The SS-3H Heavy 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. This heavy duty model may be used with antennas up to 5′. Physically the spring is about 3.0″ (76mm) tall with a diameter of 0.800″ (20mm).