Model Engine Maker
Supporting => Engine Ancillaries => Topic started by: Nerdz on December 09, 2014, 12:03:03 PM
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What are some suitable materials for a Internal combustion engine cylinder and Piston ? I know that Cast Iron is king. I see the webster uses a Steel or Cast Iron Cylinder with a Aluminum piston. I have some Cast Iron on the way (5/8 rod) But can I use it in an aluminum cylinder?
How good of a fit do I need? The piston I made doesnt slide through readily, but I can see a glint of light if I look down the bore with the piston inside. Would this fit be acceptable?
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Nerdz, personally I would use an aluminum piston in a CI cylinder which is more standard practice. What do your plans call for? Also what type of rings are you planning on using...O-rings or cast iron?
As to the fit, do you have any measurements on the cylinder ID and piston OD. Again, I am assuming that some sort of ring will be used to seal the light leak, else you probably won't have enough compression.
Bill
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Are you going to be using any form or rings? or will it just be the fit of piston into cylinder that makes the seal.
Also what sort of speeds, a slow running Hit & miss or a faster reving aero engine?
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Jan Ridders 2 Stroke (Puppy), he claims speeds of up to 1500 RPM. His Plans call for a 14mm Piston/Cylinder bore. Being American I dont have those, So I'll be using 0.5''. He uses a Aluminum Cylinder and a Graphite Piston. I reamed out the cylinder to be 1/2'' and then lapped it using a wooden rod with Brasso. The graphite stock I have is also 1/2'' but has to be polished slightly. His plans just call for the Piston to be fitted to the cylinder, with no rings. This is my first IC so Im trying to follow plans as much as possible, but I will deviate if I have to.
I have Cast Iron Rod On the way from speedymetals (5/8 rod), and it was cheap. I actually wouldnt mind making piston rings, as I am running out of graphite.
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At the risk of giving egg-sucking lessons:
You can't have an aluminium piston in a steel or cast iron liner unless (as the others mention) you use piston rings and have a suitable clearance fit on the piston, because aluminium has a higher expansion coefficient than steel or cast iron. You can run a ringless cast iron piston in a steel or cast iron liner using a lapped fit, or you can run an ringless aluminium piston in a brass, bronze or aluminium liner with a tapered bore, but the liner will need hard chrome or nickel plating if you want it to last more than a few minutes.
AS
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Looking at Jan's site (http://www.ridders.nu/Webpaginas/pagina_tweetakt_puppy/tweetakt_puppy_frameset.htm) he has cast iron for both the cylinder and piston, no mention of graphite (Grey cast iron does contain graphite but he does not mention graphite rod)
If its is iron in iron then I would lap the cylinder, lap the piston until it starts to enter the bore and then use 1000g lapping past to lap the piston into the bore. This is how he says the larger "debbie" piston was fitted to the cylinder.
If you want to save the work done on your aluminium cylinder then bore it out and fit a steel or iron liner and make a new iron piston.
J
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Looking at Jan's site (http://www.ridders.nu/Webpaginas/pagina_tweetakt_puppy/tweetakt_puppy_frameset.htm) he has cast iron for both the cylinder and piston, no mention of graphite (Grey cast iron does contain graphite but he does not mention graphite rod)
If its is iron in iron then I would lap the cylinder, lap the piston until it starts to enter the bore and then use 1000g lapping past to lap the piston into the bore.
I know it says on his website that he uses Cast Iron, but the plans call for Aluminum/Graphite. Its one of the few discontinuities Ive found.
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I have had good success with i.c. engines by making the cylinder from cast iron and making the piston from aluminum, and using Viton as a piston ring. Bore the cylinder to finished size (whatever), hone it with a 3 stone brake cylinder hone to get out any high spots, (you won't be able to do that because of such a small bore) and then finish lap the i.d. with 600 grit lapping paste and a piece of aluminum .002 to .003" smaller in diameter than the cylinder bore. Make the piston .0015 to .002 smaller than whatever the cylinder bore finished up at, and lap it into the cylinder bore, again with 600 grit lapping paste. Use one 1/16" cross section viton o-ring on the piston. I have never had a piston "grow" enough under prolonged running to seize in the cylinder bore.If you choose to go the "no ring" route, then you will have to go to almost zero clearance to seal compression, and you will have to use a cast iron piston to avoid seizing from differential expansion.---Brian.
