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I'm trying to make most of an engine from scratch (cylinder, crank case, piston). I've been reading about pistons and I've learned that many modern pistons are cam ground--slightly elliptical in shape (reference article). As I currently understand it, the reason for this is so that when the piston heats up to the operating temperature, it forms a more precise circular shape.

I only have access to a metal lathe and that got me thinking, is it possible to design pistons so that they can be machined circularly and they expand evenly? It seems that it would save on manufacturing costs; then again, why aren't modern pistons designed so they don't have to be cam ground and that they expand uniformly?

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  • $\begingroup$ The only way to make the piston "expand evenly" would be to make sure all the metal is the same temperature. There are materials with different thermal expansion coefficients in different directions, but they would be ridiculously expensive for a typical IC engine. Unless the combustion process (not just the geometrical space where it happens!) is perfectly symmetrical, the temperature won't be uniform. In any case, the cylinder block will not expand uniformly either, because of the way it is cooled. $\endgroup$ – alephzero Jul 5 '17 at 23:50
  • $\begingroup$ @alephzero are you suggesting that, at the steady state operating point, there is a temperature gradient in the cylinder which causes it to expand unevenly and the cam ground piston is designed to accommodate the cylinder? Please correct me if I misinterpreted. $\endgroup$ – Klik Jul 6 '17 at 3:30
  • $\begingroup$ There is certainly a temperature gradient in the piston. Your article says that! In fact the piston temperature will be constantly changing depending on the power output of the engine. (To be honest I think the article is over simplified - it ignores the fact that the seal between the piston and cylinder is created by the piston rings not the piston itself, and the rings are much more flexible than the piston.) $\endgroup$ – alephzero Jul 6 '17 at 5:36
  • $\begingroup$ You are misunderstanding the situation. Read any basic ICE design book. $\endgroup$ – Carl Witthoft Jul 6 '17 at 13:13
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    $\begingroup$ Carl, if it's easy and obvious and common and covered in any basic book, can you perhaps submit an answer, rather than simply denigrate every answer offered? $\endgroup$ – achrn Jul 6 '17 at 15:24
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This is my best guess - I could be wrong:

From Fig. 6 in the reference article, you can see that the piston rod has an I-shaped cross-section (presumably because the rod itself is subject not only to axial compression but to some bending while the piston head is not directly in line with the axis of the rod). Possibly, the connection between the piston head and the pinned connection with the rod has a similar configuration (although it's not shown in the Figure).

It is possible that at this connection, a great deal more restraint is provided in one direction compared with the other. I imagine that as the piston head heats up, it is inclined to expand more in the direction in which less restraint is provided by the (relatively) cooler piston rod/pinned connection. In that case, the minor axis of the elliptical piston head would coincide with this direction of least restraint.

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    $\begingroup$ Please don't post "guesses" as answers. It's not that hard to look up the actual reasons, especially for something as common and well-engineered as modern ICEs $\endgroup$ – Carl Witthoft Jul 6 '17 at 13:14

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