Find a wide array of Silicone Couplers & Boots to finish your custom intake on your 4BT 3.9L Diesel. These reinforced couplers are manufactured with durable rubber or silicone for increased reliability. Sizes from 2" to 6" in diameter are available to fit the application for the boot you need. You'll find straight, reducing, hump, and angled boots ranging from 45 to 90-degree bend to choose from. You'll find top brands such as Vibrant, PPE, Full Send, and Mishimoto brand products available at XDP.