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Brian,
How much depth is needed to seat the O-Ring in a Piston to provide a good seal?
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For a 1/16" cross section Viton o-ring, which is actually 0.070" diameter, the ring groove must be .057" deep and .093" wide.----However--for a 1/2" diameter piston, you can probably go to a 1/32" nominal diameter o-ring. --and the groove depth will really be a trial and error thing----If you want to run without rings, check out any of the posts by Gail from New Mexico. he is a master at making engines in these smaller bore sizes with no rings. I know of at least two builds he has done of horizontally opposed cylinder engines that he has made this way.---Brian
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I make the liners from cast iron or free cutting steel (more rust sensitive). The pistons from aluminium and turn them 0,0002" (0.005mm) undersize. Use lapping past to run them in. As stated by AS you would expect the engine to stall witin second but they don' (at least mine doesn't). Saves you the time and trouble of making and more difficult fitting the rings to the piston. There are many opinions on this topic, do not claim mine is based on any knowledge or so ever, i can only report what worked for me.
Regards Jeroen
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There are various material combinations that will work. A lot depends on what you want the engine to do. Some engines are made for competition purposes to give the maximum power for some sort of racing. There are some fine examples on the forum. Some are made to idle as slowly as possible for display purposes. Others are made to prove a technique and may only run for a few hours in their entire life.
I have seen running engines with brass cylinder and pistons, brass (unplated) cylinders with aluminium pistons (with and without rings), various cast iron and steel cylinders with cast iron or aluminium pistons. In high performance model aircraft engines ABC, aluminium piston in a chrome plated brass cylinder was and maybe still is popular. The thermal expansion of aluminium and brass are similar, but both are higher than cast iron and steel.
A cast iron piston in an aluminium cylinder should work. The clearances may increase a little as the engine heats up, but by then it is running :)
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A cast iron piston in an aluminium cylinder should work. The clearances may increase a little as the engine heats up, but by then it is running :)
Well is it safe to assume that if I use Vitron O-Rings, that clearance wouldn't matter? It seems that through some investigative work, Jason was correct in his assumptions. I have old plans it seems, and the engine does require cast iron cylinder/piston. Ive ordered some O-Rings from Mcmaster (0.5'' OD, .375 ID, 1/16th width) and cast iron rod/flats from speedy metals.
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Not sure as to the type of balls shown on youre drawings?
I know they are now using 3/16 viton
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Not sure as to the type of balls shown on youre drawings?
I know they are now using 3/16 viton
The original drawings show a steel ball. I was going to use a .177 BB. I found 3/16 Viton Balls. Thanks for the Heads up!
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On such a small piston if you do opt for O rings of 1/16th nominal make sure you have the the wall thickness to fit them, would be a pity if you inadvertantly parted off half the piston while cutting the groove.
Although clearance with rings is less critical you don't want masses as the piston will start to twist in the bore if there is a lot of play, again a short piston will suffer more than a long one.
J
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would be a pity if you inadvertantly parted off half the piston while cutting the groove.
J
Been there, done that :toilet_claw:
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I have no experience with low performance engines. High performance engines transitioned from cast iron pistons and liners a long time ago. Duke Fox wrote a comparison that's still valid today. The largest bores suitable for ABC or AAC pistons and liners is around 25 to 30 mm on glow fuel. Ringed pistons do better on gasoline. See Jim Allen's thread http://www.modelenginemaker.com/index.php?topic=3890.0 for what it takes to build a seriously high performance piston and liner.
Lohring Miller
Duke Fox on pistons and liners:
Now, to the piston and cylinder. For years the most popular model airplane piston materials were iron in its various forms, (gray cast, ductile, and Mehanite). The advantage of an iron piston is its low expansion, its dimensional stability. and the fact that it does not soften noticeably with the heat reached in model engines. The primary disadvantage is its weight, which is about 3 times as much per cubic inch as aluminum. This is somewhat offset by the fact that iron does have a higher modulus of elasticity than aluminum, and it is possible to make an iron piston that's thinner than an aluminum one and still maintain its shape. Iron pistons are usually run in soft steel liners. Most of the model engines in the past 50 years have been built in this combination. The iron can be fit very closely in the steel liner. The expansion co-efficient of both are about the same, and should the parts rub too hard, the iron tends to burnish and not gall. To the user, an iron piston/steel liner motor has a freeness and a snap-over compression not readily achieved with any other combination. The primary disadvantage of the iron piston and the softliner is that as you get into larger and larger out-put motors, it becomes more and more difficult to keep the expansion of the cylinder liner and piston matched. At one time we produced the 59 with an iron piston and steel liner, but today our 40 Standard is the largest that we feel is practical.
