Simple 12-Valve Cummins Upgrade With the Fleece Fuel Block-Off Plate

AS SEEN IN OUR MAY 2026 ISSUE – Buy Now!

Installing a Fleece Performance Fuel Block-Off Plate on a 12-Valve Cummins

Few engines in the diesel world have earned the reputation and loyalty of the 12-valve Cummins. Simple, mechanical, and brutally effective, the 5.9L has powered everything from work trucks to all-out competition builds for decades. But as many owners modernize their setups with electric lift pumps, certain factory components are left behind. One of the most common is the stock mechanical lift pump.

When the factory lift pump is removed, the opening in the engine block must be properly sealed. Most of us just go ahead and grab one off the shelf at the part store and grab a tube of black RVT to seal it up. But that’s the thing—it never does seal, does it? That is where the Fleece Performance fuel block-off plate comes into play. It is a small component, but one that serves an important role in both reliability and appearance when you’re changing the starter on it. We recently installed this part on our 12-valve Cummins to show just how quick and worthwhile this upgrade really is.

What You Will Need

This job can typically be completed in under 30 minutes once the turbo is out of the way, making it an ideal upgrade—even for those working in the driveway. The install itself is very straightforward and can be done with basic hand tools. You will need the Fleece Performance fuel block-off plate kit, which includes the plate and O-ring seal, along with a ratchet, appropriate socket, and a clean rag. At this point, it is important to clean the mounting surface on the block. Any leftover gasket material, dirt, or oil residue can compromise the seal of the new block-off plate. A clean, flat surface is key.

Installing the Fleece Block-Off Plate

With the mounting area prepped, install the supplied O-ring onto the Fleece block-off plate. Lightly lubricating the O-ring with clean engine oil helps ensure it seats properly without pinching.

Position the plate over the opening and align it with the factory bolt holes. The plate is designed to sit flush against the block, so there should be no force required to set it into place. Install the bolts and tighten them evenly, snugging them down to factory-style torque. There is no need to overtighten, as the O-ring does the sealing work.

Once installed, give the area a final wipe and visually confirm the plate is seated evenly all the way around.

Final Checks and Benefits

After installation, start the engine and check for any signs of oil leakage around the block-off plate. When installed correctly, the Fleece plate provides a leak-free seal and eliminates one more potential failure point on the engine.

Beyond functionality, the visual improvement is immediately noticeable. The CNC-machined Fleece plate gives the side of the engine a clean, finished look that matches the quality of modern performance upgrades. For builds that see the show circuit or simply pride themselves on attention to detail, it is a subtle but meaningful improvement.

A Small Upgrade With a Big Impact

In the world of diesel performance, it is easy to focus on big horsepower parts and to overlook the small details. The Fleece Performance fuel block-off plate proves that even minor components play a role in reliability and presentation. For any 12-valve Cummins running an electric lift pump like a FASS or AirDog, this is one of those no-brainer upgrades that checks every box.

Simple installation, quality construction, and peace of mind—that is exactly what Diesel World readers expect, and exactly what this upgrade delivers. Shout out to Fleece Performance for keeping great-looking and great-working parts for these old trucks, including Stock to Not!