Reader Question: Square Cylinders?

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Here’s the latest reader question, along with my reply!

BD asks: Yesterday I had an interesting thought. What if the engine’s cylinders were square instead of circular? Yes, I understand they would cease to be “cylinders,” but it’s about what they would gain. The cross section of an engine block is rectangular, so square cylinders could accommodate more area, and thus, more volume for a given cylinder length. The area of a circle is (pi)r^2, and that of a square is 4r^2, the ratio is 4/pi, which is ~1.273, so a square “cylinder” would gain you 27.3% more volume! That means a 350 small block Chevy could be increased to ~446 cubic inches without changing the size of the block. My question is, have you ever heard of this?  Has any engine been built this way?  I reckon the primary difficulty would be “boring” the cylinders, but construction shouldn’t be impossible.

My reply: I’m buzzing along on too much coffee and not enough sleep but, if I recall, this has been tried and for the reasons you lay out – but the main problem is effectively sealing the right angles (i.e., the “rings”) of a square. With a circular piston the rings can expand and contract to uniformly bridge the gap between the piston’s surface and the cylinder wall, necessary for both compression and oil control.  A related problem could be differing thermal expansion of a square vs. a cylinder and flame propagation characteristics one vs. the other.

Machining is another issue. Here’s a good video:

It’d be fun to cast some and see, though!

. . .

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  1. On a round piston and cylinder, rings are spaced almost evenly around the piston keeping the piston in the center of the bore. A square piston would need to operate perfectly up and down even if the ring problem in the corners were solved. But then you still end up with cornering forces and keeping the engine perfectly level.

    I’d take a look if someone wanted to cast one and set it up with square holes and pistons but don’t want to put my own money into that experiment which has probably been done before.

  2. Eric,

    All of that makes sense. Also, this man was quite entertaining with his explanations. Thanks! I guess it’s tritely back to the drawing board.

  3. I would be worried about stress points in the corners, and possible poor ring/cylinder seal (resulting in excessive blowby) in the corners.

  4. “expand and contract to uniformly bridge the gap between the piston’s surface and the cylinder wall”

    I recall that was a similar problem with Honda’s oval pistons. While OK for sprints in expendable racing engines, their longevity in street machines wouldn’t have made customers happy.

    Not to mention Honda trying to sneak an engine with eight piston rods past governing bodies as a four banger; another chapter in Honda’s Hilarious History.


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