datz510 made a tank using a moroso weld in fill cap like this. The drawing for the tank is here and somewhere towards the end (pg 16) is his finished tank. This is one of the bikes featured on Sick Bike Parts page. The tank is really a nice one and the fill cap is quality.

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LennyHarp of Lenny's Bikes & Things
http://lennyh.com
The true value of a man is not judged by what he has, but rather by what he can do without.
"We are what we repeatedly do. Excellence, then, is not an action, but a habit." -- Aristotle