THE DIAMETER IS 96 MM.
Wolo 357-12 Industrial Series 370 Disc Horn, 12 Volt, 385 Hz INDUSTRIAL SERIES 370 DISC HORN ™, 12 VOLT, 385 HZ O. ALL STEEL CONSTRUCTION, PAINTED BLACK ENAMEL FINISH. UNIVERSAL DISC HORN, AVAILABLE IN 12, 24, 36, & 48 VOLT, 2 TERMINAL SCREW TYPEEASILY FITS INDUSTRIAL EQUIPMENT MOTORCYCLES, CARS AND TRUCKS. COMES WITH STRONG ZINC PLATED 90 DEGREE MOUNTING BRACKET.