Blew a hole in the rear piston - why?

[1970_mgb]

Sportster LE 2000 / 951 CC engine

Long story short, pulled the head to investigate zero compression in the rear cylinder and found a hole in the piston at the circumference at about 4 oclock position looking down from the front of the boat. The hole was triangular in shape, about 3/4 inch around the cylinder wall and 1/4 inch towards the centre of the piston. Smooth surface.

Small bits of ring stuck to the cylinder dome, but otherwise the piston looked normal except for the hole, ie no alumimum spatter, spark plug was dirty but not burnt. No scoring on the cylinder wall. The other cylinder looked fine.

Boat was running fine up to that point with the exception of some hard starting issues. WOT it was running normally. On the way back it just lost power.

My questions are:
1. Why might this have happened?
2. Can I just rebuild the top end or might I need to do something else? Obviously I need to find and correct the underlying problem.

Additional info. At the beginning of the season , had contaminated gas. Would not start.
Had to drain out and replace gas. After that no issues.
Engine has less than 100 hours.

Thanks

 

[Seadoobuddy]

A reason might be that it was running too lean.
Let's see what the experts say about this problem.
give them a chance to answer.

 

[94greenXP]

Went lean. Check ur carbs. I bet one has a bad diaphragm. Also many bad fuel lines. So on.

 

[1970_mgb]

Thanks, In the back of my mind Im thinking cracked crankcase. arrrg !
Had lots of rain water water freeze solid in bottom of the engine last fall , but after head was winterized with anti freeze. Wondering if that might have cracked something allowing an air leak. Might also explain the contaminated fuel.

 

[rotaxrule1]

So did rain water surround the engine in the hull or did it get into the case? Water should never be in the bottom of these engines, that would mean disaster if not quickly rectified....

 

[1970_mgb]

Well I winterized the boat , had the cover on but forgot about taking out the drain plugs. We had torrential rains followed by a solid freeze so the engine was surrounded by a solid block of ice, should not have gotten inside the crank case since it is sealed unless it cracked the block. In the spring I removed the plugs and cranked it to get rid of any water / fogging oil but.. couldnt start it till I replaced the gas.