The Wera 3950 PKL in 4mm is one of the most commonly specified sizes in the range, ideal for M5 socket head cap screws used extensively in general engineering, machinery maintenance, and structural assembly applications. Constructed from high-grade stainless steel, this key prevents the transfer of extraneous rust to stainless fixings — a key benefit in corrosion-sensitive industries. The Hex-Plus profile delivers a larger contact surface inside the screw head, substantially reducing notching and protecting the recess integrity across multiple tightening cycles. The ball end on the long arm offers the flexibility to drive at an angle, helping you tackle fixings in tight or partially blocked locations with ease. Features and Benefits: • Stainless steel construction prevents extraneous rust on stainless fixings • Hex-Plus profile protects screw recesses and extends screw life • Ball end on long arm for angled “round the corner” screwdriving • Metric sizing Contents: 3950 PKL L-key, metric, stainless, 4 x 137 mm Specifications: Key Size: 4mm Overall Length: 137mm