The shells and sheaves are made of high dielectric, fiberfilled natural nylon. The shaft is of silicon bronze and the sheave bearings are oil impregnated bronze. Blocks are available with either polypropylene or Poly-Dacron rope for minimum stretch and long service life. Dielectric Strength: Dielectric rating in dry weather is 30,000 volts between the bearing and the mounting bolt nut. Maximum 1/2" rope.