Yeah, I have used JB Weld to fix holes in blocks, too. I have used it to fix the block where the chain broke and put a hole in the block just like yours did. Just clean all the oil off the block real good and it will hold. If it is a large hole, just put some JB Weld around the edge of the hole and then cut a piece of screen wire or fiberglass cloth and lay over the hole onto the JB Weld. Now put more JB Weld over the screen wire or fiberglass cloth. Smooth it out as good as you can. Let it dry for the full amount of time specified on the tube or package. When it's dry you can sand it and paint it. If you do it right, it will hold and you won't even hardly be able to tell it. I've fixed 3 or 4 like that and they held, too. Didn't leak a drop, but you have to make sure you clean the oil off the block really good or it won't hold. It's best to use Acetone to clean the oil off, because it dissolves oil real good and it drys quick and it don't leave any type of residue or film behind.