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Cummins 12MM Banjo Seal Washers (6)



Banjo seals are known by many different names and we've tested them all! The most common names are banjo bolt washers, unit rings, coated washers, bonded seals, banjo washers, seal washers, O ring washers, 12MM washers and sealing washers to name just a few. Some Cummins diesel owners have even used nylon washers in an emergency fix. I don't recommend this, as I will explain later.

macrobsb001.jpgCummins and many other engines use sealing washers on their fuel system connections. The design and construction of the banjo washer may seem insignificant and not worth the time to discuss it. After reading this, I think you will agree that the Cummins sealing washer is a critical part of your diesel injection system. 


We have a box of used seals that we have tested from all different sources. Some were "renamed" industrial seals with a diesel name on the package. Others were hydraulic seals commonly used in Europe. Several of the test pieces didn't make the grade while others worked quite well, but they cost more than $4.00 each. 

Take a look at the cross section image of our sealing washer, to the right. bonded-seal-0001[1]-0001.jpgThe white area on the inside is the steel core. Without it, the rubber would be squeezed out whenever you tightened the banjo bolt. Why not use a copper, aluminum or nylon washer? We found that if the tapped hole is not perfectly square with the face of the part, these three seals can leak. Surface finish and corrosion pits make sealing with these washers difficult, at best. More on that later.



Why do you want flared lips on the Cummins banjo seal? Why not use a flat bonded seal washer or Unit ring? There are two very important reasons why and one of them is not very obvious. What seals the fuel from escaping, whenever you run your Cummins engine? The flared lips do and to a lesser extent, the flats of the seal. In the Cummins, the fuel pressure to the injection pump is from 10 to 32 PSI and as high as 65 PSI in P pump perfomance applications. The fluid pressure acts on the radius and forces the lips apart, which seals the banjo fitting. The higher the fluid pressure, the better the seal.

Why is this style of seal the best for your Cummins engine? After you shut your engine down, the fuel pressure in the system will fall to zero and then it will show a vacuum reading on a sensitive pressure/vacuum gauge. The vacuum is created from the weight of the diesel fluid column, from the injection pump, to the fuel tank. The diesel fuel, by its weight, wants to flow back to the tank. Introduce even a pin hole leak into the system and the diesel fuel will flow! Park on an incline and the problem gets worse.


Let me illustrate the importance of a good seal with a true story. I spent several phone calls with a man from Ohio, trying to help him figure out why his Dodge Cummins was a bear to start. It would stall on occasion, too. Bob checked for bad hoses and replaced them ALL,from the tank module to the Bosch P7100 injection pump. He replaced the square cut rings on the fuel strainer/heater assembly and replaced the fuel return line located under the intake manifold. He checked the tank module for rusted fuel lines. They were OK. I suggested that Bob install a clear hose between the filter and P pump which he already did. NO VISIBLE AIR BUBBLES!


Guess what caused all the problems? He replaced a Cummins seal on the fuel filter with a hardware store nylon washer. The fact of this story is, a nylon washer will in most cases seal under pressure (no visible leaks), and fail under vacuum. Even a pin hole leak (or vacuum leak) can make your Cummins diesel difficult to start by allowing the fuel to flow back to tank. Remember, if the tapped hole is not perfectly square with the surface, a non standard seal can leak. Our seal with flared sealing lips is much more tolerant of surface imperfections and non square tapped holes.

On your Cummins fuel system, you need the flared lips to seal under pressure and seal under vacuum! The seals that we stock are not standard 12mm Cummins banjo seals. We have them molded to our specs with some slight changes. Mainly, the durometer (hardness) of our seals is different from off the shelf items. The seal lips are super smooth which helps in sealing the banjo fittings and the Cummins fuel system. Unlike poor quality seals which show metal through the rubber covering, the BS12MM seal washer is covered with 100% rubber. They seal perfectly.

Part number: 6xBS12MM 
Superseded part number: BS12MM

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