For the 1957-1958 Pontiac Bonneville, this lower antenna mounting base measures 2-3/4" O.D. (base) x 1" I.D. x 7/8" high, making it a perfect fit for your classic car. Designed with high-quality compounds, this part is built to last for years, offering excellent chemical and UV resistance to withstand harsh conditions. Its sleek black finish complements the vehicle's exterior while ensuring a secure and snug fit for the antenna. This mounting base is easy to install, sitting neatly at the base of your antenna to provide stability and protection. Backed by Metro’s Platinum Warranty, this part is covered for 30 years, giving you peace of mind and confidence in your purchase.