ASK THE MXPERTS; IS IT TRUE THAT PISTONS AREN’T ROUND?

Dear MXA,
   Is it true that piston aren’t round? A kid in auto shop told me this, but I have looked at a lot of pistons and they all look round to me. Are they round?

They aren’t even close to round. Pistons are egg shaped from front to back. This egg shape is called “camming” in piston speak. Camming is hard to measure because the sides of the skirts aren’t there, but if you could measure them, you would find that many pistons are as much as 0.010″ out of round. Camming helps resists rocking. The extended fore and aft skirts aren’t the only thing that resist the tilting load placed on the piston.

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