What the Weight of Pisten Required.

Ideal weight for all reciprocating parts is zero.

Do you want smooth, or efficient? With zero weight components there will be a pronounced decrease in smoothness due to the fact that there is no mass to adsorb the shock of the firing pulse.

The question is ambiguous.

There's always the flywheel ...

Touche'.

It doesn't reciprocate, does it?

Indeed - as you know, the mass of the crank pin can be balanced by shaping or weighting (OK, removing or adding mass from/to) the flywheel (which must have mass to ensure that there's an "upstroke"). With no piston (or piston rod) mass, the engine could be perfectly balanced (if, e.g., the crankshaft were rotated by an external torque (applied at both ends?)).

Unfortunately, it's not as "easy" to get rid of the bending moment applied to the crankshaft by the piston rod, particularly during the compression and ignition strokes.

(Sorry, I'm waffling ).

Pistons are typically made from forged aluminum with as much material removed as possible to minimize the moving mass and still retain enough physical strength for reliable operation. As stated, the ideal piston "mass" is zero, but I am not aware of any suitable material that has zero mass.

If anyone has such a material (see Unobtainium), I would gladly purchase the entire stock using an equivalent mass of gold as payment.

Weight mass - my bad.

Is waffling anything like knurling?

The ideal weight of a piston should be not 1 tiny bit heavier than is required to perform efficiently for one life cycle.

The weight for a formula 1 Engine would be manufactured to different standards than a Briggs and Stratten of the same displacement. The piston designed for a 350 Olds diesel was way different from the 350 Olds gasoline engine.

What is the correct length for a connecting rod?

Correct length of connecting road is depend upon the weight of piston. If weight of piston is more the length is required more for best performance.

On what planet?

"Correct length of connecting road is depend upon the weight of piston"

The "Correct length of connecting road" is a straight line between two points.

The correct length of a connecting rod is generally as long as practical. (For max power)

ok Bob how long?

shorter should be better, less mass

long enough so the skirt of the piston doesn't hit the throws of the crank

I like to have waffles after curling, but not the frozen kind, as I would already be cold enough

I agree that the lighter the piston the better. Obviously it must be able to withstand the stress, wear, possible fatigue, and heat especially at the crown.

If retrofitting lighter pistons to an existing engine, reciprocating assembly balance must be considered. Generally the piston mass (and a portion of the rod mass) gives an out of balance spike at the top of the stroke and at the bottom of the stroke. In a single cylinder engine the crank weights counter some of this but since the crank weights go around and around, the cost of cancelling some of the piston/rod mass acceleration is that there is out of balance elements added by the weights at other than TDC and BDC positions. In multi cylinder engines often times the crank throws are positioned so that the mass of the piston/rods cancel out as much as practical of the other piston/rods. That is why horizontally opposed engines are generally smooth.

The reason I mention balance is that when changing piston mass the balance must be considered. The companies that sell light weight aftermarket pistons have considered this and know how much lighter they can go without seriously affecting the balance of the reciprocating assembly. Some aftermarket crankshafts are also designed to be used with specific non standard piston and rod masses.

I would like to also say something about connecting rod length since it was brought up even though the original post did no mention it. The shorter the connecting rod, the higher the loads are between the piston skirt and the cylinder side wall. When the piston is one half way between TDC and BTC during the power stroke (crank at 90 deg) the rod "big end" is at its maximum offset position. As the piston pushes down during the power stroke there is also a side load between the piston and cylinder wall. This side load obviously has limits due to friction and heat. Typical piston engines have long enough rods that this force is not a serious issue but if you put a stroker crank in an engine (to increase the displacement) often times you would put in shorter rods so the piston to block relationship stays the same when at TDC. There is a limit to how short a connecting rod should be used for this and other factors.

Some singles & twins use a gear driven balance shaft to cancel out part of the vibration,

unless of course it's a harley & then the vibration is encouraged by having both connecting rods on the same throw of the crank

As Dave mentioned

short connecting rods increase the load on the piston skirt,

opitimal connecting rod length is a compromise

Yep, the shared crank throw also gives the syncopated power pulses.

Bang bang ......................bang bang ........................bang bang....................

Is that a mechanical stutter?

Not taking chamber pressures etc into account and concidering undeformable materials of course the smaller moving to non-moving parts ratio the better, the longer the connecting rod the better (gives closer to sinusoidal piston speed and acceleration) but then in real world compromices are made. Not much HP from a tin-foil piston and a mile long needle sized connecting rod. And by "moving I mean only parts having some linear motion. Crankshaft for example is not "moving" just rotating But for a given configuration piston plus a fraction of connecting-rod's mass must be "matched" with crankshaft's fixed per-cylinder imbalance 53 2E 4D 2E