Cummins Captive Isolator Screwcap 3959400

This high-quality Cummins 3959400 captive isolator screwcap is engineered to securely fasten the valve/rocker cover while helping seal out oil leaks and contaminants in demanding diesel environments. Built to OEM specifications (M8 x 1.25 x 66), it delivers durable, repeatable clamping force and proper fitment on compatible Cummins engine platforms used in on-road, off-road, and industrial equipment. The captive isolator design keeps the fastener and isolator together, simplifying installation and service while reducing the risk of dropped or lost hardware during maintenance.

Key Features

•    Genuine Cummins OEM part number 3959400 for reliable fit and performance.

•    Captive isolator cap screw used as a valve/rocker cover bolt on many 5.9L and 6.7L Cummins engines (commonly 2006–2018 era applications; check your exact fitment).

•    Thread/size: M8 x 1.25 x 66 for correct engagement and clamping load on the valve cover.

•    Designed for harsh diesel operating conditions in trucks, industrial, and off-highway equipment.

•    Ideal as a replacement for worn, stripped, or missing valve cover bolts to help maintain proper sealing and appearance.