The second, very popular, piston and cylinder combination is the use of a hardened steel liner and an aluminum piston fitted with 1 or 2 piston rings. A lot of motors of yesteryear used this combination. The McCoy and the Hornet were outstandingly successful. This combination worked very well and had a light piston which was relatively vibration free. However, the success of this combination depends on having a quality piston ring. Unfortunately, only one piston ring company seemed capable of producing an acceptable quality ring, and when they were swallowed by a conglomerate, quality model size rings became unavailable. K&B solved their problem by developing the Dykes ring to an acceptable quality. We solved this problem by developing a new method of shaping conventional design piston rings. Licensing of this patent is now available to interested ring manufacturers. The primary advantages of a steel liner-ring piston motor is its ability to accept abuse - over lean runs, dirt, no warm up. etc.. and the fact that it can be flown out of the box without fear of damaging it or seizing the piston. About the only disadvantage is that the cylinder webs reduce the power output slightly over the other two types.
Now, to A.B.C. or A.A.C. type cylinder/piston combinations. A.B.C is an abbreviation for Aluminum (piston) Brass (Liner) Chrome (plated). The problem of getting a good piston ring was probably a substantial cause for the increased popularity of the so called A.B.C type piston anti cylinder combinations. This amounts to an aluminum piston with no rings, but cast out of one of the modern high silicon, low expansion aluminums which is fitted into a brass liner which has been chrome plated. The chrome plating produces a hard wearing surface to keep the cylinder liner from wearing out. An aluminum piston ran in a brass liner with no plating would be completely worn out in 10 or I5 minutes. However, hard chrome is expensive, difficult to apply evenly, and almost impossible to hone once its on. In order to make a very small shape anti surface improvements, we had to resort to diamond and borizon honing stones to make a few tenths corrections in our A.B.C. lines when they come back from plating. Now not everybody claiming an A.B.C cylinder really has hard chrome on it. Some cylinders have used polished chrome, like that put on auautomobile bumpers. This is much softer and can be honed. but also it wears out quicker. It is cheaper however. AIso, there have been same cases, where manufacturers have used electroplated nickel or electroless nickel. Nickel, compared to chrome, is very soft. Furthermore, nickel has moreadhesion problems to the base metal.
Our advice is to stay away from cylinders that are nickel or polished chrome plated. For an A.B.C cylinder and piston to work right, a cylinder should be either a low expansion aluminum of the #390 alloy variety, or brass, and should be hard chrome plated. The piston must be one of the high silicon types, the most usual being the #390 series. The advantage of the A.B.C is that when it is properly fitted, the motor will run slightly faster than the ring motor, primarily because the webs in the cylinder which retain the rings on the ring motor can be removed and leaves a little more porting area. The disadvantages are first, one of cost, and second, that the motor must be handled mare carefully than a ring motor. The warm up period, particularly, is vulnerably. If an A.B.C motor that is cold and still relatively new is started and run at full power immediately, the piston heats up faster than the cylinder and, consequently, expands faster. It is not unusual for the piston to stick in the top of the cylinder with such force that the inertia pulls the rod apart. However, an A.B.C that is carefully handled and is operated properly will last a very Iong time and runs very smooth.
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Awesome info, Thanks everyone! I got some Viton O-Rings from mcmcaster and figured out (by trial and error) the correct depth. Turns out to be 0.110'' to 0.115''. One last question, if I use an O-Ring would it matter on the material selection still? I have Cast Iron on the way either way :)
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.110 deep groove for a 1/16" diameter o-ring?--That doesn't sound right. Did you mean .055" deep?--That would be awfully close to the 0.057" I gave you earlier. Material selection for the piston won't matter that much. Either aluminum or cast iron will be fine. People use aluminum if it is going to be a really high revving engine because it has lower mass, and consequently less inertial forces are placed on the con rod and crankshaft, but for most "normal" speed model engines it isn't going to matter.---Brian
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We are both correct. I took off 0.110 to 0.115 on my lathe, but measuring the grove it is 0.060.
ADD: Is there any benefit or disadvantage to more than one O-Ring or Piston ring in a model engine?