A side pocket mandrel is of hollow metal tubing having a cross-section that is symmetrical about an axis and in which one of the symmetrical cross-section halves reveals a smoothly continuous convex curve, comprises a first, second and third circular arc portions of different radii and different centers of gyration; the first having a radius r.sub.1, the second r.sub.2, and the third a transcending radius r.sub.3 with r.sub. 1 greater than r.sub.2 and with r.sub.3 being less than three times the sum of the radii r.sub.1, r.sub.2 and the distance between the centers of the first and second portions, but no less than one-half of such sum of r.sub.1, r.sub.2 and that distance between the centers.