The results of the study of the impact of the component composition, mass and configuration of powder-briquetted modifiers on the structure and properties of high-strength cast iron obtained by the method of intraform modification are presented. Modification of the melt is provided at the expense of modifying inserts—cylindrical bushings of two configurations—with an axial cylindrical opening and a hole in the form of a conical surface (diaphragm) pressed from two variants of mixtures of magnesium, FeSi, Fe and CaF$_{2}$. The mass of modifying inserts varies from 25 to 100 g. The results of the studies show that the use of a modifying insert in the form of a diaphragm provides an increase in the average content of magnesium in casting and strength of the resulting cast iron compared to the smooth sleeve for all values of mass inserts, besides increasing of uniform distribution of magnesium along the length of the casting for variants of high-mass inserts (75 and 100 g). Increasing the weight of the modifying sleeve leads to an increase in the level of alloy strength.