Blew a hole in a piston?

[L_C99]

I have a 85 SS440J and I was riding today, not fast or hammering on it, and I lost a cylinder. Long story short I limped home on one cylinder and took it apart and there is a big hole in the piston on the clutch side.
So my questions is what causes this? Was it just too warm of a day for an air cooled (50F)? Do I need to use premium cause I just filled up with regular before this happened?
Basically I don't want this to happen again if I rebuild the top end so I want to know the cause. I thought I read a thread somewhere about this but I couldn't find it again.
Thanks

 

[Crazy Axe]

Lean fuel mixture causes detonation, which blows holes in pistons. Check for cracks in the carb boots. Or it could be a clogged carburetor jet or even bad crank seals. I would bet it's one of the first two though... Could also be caused by a hole in the exhaust which you should look for but I doubt that's it.

 

[terret725]

if you could post a picture of the pistion it would help. lean burns are usualy on the exhaust side of the pistion. usualy rounds off the leading edge of the pistion till it gets to a ring and snaps it off. det. is usualy the top or all sides of the pistion depending on what the combustion champer looks like. its usualy caused by low octane. the fuel air mixture on the outside of the chamber fires befor the spark plug does and when the 2 flame fronts meet it creates a hot spot that burns one small piece at time untill it burns all the way thru. this is what yours sounds like. with the high temps and low octane it was probabaly a combination of the both. the motor was runnning at a higher temp then normal and then low octane began to fire on its own befor the spark.

 

[L_C99]

As requested here is a picture of the hole in the piston. The exhaust side is on the left and intake on the right.

Based on your description it is probably a combination of both. It looks like there are some "lean burns" on the exhaust side of the piston, but the hole is in the middle of the piston so the lean air/fuel mixture plus the higher than normal temps is probably what cause the problem.
So what should I do to prevent it from happening again? Colder plugs? Turn out the needles some? Premium gas only? Combination of thoes things?

Also I was looking at some rebuild kits and there are generic that cost about $100 and Wiseco which costs more like $200. So is it worth it to go for the Wiseco?
Thanks for your help.