Hard Blok is a specially formulated, cement-based product with refined, size-graded iron particles combined at an optimum ratio to provide maximum strength, vibration dampening, and heat dissipation required in high performance race engines. Its coefficient of thermal expansion is identical to cast iron engine blocks and it has been tested and proven effective in aluminium blocks. One tall fill unit will fill most big blocks to the bottom of the water pump connection. In certain tall deck blocks, the fill level may be an inch or so lower. This level is adequate to stiffen the cylinder walls. Use the tall fill kit for 350 cubic inch and larger blocks. Use the short fill unit for circle track and street rod applications.