Author Topic: Piston Materials and Fits For IC engines  (Read 8888 times)

Online Jasonb

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Re: Piston Materials and Fits For IC engines
« Reply #15 on: December 10, 2014, 07:57:26 AM »
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

Online Roger B

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Re: Piston Materials and Fits For IC engines
« Reply #16 on: December 10, 2014, 09:47:12 AM »
would be a pity if you inadvertantly parted off half the piston while cutting the groove.

J

Been there, done that  :toilet_claw:
Best regards

Roger

Offline lohring

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Re: Piston Materials and Fits For IC engines
« Reply #17 on: December 10, 2014, 01:43:26 PM »
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.

Offline Nerdz

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Re: Piston Materials and Fits For IC engines
« Reply #18 on: December 11, 2014, 12:00:25 AM »
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 :)
-Chris

Offline Brian Rupnow

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Re: Piston Materials and Fits For IC engines
« Reply #19 on: December 11, 2014, 12:06:22 AM »
.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

Offline Nerdz

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Re: Piston Materials and Fits For IC engines
« Reply #20 on: December 11, 2014, 01:10:40 AM »
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?
« Last Edit: December 11, 2014, 02:14:21 AM by Nerdz »
-Chris