These glass-filled nylon mounts are engineered to give all the support of an aluminum mount with half the weight. The webbing design gives rigidity, yet the mount will still break in a crash (thus saving expensive engine repairs). This is a round base mount, which is ideal for sharply pointed-nose planes. This mount fits most .40 size engines with rear exhaust. * Distance between arms: 1 5/16" Diameter of base: 2" Length of arms: 2 15/16" updated NMH 6-30-93 updated pcd 12/1/95 Fits crankcases from 1.300 to 1.330 inches wide (6/18/95 RDT).