Which function allow it to hit through a block (like water/lava)

Because i do not want that you can break my gas^^"

You'll want to override the following functions:

//This disables the ability to place torches/redstone/etc on this block. Use it on non-solid blocks or blocks with custom renderers.
public boolean isOpaqueCube()
{
    return false;
}

//This removes the collision box from the block, making things pass through it.
public AxisAlignedBB getCollisionBoundingBoxFromPool(World par1World, int par2, int par3, int par4)
{
    return null;
}

Thanks but already solved. And you was wrong.

There is a function isBlockColideable()

that makes that you can hit a block or not