Many pistons aren't round. They are actually slightly oval. This is to make up for the piston pin boss, and the expansion of the piston when it gets hot.

Good luck,
David Duffield

beepbeep, actually the real reason for the hemi engie design was to allow the largest possible valves in a give bore size. The hemispherical head has the greatest area and therefore them most room to fit the valves. It also allows the valve to be at a steeper angle to the top of the piston which straightens out the intake and exhaust runners as well as improves airflow within the cylinder before and after combustion. I'm in agreement that rings are probably the singe biggest reason for round pistons with complete combustion a close second.

edit: old_school is correct on that MOST pistons aren't round, at least at ambient temperature, it addition the sides aren't parallel either. Pistons are design to become round once they've heated up which is why you can sometimes here piston slap from forged pistons as the warm up since forged aluminum expands more than cast or hypereutectic.

Interesting. Especially Superman's sexual explanation. Oh, the PNW...

I'm not fully convinced that, as Jim Sims says, "The shapes and forms of machinery can be traced to the process of satisfying the requirements of statics, kinematics and other physical laws of motion in the most efficient ways possible." If that's true, Mechanical engineering would be a field where there's no difference between ideal and reality. But that's probably not a discussion for the Technical Forum
If, as you so delicately put it, these topics are covered in sophomore ME, could you point me to a standard textbook?

Please note I stated: "most efficient ways possible." "possible" is what separates the "ideal" from "reality". I remember distinctly a problem given to us in the introductory kinematics course that resulted in an infinite acceleration being calculated (I think it was a cam system). The professor let us a ponder it a bit before he pointed out that infinite accelerations meant infinite loads and that materials have finite stiffnesses and strengths and that something would break. Such is the difference between the "ideal" and "reality".

If, as you so delicately put it, these topics are covered in sophomore ME, could you point me to a standard textbook?

"Kinematics and Dynamics of Machines" by George H. Martin, McGraw Hill Series in Mechanical Engineering. It has a nice section on balancing reciprocating masses included opposed configurations like the 911 "boxer" engine.

Cheers, Jim

Thanks, Jim!