I picked up a 21' day cruiser last fall and ended up having to winterize before I could take it out for a spin. While winterizing, after pulling the exhaust manifolds and removing the drain plugs on the block, I discovered a hard solid material behind both plugs. My best guess is this is Block-fill. What I don't get is why would this be done to a marine 454 Chevy? The prior owner didn't know about it and so far doesn't have the information on the building of this engine. If anyone knows why fill a block and what benefits or liabilities of having a filled block there are, please reply to this post. Thanks..