An arrangement in a pulp grinder comprising a grindstone, a feed chute extending to the grindstone, a piston moving in the feed chute, an auxiliary piston and closing members. In the arrangement, there are recesses on the front of the piston and apertures in the auxiliary piston so that the closing members can be pushed from both sides of the feed chute through the auxiliary piston to the recesses of the piston so that they settle behind the wood pressed by the piston.