an annular groove and the relief piston is formed by at least one annular recess a channel extends transversely through the relief piston at said annular groove an axial blind channel terminates at said transverse channel these two channels form part of the flow path for the fuel the upper edge of either the annular groove or annular recess and the lower edge of the other define therebetween a maximum flow through area for the fuel when the relief piston is in the upstream position this maximum flow through area is substantially equal to the cross sectional flow through area of the discharge conduit application_number US 65665176 A description referring now to fig1 there is depicted an injection pump constructed in accordance with this invention the upper portion of the pump piston 3 is provided with a longitudinal groove 31 and with an underlying recess which cooperates to form a profiled regulating edge 32 of helical form the piston 3 is supported for rotation about and for reciprocation along an axis 100 in a cylinder 2 having a radial channel 21 which communicates with a fuel chamber 11 into which fuel is supplied via a supply channel 12 the latter channel as well as the fuel chamber 10 are bored into a pump casing 1 having a bore 1a into which the pump cylinder 2 is tightly fitted a gasket 4 having a central aperture.