Theory and Analysis of Single Piston Melting In 2 stroke Outboard Motors

[Fishinmymission]

Hey guys I work as a failure analysis engineer and decided to tear down two optimax v6 engines with a single melted piston for my masters thesis project. This is the presentation that I created and I thought that it would be cool to share with others.

[gmorgan]

Very informative article. As long as I have been messing with two-stroke outboards, I never thought about the fact that two-strokes fire twice as much as four-strokes, so they produce more heat. Duh!!!
everybody can gain something from reading this.

[PanRonnie]

Great read but i could think off piston quality
when I broke the promax with the original pistons they looked like styrofoam aluminium they were melted
With the wiseco piston last time it desintregrated like a piece of rock no melted pieces
I am surprised the did not put the diverters in the cooling channels?
They did in the sst 200xs

[Fishinmymission]

So for any metal failure at work i would also take chemical composition and hardness i just didnt have access to that equipment for this at the time. This would be critical for precipitation hardened aluminums but it would not make sense to use a heat susceptible alloy in a place like a piston. Hardness and composition would let me know that the right alloy and heat treatment were used. I would also likely look at a polished and etched cross section to examine the microstructure. A melting failure especially after a long period of time has little correlation to material quality in my opinion. Reaching melting temperatures is simply a matter of reaching melting temperature which is why i looked for melted morphologies on the scanning electron microscope. The other things we look for with an SEM are if cracks are fatigue (cyclical stress slowly propogating) ductile overload (think high stress and breaking away) or intergranular(grains falling apart due to stress corrosion cracking, liquid metal embrittlement, or for steels hydrogen embrittlement). On the piece I cut then fractured off to fit in the SEM I saw typical ductile overload than melting on the other surfaces.

Nothing suspect of the material quality with the data I had and things I saw.