For motorcycles that carry the fuel tank directly above the engine, seepage or leakage from the fuel cock or adapter that takes out the fuel can lead directly to dangerous problems. Age-related deterioration of the O-rings and gaskets incorporated in the mounting area can cause leakage, but in such cases, it is important not only to retighten it, but also to check the condition of the seating surface and review it from the base.
Reuse of hardened, skinny gaskets is the biggest cause of fuel leaks
The fuel cock on this motorcycle is attached to the frame, and the fuel tank comes with an adapter with two pipes running through it. The female thread on the tank side penetrates into the tank, and a gasket is built into the bolt that holds the adapter in place. It is thought that the double thinning of the gasket on the adapter side and the gasket on the bolt due to aging leads to fuel leakage.
The gasket on the oval has a partition separating the mounting bolt area from the strainer area. The paint film on the tank is peeling off the inner part of the gasket, but it is unclear if this has been the case since new or if it was peeled off when the adapter was removed in the past. Since it looks halfway, either way, the peeling area should be enlarged so that the gasket does not touch the paint film. If the new gasket is in direct contact with the tank substrate, the risk of oozing or leaking is greatly reduced.
The gasket that is incorporated into the bolt that holds the adapter in place should also be replaced with a new one.
No sealant is applied to the gasket that goes between the adapter and the fuel tank, but a thin coat of fuel-resistant sealant was applied to the gasket at the bolt in lieu of insurance. Some say that sealant is unnecessary if the new gasket is airtight.
There is nothing strange about this for riders who attach and remove the fuel tank for carburetor or engine maintenance, but a motorcycle with fuel directly above the engine, which is the hottest part of the engine, is in a sense a very dangerous vehicle.
While most automobile fuel tanks are completely closed at the bottom and the fuel pump is attached from the top, many motorcycle tanks, whether carbureted or injected, have the fuel cock and pump inserted from the bottom of the tank, so the probability of leaks or oozing is higher than for automobile tanks. In addition, when the packing of the cock itself hardens or deteriorates and leaks, it is often easy to notice when the rider operates the cock, but it tends to be difficult to notice abnormalities that occur at the tank's attachment point.
For users of discontinued models and restoration enthusiasts removing rust from fuel tanks is one of the essential tasks. In such cases, rust removal chemicals are often used after the fuel cock and adapter are removed, but it is strictly prohibited to reuse the O-rings and gaskets on the joint surfaces when reinstalling it.
It is understandable that it is tempting to want to restore a rusted tank quickly while new parts are not ready in time, or to decide that it looks okay because it still has elasticity, but rubber parts that have been in contact with fuel for a long time and subjected to pressure will always deform and harden. If the gasket is reused, the surface pressure of the contact area will be reduced, and sealing performance will be compromised, causing highly permeable fuel to seep through. Some riders may have experienced overflow during carburetor overhaul due to deterioration of the float chamber gasket that was reused when the carburetor was still usable, and the mechanism is the same.
The surfaces of the gas tank and cock where the O-rings and gaskets meet are not perfectly smooth. The rubber follows the slight irregularities to ensure airtightness, so hardened rubber-like plastic cannot adhere tightly.
Also, when a new O-ring contacts the tank or cock, if you look at the contact area, it is a single line. When it is crushed and permanently deformed, the contact area becomes a surface, and the pressure generated in the contact area decreases, which can also cause fuel seepage and leakage.
While it is important to remove rust from a tank whose interior has rusted, it is a real mistake if you neglect to replace the O-rings and gaskets, causing fuel leaks. It is important to make it a habit to always replace rubber parts at the same time when removing cocks, adapters, fuel pumps, and fuel gauge sender units from the fuel tank, rather than judging that the purpose is to remove rust and not to replace O-rings.
POINT
Point 1. Replace rubber O-rings and gaskets that have been used for a long period of time with new ones instead of reusing them, as they become deformed and their airtightness deteriorates due to hardening.
Be careful of paint film swelling on the fuel tank side for discontinued motorcycles.
The seat surface of the fuel gauge unit is also painted in the same way as the fuel adapter, but the paint film is not damaged at all, perhaps due to the disk-shaped gasket instead of an O-ring type, or perhaps because the paint film is tough, but if the paint film is in good condition, there is no need to remove it. However, if the coating is in good condition, there is no need to peel it off.
As mentioned in the previous section, it is important to replace the O-rings and gaskets with new ones when the fuel cock or adapter is removed, but it is also essential to check the fuel tank side. Especially for tanks with painted seating surfaces for the cock and adapter, deterioration of the paint can cause fuel leaks and seepage.
The tank in the image is a 1987 model. The paint film around the adapter attachment area connecting the tank to the fuel cock had become crusted over by fuel and was peeling off easily. The inside of the gasket on the oval has an exposed steel plate, so it may have been painted with masking at the gasket, but the fuel that gradually seeped through the gasket itself as it became less airtight must have swelled the paint film.
On some tanks, the fuel cock and adapter mounting surface may also be painted. In this tank, too, the seating surface of the fuel gauge unit was painted and fitted with a round gasket. However, despite the same age, the paint here was not damaged and there was no fuel seepage.
Unlike simple lacquer paint, the urethane paint used in factory paint is not easily attacked by fuel, unless it has been exposed to fuel for more than 30 years since the 1980s. Fuel penetrating the gap between the tank substrate and the paint film can cause the paint to swell and reach the outside of the gasket, tunneling under the paint film.
In such cases, removing the fuel cock or adapter so that the gasket is in direct contact with the tank substrate and stripping the paint film where the gasket comes into contact with it will prevent subsequent leaks and seepage. It goes without saying that new gaskets should be used when restoring a tank with stripped paint film.
When a leak or ooze is found at the fuel cock or adapter, simply retightening the screws and bolts to compensate for the reduced airtightness due to gasket deterioration is not the solution in most cases. Not only that, over-torquing can further crush the gasket and induce distortion of the tightened area.
Although there is a fear that some genuine parts for discontinued or old motorcycles are no longer available, it is recommended that O-rings and gaskets be replaced with new ones when fuel system components are removed.
POINT
Poin1. If the paint on the fuel cock or adapter mounting surface is swollen, causing fuel to ooze or leak, peel off the paint film on the gasket contact area and assemble.
