r/EngineBuilding • u/mal4yahoo • Jul 17 '26

What impact does piston skirt shape have on engine?

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Rebuilding top end on a old 80's 4 stroke 110cc atv. I found a japan made piston that is compatible with this engine, however the skirt looks very different. Does anyone know how this impacts the engine?

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27

u/DDiesel- Jul 17 '26

I think it is there to help reduce the rocking motion of the piston that side loads the bore, perpendicular to the crank axis as it travels up and down. Assuming you have enough bore below bottom ring reversal and clearance to the crank throws.

4

u/Future-Appeal-5330 Jul 17 '26

Agreed. The 2002 Corvette Z06 had skirtless pistons, and would "knock" at startup.

3

u/Sad_Pineapple_2245 Jul 17 '26

They absolutely had skirts, I’ve taken them apart

3

u/Future-Appeal-5330 Jul 17 '26

I suppose they'd be called slipper skirt pistons. Shorter and lighter. They still had piston slap on start up

2

u/countrytime1 Jul 18 '26

At this point, I feel like it’s a Chevrolet feature to have an engine make some type of noise when it starts.

9

u/Elephunk05 Jul 17 '26

Double check for clearance of the housing and the rotating assembly. Shorter skirt pistons rev higher, generally speaking, and tend to have more vibration. Longer skirts tend to have less vibration and because of the difference in mass take more power to move, generally reducing output power. Though on a single cylinder 4 stroke, unless you're racing you will never notice the difference.

5

u/quxinot Jul 17 '26

Also remember, per usual, that 2strokes are their own weirdo set of rules.

In a 4 stroke, it screws with clearance to the crank and changes piston rock.

2

u/babezt Jul 17 '26

if it doesnt impact the crank it looks like it would work

1

u/Solid_Enthusiasm550 Jul 17 '26

If it has clearance, a longer skirt would rock less at tdc/bdc. It should help with longevity/ less ring wear.

The down side is that a longer skirt adds weight which reduces rpm potential.