Piston inspection diagnosis

Just to update this post, I have shown the piston to an experienced 2-stroke mechanic who said that this is the result of blow-by. Carbon particles gettingtrapped between the piston and cylinder wall cause the scratches.

He also pointed out to me that there were marks at the bottom the skirt that indicate the beginning of piston slap. (I'm not sure if you can see them in the photo.)

The flakiy nature of the carbon on the head of the piston is caused by infrequent running. Apparently the inital carbon layer absorbs moisture while the aircraft is laid up, crystallizes and then the next layer has limited adherence to it. 

He also agreed that 2-stroke oil can go off with time if the container is open to the atmosphere. Mine was stored in a 10 litre barrel.

However, if there had been an oil failure one would see seizure marks rather than scratches.

Edited: to add that the label on Shell VSX says, "Shelf life: 4 years from the date of manufacture if kept covered, stored ina dry place and protected from heat."

Here is a better picture of the early-stage piston slap on the bottom of the skirt.

Here is a better picture of the early-stage piston slap on the bottom of the skirt.

There is also a small cold seizure mark on one 'corner' of the piston on the intake port side.

I hope this information is of use to people in future.