Piston vs Cylinder wall.

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jacobball2000

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Any one know what the bear minim would be needed on a cylinder "WALL" to a given piston size like 1/2 inch.
 

larryg

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More details needed like is this an internal combustion, external combustion, pneumatic cylinder? Bottom line is what pressure and temperature is the cylinder going to see and what is the cylinder material?

lg
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jacobball2000

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Piston and cylinder are made out of steel. fuel will be gas or nitro methane.
 

tornitore45

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The question may suggest you desire to lean against a bear minimum rather that what established wisdom recommends. If that is the case the standard answer is: When a design call for "heavy" compromise is time to question the premises and find a better approach. The operative word here is "heavy". Compromise is not undesirable, is actually the core concept in engineering.
That said I would not go below 0.040" wall thickness, I say this not as a result of calculation but off the seat of my pants based on experience gained on making a dozen engines.
Since much strength and stability is supplied by the jacket surrounding the sleeve one can go a little thinner but then the problem is how to machine it an hold it without distortion. One approach may be to press a sleeve with a much thicker wall and THEN bore to a thinner wall counting on the jacket to add stability.
 

777engman

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I would also strongly recommend a cast iron piston to run in a steel liner.
Regards
Dean
 

almega

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Does the piston really matter? Isn't it more a concern with the rings? I thought the piston itself (at lease in an I/C engine) would not contact the cylinder walls.
 

JOHN DUNCKER

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A 1/2 inch bore piston for an IC engine would not normally have a ring. Off hand I can not think of any production engine with that size of bore with a ring Bump the piston dia up to 3/4 inch plus then rings are common, usually on aluminium pistons.
 

Cogsy

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Bob Shores' Peewee V4 uses cast iron rings on a 5/8" piston so I guess you could put rings on a 1/2" bore. Getting tricky at that size though.
 

dieselpilot

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The Whittle V8 (and I assume his other designs of similar geometry) has steel cylinders of .460" bore and .500" minor diameter for fins of .660" OD. The pistons are ringed.
 

lohring

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Our custom race engines run chromed thin steel sleeves with aluminum pistons and steel rings. Bore sizes run from 32 mm to 38 mm. Smaller engines run aluminum pistons in chromed thick brass or aluminum sleeves without rings. In the past these engines ran ringed pistons. They were easier to make, but lower in power. You can buy commercial rings for these engines.

Lohring Miller
 

Chris Murphy

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The US made McCoy 9 glow engine of 1950 had a bore of 0.5" and was ringed in the earlier models (the later variants used a lapped piston)-here's a link to the test on the original model
http://sceptreflight.net/Model Engine Tests/McCoy 9.html
personally I'd say it is a lot of trouble to go to for a bore of this size-and a lapped piston makes far more sense-and the manufacturers quickly came to the same conclusion! Futhermore-modern commercial engines of this size 1.5-2cc-generally use a high silicon aluminium piston in a tapered chrome or nickel plated brass cylinder liner..the very best using a chromed aluminium liner-but this is very difficult to achieve good results with in terms of consistency-which is why the majority of commercial manufacturers stick with chromed or nickel plated brass. Steel in steel is a very poor material choice....

Chris Murphy
 

jacobball2000

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I was thinking to have my cylinder wall 1/16 of an inch or .060 this would be my minimum. So the bore .050. the piston .005 under the bore. And the wall of the cylinder .060 But this would be my first engine. Aluminum piston you have to give for expansion.
 

Cogsy

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That sort of piston clearance is huge and would be considered a 'rattling fit' for a piston of that size. Aluminium does thermally expand at a greater rate than steel or cast iron, but at the size you're considering the difference is absolutely tiny. I've done some math for you to illustrate:
Coefficient of thermal expansion for aluminium ~24E-6m/mK (upper limit). For a 1/2" piston (12.7mm), this means your piston will expand ~3.05E-7m/K. If you heat your piston up from room temp to twice the temperature of boiling water, you will get an increase in piston diameter of roughly 5.33E-5m. In easier numbers, it will increase diameter by ~0.053mm or about 0.002". So if your bore did not expand at all, even with a massively overheating engine you would only need 0.002 for expansion. In reality, your cast iron or steel bore will expand about half that of the aluminium giving even more clearance, and it's unlikely you're going to run at such a high engine temp.
 

jacobball2000

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The .005 is from a machinist friend. but we were talking about steam engine at the time. like I said this will be my first engine. Like the Right brothers a lot of trial and error.
 

Cogsy

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With thermal expansion it's all about the size of the item itself. The bigger the part, the more it's going to expand. As you go down to tiny sizes you need to adjust the fits to suit.
 

David Shealey

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Does the piston really matter? Isn't it more a concern with the rings? I thought the piston itself (at lease in an I/C engine) would not contact the cylinder walls.
Steel on Steel is a really bad idea! The rings only seal the gasses from bypassing the piston, not keep it centered. When the piston is going down, the angled connecting rod pushes it hard against the wall. One of the first things one learns is that you never use the same material for two sliding contact parts. Seems you are not worried about weight, so instead of aluminum you could use brass, or even bronze for the piston. Cast iron would be most people's second choice after aluminum for an I.C. engine though.
 

lohring

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Hard (stainless?) steel rings on a chromed or Nikasil bore is what modern two strokes run.

Lohring Miller
 
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