Bob, Thanks for your help. I realy appreciate it.
The problem is the Engine is new, Yanmar installed it in June, so it is still under the warranty for some more years. And one of my crew is the guy that installed it!! He is the Piano Man, because he is so big!!!
They took the stud with them, and I could almost swear thet they new what happened and didn't want to talk about it, too much.
I alm also inclined towards CP's comment " weak stud that was over-torqued at installation".
I had many options for engines when I had the boat built, and Yanmar was the winner (not cost related). I am sure that they will support their product.
I posted to see if it had happened to anyone else out there.
If I change the studs as you very well suggest, I will void the guarantee. So this is a case of sit back and watch.
The bolt was all smooth at the broken face.
PS Have you been to member galleries? I am posting some photos we did when the running rig was being installed, and of the testing. she still had the EXP logo on the side.