A wood pulp grinder includes a grinder housing for supporting wood to be ground above a grindstone. A main press chamber in the grinder housing supports a main press ram which moves toward and away from the grindstone. When the main press ram is upraised, the main press chamber is charged with wood to be ground. Adjacent one or both circumferential sides of the main press chamber is disposed a respective auxiliary press ram with is movable toward the grindstone independently of the motion of the main press ram for retaining pressure upon the wood in the grinder housing while the main press ram is not pressing upon the wood. The auxiliary press ram or rams are relatively long in a lengthwise direction, having a length equal to fifty percent and preferably at least eighty percent of their active press stroke. The active lower surface of the auxiliary press ram may be inclined generally in a direction tangent to the surface of the grindstone or may be curved convexly toward the grindstone. When the main press ram is being retracted for reloading the press chamber, a closure plate moves across the bottom of the press chamber for preventing reverse movement of the wood.

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