Firestik heavy duty springs are designed to be used with antennas up to 5 feet tall. For 2 foot FS and KW antennas and 3 foot Firefly and Road Pal antennas, use a medium duty spring. These 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 3/8" x 24 threaded CB antennas. Each spring is about 3.0" (76mm) tall with a diameter of 0.800" (20mm).
You can find more information on CB antenna springs here.