Cast iron for piston rings, model engines

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lensman57

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Hi,

Could someone please tell me what is the best grade of cast iron for making piston rings using the cold turn method? The only two grades that I can find easily here in the UK are GR17 and SG (GGG40).

Many thanks for your help.

Regards,

A.G
 

I. Klemetti

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Not knowing what a "cold turn method" is, I'll tell my story. I made a piston ring from ductile cast iron for a Super Tigre 40 glow engine. The diameter is about 20 mm and cross section 1x1 mm.

I got a bunch of odd shaped scrap pieces of ductile iron from a local foundry. Free :). The material has higher tensile strength than GGG40 but it has no significance.

At first I turned an oversize ring. Then a gap was cut on the ring and it was attached in a fixture holding it in compressed state. The final outer diameter was turned.

Well, the engine started with first flip. The power of the engine was far better than it used to be. I could change for a larger propeller. After three active flying seasons, the engine does not show any signs of wear so I am convinced that this material is suitable for piston rings.

-ilkka-

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Jasonb

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I would not use SG Iron for piston rings. You want what in the UK is generally termed Meehanite in Grade 17 (250)

Just about all the model engineering suppliers will have this grade such as college eng.

http://www.collegeengineering.co.uk/CastBar.htm

SG (spheroidal graphite) is a ductile iron that is more like steel to work and is generally used for stressed parts, on my traction engine the crank and front axle are SG Iron as normal would likely snap.

These are all grade 17 (250).









J
 

lensman57

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Hi,

Many thanks to all of you who replied. I have turned the rings exactly as described without heat teatment for normalising. I have a slight concern over the size and the thickness of these rings but that is another matter.

I will source some GR17 as soon as possible. The last piece of cast iron that I recieved looked very different to the stuff that I had before, it has a sort of greyish appearance and the surface is full of pebble shape artifacts, so I guess that I will not use this one untill I have managed to get it cleaned up and see how it turns on the lathe.

Warm regards,

A.G
 

superlionb10

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Hi I have made many rings for both Internal combustion and Steam engines,I have always used S.G .Centrifugally Cast,these work a treat.
 

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