This pump also works on relatively small volumes of water. The inner was just big enough for the bolt taking into account that the thread will be filed off the bolt inside the guide tube , but did not have enough substance to split it into four extended legs to support it above the plate, but the larger tube served that purpose. I suppose, one large hole should work as well, as long as it is a smaller than the valve disk to allow the sealing overlap. As the momentum increases, the pressure of the water will drag the impulse valve shut.