Leaking gas cap - 1964 cb77

[cknight]

Greetings,
For something that seems so simple, this really requires a multitude of factors to work reliably.
First, get a flat piece of class (or clear plastic) about 2.5" diameter. Hold it against the gas cap gasket sealing flange, and probe around the circumference of the flange with a .0015" feeler gauge. Using a clear plate will help indicate the severity of the gaps. If there are any gaps, there will be leakage. Use a piece of hardwood inserted in the filler hole as a pry bar to flatten out the surface, eliminating any gaps.
Wrap some 600 grit sandpaper around a hard flat surface (not rubber), sanding the sealing flange gently, until the flange is polished all 360 degrees around.
Have someone that really knows vintage motorcycles paint the tank. Even with the leakage, the finish shouldn't have been destroyed that easily. There should be no paint on the flange, or inboard of it. Most folks nowadays just "paint it" without using the correct paint and clear coat.
Use the later replacement gas cap with the rubber gasket. The venting and baffling system is far superior to the original cap.
Turn the cap over, and note the vent hole just outboard of the gasket. It's best to have this hole pointing to the left when the cap is tightened, making it as far above the fuel level as possible. A small mark with a Sharpie, aligned with the hole, on the knurled OD will help with this.
When filling the tank, keep the fuel level about 1" below the top, to reduce the possibility of gas getting sloshed into the venting passages.
Yes, this all might sound like a bit of overkill, but it can save you a lot of grief in the future.
Regards, Chase

[02z06dave]

Greetings,
For something that seems so simple, this really requires a multitude of factors to work reliably.
First, get a flat piece of class (or clear plastic) about 2.5" diameter. Hold it against the gas cap gasket sealing flange, and probe around the circumference of the flange with a .0015" feeler gauge. Using a clear plate will help indicate the severity of the gaps. If there are any gaps, there will be leakage. Use a piece of hardwood inserted in the filler hole as a pry bar to flatten out the surface, eliminating any gaps.
Wrap some 600 grit sandpaper around a hard flat surface (not rubber), sanding the sealing flange gently, until the flange is polished all 360 degrees around.
Have someone that really knows vintage motorcycles paint the tank. Even with the leakage, the finish shouldn't have been destroyed that easily. There should be no paint on the flange, or inboard of it. Most folks nowadays just "paint it" without using the correct paint and clear coat.
Use the later replacement gas cap with the rubber gasket. The venting and baffling system is far superior to the original cap.
Turn the cap over, and note the vent hole just outboard of the gasket. It's best to have this hole pointing to the left when the cap is tightened, making it as far above the fuel level as possible. A small mark with a Sharpie, aligned with the hole, on the knurled OD will help with this.
When filling the tank, keep the fuel level about 1" below the top, to reduce the possibility of gas getting sloshed into the venting passages.
Yes, this all might sound like a bit of overkill, but it can save you a lot of grief in the future.
Regards, Chase

Thanks chase.
It looks like I have several gaps and the top of the tank is even dented down slightly. It was hard to tell except I kept staring at it trying to figure this all out. I appreciate your feedback, you've put me on track.
Thanks,
Dave