Can the coefficient of expansion for a mat'l change? 1

[Durkee]

I do a lot of 2 stroke engine work and just lately we have been struggling with an issue about heat cycling.

Most modern 2 stroke engines have cast aluminum pistons along with a water cooled cast aluminum cylinder which is nicasil coated then bored and honed to size. We know that if we use an aftermarket forged piston, that we need a larger clearance due to the fact that a forged piston expands due to thermal stresses faster than a cast piston. *We should be able to use the same clearances with both pistons if the engine was allowed to warm up thoroughly before getting pinned. But we have to consider the customer and their warm-up practices, which most of them don't have. I just want to build a reliable engine and keep happy customers and if heat cycling the engine several times before getting it back to the customer would allow me to run tighter tolerances, that is what I want to do.

There is a lot of talk about heat cycling the engine to stabilize the materials used. Many say that after the engine is heat cycled many times that the parts don't expand due to the heat as much as they did previously. I tend not to agree with this. I don't think that the coefficient of expansion for a material can change. The reason that I heat cycle a new engine is to allow the grease (the crank bearings are packed with isoflex grease) to ooze out of the bearings a little at a time. Otherwise the grease may expand to quickly and cause a bearing siezure.

The question is this: Can the coefficient of expansion of the materials used in engine construction decrease as the engine is heat cycled?

Sorry about the length.

Keith Durkee

http://www.toolgrinder.com

 

[TVP]

The coefficient of thermal expansion does not change over time or the number of thermal cycles. The changes that occur are due to wear, with the rings, piston, & cylinder liner all subject to change in dimensions. Residual stresses that may be present in the forged components may be relieved due to thermal cycling, which may or may not have an effect on distortion/change in dimension.

 

[EdStainless]

I agree with TVP, in cases where I have seen non-reversible or non-repeatable thermal expantion it was mostly dure to residual stresses in the parts. Once the parts stress relive they are stable.

It may be that the grades of Al that you are working with have significantly different expansion rates. I don't know the alloys involved, but the expansion of Al alloys ranges from 11-14 x10^-6 in/in/F (RT to 400F)

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Corrosion never sleeps, but it can be managed.

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[rd400guy]

I'm not sure exactly what temperature the pistons will reach while heat cycling the engine prior to delivery, but can you heat cycle (stress-relieve?) the piston before you install it in the engine, to minimalize dimentional change?

Or, you could do an experiment where you very accurately measure an as-purchased piston, do your normal heat cycle, then take the piston out and see if there is dimensional change. If so, try to replicate the experiment with the piston before it's installed, and see if tolerances are held better? I'm just spit-balling here.

 

[EdStainless]

You could try both. You are not being too far fetched. The problem is that is service you are also creating new residual stresses everytime the part cools.
It might be good to carefully dimension some pistons, run them and then re-measure. Maybe you could get the size a little closer.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

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