miss I would use a lapped cast iron piston if your liner is cast iron with a couple of oil grooves and no rings.

 

[stevehuckss396]

Strictly IC

Vol. 2 No. 7
Vol. 2 No. 8
Vol. 2 No. 9

These 3 issues have all the info for the George Trimble method. The 3 part article goes in depth into the research he did to come up with the method. It covers how to properly design a fixture for your rings based on diameter. It also goes into how to use the fixture to heat the rings properly. It covers everything.

This will blow your mind but using his method, the rings for a 3/4 bore should be .017 wide and .032 deep. The modern trend is to make the rings square. .032 X .032

Many people have had good success making rings this way. The fixture can even be heated with a torch if you put some kind of goop on the rings to keep the oxygen away. I have not done the goop method because I have an oven.

If you ever plan to make rings some day, get the issues and read them and you will be see this is a very good method.

 

[Brian Rupnow]

For a hit & miss I would use a lapped cast iron piston if your liner is cast iron with a couple of oil grooves and no rings.

Nick---That is a very interesting thought, and I have heard others make reference to running a hit and miss engine with no rings. The nice thing about that is that the contact area would be spread out over the ful length of the piston so would be less apt to damage the 316 stainless cylinder. It would be easier for me to machine a piston from cast iron than it would be to make rings, but I wonder if I would get enough compression that way for the fuel mixture to ignite properly. Who knows more about this idea of running a hit and miss i.c. engine with no rings at all??---Brian

 

[mu38&Bg#]

I forgot the ever useful Ron Chernich.

http://modelenginenews.org/techniques/piston_rings.html

 

[stevehuckss396]

2. Piston diameter (for pistons with rings) should be (0.98*bore) + 0.001".

Are you sure about this one? .750 * .98 + 1 = .736

I wouldn't put a piston in a bore that is 14 thou smaller than the bore. That seems way to small to me. I would think you would want to be about .7485 - .749 assuming 6061 aluminum. Where did the .98 come from?

Also the ring dimentions seem waaaay to large for a 3/4 bore.

Seem flat out wrong to me, what am i missing?

 

[Lakc]

Edit 04: (0.98*bore) + 0.001" seems to be an excessive amount of clearance between the piston and the bore, but that is what "they" say. Who is "they". Other HMEM members.

We need concensus. Everyone weigh in on your idea of what the piston size should be, as well as the other dimensions.

It will be the difference in coefficient of expansion of the piston material and bore + thickness of lubricant filmx2.
A quick swag for an aluminum piston in an iron cylinder would be about .742 at the top and taper it to .747 at the back

 

[stevehuckss396]

A 3/4 inch bore, I would shoot for

OD .750
ID .686
Width .025
Thick .032
Gap .003

Ring groove

.680 diameter (.035 deep)
.0265 wide

This is my personal opinion and exactly what I would do if I were in your shoes.

 

[Maryak]

I think the spreadsheet you are referring to is one I posted some time ago. the 0.98D for the piston size comes from work done by Tubal Cain and Professor Craddock.

For a 0.75 bore this equals 0.735 which is 0.0075 radial clearance with a ring of .045 radial depth this means 0.0375 - 83% of the ring is still in the piston when cold.

Hope this helps

Best Regards
Bob

 

[stevehuckss396]

This test proves that you do not have to heat treat cast iron rings to prevent breakage.

I think the heat treat is to relieve stress while the ring gap is being held open. Also, all my dimentions were finished dimentions. I have never made a ring using the "finish under compression" method.

Don't let the size scare you. I have an engine with a .850 bore. I made .0185 wide rings for it.

 

[NickG]

There are many advantages to just making the piston from cast iron. No rings, easy to machine, much lower friction (esp useful in case of a hit & miss). If you get a good finish on the bore and piston it'll have a good seal and give less wear to your cylinder liner.

Gail has started a topic on using graphite for his tiny i.c. which as the added advantage of light weight, even less friction and no lubrication. It's also easier to obtain a good fit / seal. He is going to try that. I think one of these are the way you need to go for a small hit & miss.

Nick

 

[stevehuckss396]

The Jerry Howell article agrees with Steve's dimensions for this size ring.
The article also states that if you do the heat treating, you must use the correct temperature.

He must be using the Trimble method also. What did the Howell article say about the temp. 950 - 1000F?

Trimble also said that increased thickness adds friction but does not extend wear. In his opinion increasing the width does not have any benifit.

I would'nt trust that spread sheet for a gas IC engine. I wonder if it was supposed to be used for a steam engine or some other type of engine. The pistons seem too small and the rings seem too big.

The pistons in the Peewee are 1 - 1.5 smaller than the bore. If I plug the bottom of the bore, and put the piston in the bore, the piston is suspended by the trapped air below it. When I unplug the bore the piston falls right thru. I used 6061 aluminum. I hear if 7075 is used you can go even tighter but it costs about 2.5 times more. I can't imagine why you would want to make a piston .015 smaller than the bore in a IC engine.

