This piston (and the block it was in) was new less than 300 miles and 6 months ago.
Its from a 427 gas that'* in a triaxle grain truck...too much truck for the engine, but still. I still find it hard to believe that a mixture problem could do this in less than 300 miles. Two other pistons from each bank were scarred as well.

thats messed up!

Wow, that is hard to believe.

I did some searching around, but I couldn't find my answer. Why is running lean so bad (why does what happened to that piston occur?)? I understand rich will leave carbon everywhere...but what happens when lean>?

Logan, exactly what you see there. Running lean will blow holes in your pistons.

Running lean is A LOT hotter then running at a good stoich mix, or rich.

Your EGR system follows this principle. The EGR is there to cool the combustion chamber, and while it does add hot exhaust gas into the mix somewhat, it richens up the mix by taking up space that fresh air would normally be in. This makes the fuel:air ratio higher, and richer, therefore running cooler.

Detonation. Very bad.

When we had the engine rebuilt last year, we re-used the old carb...bad idea.

I got the new carb today and put it in. It'* a Holley Truck Avenger 770 cfm and its purdy as can be.

A 427 in a tri-axle? wow...that is underpowered.

My dad'* single axle propane bobtail that he drives runs a 454 that runs solely off propane and that thing is gutless...top speed of literally about 65-70, and takes forever to get there.

Yeah. the 427 is supposed to have more torque than a 454, but big whoop. This thing has both a 5 speed transmission and 4 speed gearbox coupled together, and it still tops out at 55 empty.

So too lean causes too fast, which causes too hot, of a burn? Then the increased cylinder temps causes a pre-det?

And is that an indentation of the valve in the back there?