Du-Bro Spring Steel & Solder Kwik-Links \(4-40\) Part No: 304. Spring Steel Kwik-Links are the tried and true top quality Kwik-Link. Ideal for control linkage hook-up. Use with threaded rods or threaded couplers. 4-40 are for use with .093 dia rods, spring keepers are included. Solder Kwik-Links are nickel plated making a strong, attractive and easy to solder connector for servo or control horn. 4-40 are for use with .093 dia rods, spring keepers are included.