All I can suggest is, decide what method you are going to use and search for a thread that covers that method. If there is not one already, start a thread and ask specific questions. Then follow the advice of the people who have tried it and made successful rings using that method.

Good luck Pat!!

 

[Dave G]

Hi guys, my two cents. I once had a discussion with an engine builder about piston rings that I found very interesting. He recommended an absolutely square corner on the outside edges(upper and lower) and a 45 degree chamfer on the inside corner on the top. This will help force the ring out under compression and ignition. He checked every cylinder on his engines with a scale attached to the connecting rod assembly, then drug through the bore to check for the correct tension on the ring. Seems using the least amount of tension on the ring and using the combustion pressure to help seal the ring made the most horsepower. When I make my rings I use common grey cast iron from the many steel suppliers. I have had good luck using the Trimble method previously discussed. When turning the blank ring I finish at .001 inch larger than my finished bore size,then part off .001-.002 long. Before parting, I face the ring square and chamfer the bore. This becomes my top surface.
After parting off I sand the parted surface on a flat plate using 400 paper with the ring set into a fixture that is easier to grasp. It is just a piece of round stock with a recess turned into the face to fit the ring blank. The depth of the recess should be just slightly less than the rings finished width. I lap to almost finish thickness often checking with mics to keep the ring parallel. Then the ring is flipped over and lapped to finished dim. I saw the gaps in with a .006 inch thick slitting saw(.006
is the thinnest I could find). I clean up the edges with a fine file before heating with a torch. I figured making the OD .001 larger then cutting a .006 slot would give me an end gap when installed in the cylinder of around .002-.003 inch but it didn't happen that way. After heat treating the ring,( basically we are annealing the cast iron when we heat it but don't quench) it wouldn't go in the bore without me widening the gap. The cast iron must expand some during heating and doesn't return when cooled. I try to cut the ring grooves in the piston .0015 wider than the ring to help instal the ring on the piston. Any narrower and the ring doesn't like to go in the groove well. I normally make my rings more like an automotive ring, thin in cross section with a ratio of 1 to 2 with the depth of the ring. .025 to .050 on an one inch ring. I cut the grooves in the piston as close to the top of the piston as possible to help ring instalation also. One or two back and forth pulls on the piston with the rings in place then removed from the bore will give a good indication on the outside surface of how well the ring will seal, if it looks like it is rubbing the cylinder all the way around then its a good ring. Lately I have been making rings that seal pretty good which was not always the case, so I have been only using one ring on each piston and once the ring is wore in the crank will bounce quite easily. I hope this helps , Dave

 

[moconnor]

Hello All:

The .736" diameter on the spreadsheet "Piston Diameter" may be referring to the diameter of the top of the piston above the oil ring which supports the ring lands, not the skirt diameter which would be more like .7485" or so for a .750" diameter bore. The smaller diameter near the top of the piston is full size engine practice where the diameter above the oil ring is reduced in diameter to eliminate scuffing and or galling due to the top of the piston having the highest temperature in operation. Also FWIW, some modelers may not know that the piston skirt is not round in full size practice, but rather elliptical with the largest diameter perpendicular to the piston pin. Much effort is put into controlling the heat transfer to the skirt so as to be able to use the minimum clearance possible to keep the ring package square and sealed and to eliminate the 'slapping noise' associated with excessive piston to bore clearance. Of course this practice is not normally found on a model, but the reduced diameter above the oil ring, which is what I am guessing the .736" diameter on the spreadsheet to be, may be useful information to someone building a higher output four-stroke model engine.

Regards,
Mike

 

[Troutsqueezer]

Brian,

Thanks for asking the question. It's uncanny, as I'm preparing to make the piston and rings myself for the Upshur, your post popped up with all the same thoughts that ran through my head over the last three days.

From all the replies, there seem to be a number of possibilities. Now, which one to use? scratch.gif

I kinda like the piston-from-cast iron idea although I'm really into this hobby to learn and that would circumvent some of that learning.

I did decide to make the gears from scratch (thanks to one of your previous threads) so perhaps that bit of learning will suffice for now.

 

[Brian Rupnow]

I knew that asking the question would stir up a storm of controversy. I'm still not certain which way to jump. I think perhaps at first I will start by removing one of the two Viton o-rings I am using. The goal here is to eliminate a lot of sliding friction, but still have adequate sealing to create enough compression for the engine to fire properly. I am surprised about the information that has come to light about many hit and miss engines running no rings at all. I didn't think you could do that on an i.c. engine!!! The people who are running no rings seem to be using a cast iron piston lapped into a cast iron cylinder. I have a 316 stainless cylinder which I can not change, although I could certainly make a cast piston to work in it. I wonder if that would work for me----A cast iron ringless piston lapped into my 316 stainless cylinder. I don't know whether you can do that or not.---Brian

 

[NickG]

It would be worth a try Brian, it's not that difficult or time consuming to make a piston.

I know Jan Ridders does this on all of his engines, I think he sometimes uses chromium steel cylinders and a cast iron piston. Similar rates of expansion so should be no interference issues.

Nick