Open Deck Block
What is an open deck block?
An open deck block has very large water galleries surrounding the bore with not much metal holding the bore in place, see image below.
The bores on an open deck block are likely to collapse on them selves under high boost and revs, due to the lack of metal holding the bores in place.
Since most cars now days have open deck blocks (cheaper to manufacture). To get high performance without buying a closed deck block, you can get the close the deck by welding rods into the water galleries, see image below for a rough idea on how it is done (marked in red).
