The SS-3M Medium Duty SS Springs are made from long lasting, top grade electro-polished stainless steel. The upper and lower inserts are made of chrome-plated machine brass and have standard 3/8″-24 threads to fit all Firestik antennas, mounts and accessories. The medium duty model works especially well with our 2ft “KW” & “FS” series antennas as well as the 3ft “FL” or “RP” antennas. The heavy duty model may be used with antennas up to 60″. Physically, each spring is about 3.0″ (76mm) tall with a diameter of 0.800″ (20mm).