Why did my cylinder block break

[jamesmerc]

Hey guys,
A few weeks ago, I posted about my cylinder block breaking. I have a 1990 25hp mercury. I'm going to buy a new block but I just wanted to know why this would have happened. Someone mentionned that I may have overtorqued my plugs, but that doesn't seem likely to me.
Please let me know what you think

 

[MTboatguy]

The only time I have seen a plug hole blow out that way, was on one of my ATV's and I put a plug in that was to long, it cracked the top of the piston and blew the plug out just like your motor looks. Have you checked to make sure the right plugs were in it and have to checked the piston top? What does the bottom of the plug look like?

 

[jamesmerc]

The plugs are NGK BUHW, I was given them by the mechanic who had worked on the motor for me. The piston tops are fine, no damage but a bit sticky. The bottom plug is also ok

 

[racerone]

I have a block with an exact / identical failure.--------Perhaps overheating and a flaw in the casting is the failure mode.

 

[MTboatguy]

Like racer said, it could just be a flawed casting..although rare, it does happen, with a 25 year old motor, unless you have owned it since day one, you don't know what it has been through.