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This rule seems simple, but it’s not. It says that you can replace the stock fuel tank as long as the replacement doesn’t weigh less or hold less fuel. Plus, carbon fiber fuel tanks are not allowed.
What makes this rule, as with many AMA rules, suspect is that when the rule was originally written, it expressly said that the stock fuel tank had to be used. The AMA production rule was written in 1985 to help level the playing field for privateers and to achieve that by making the manufacturers put more emphasis into making their stock production motorcycle more race-worthy (since factory team didn’t race the stock production motorcycles, but instead had hand-built works bikes). The idea was that the factory teams would have to race with a production bike and the manufacturers would make that production bike better to help their high-paid stars—and, in the process, help the privateers. However, in practice the squeaky wheel gets the grease. And since privateers aren’t allowed the squeak, only the factory teams are heard.
When it turned out that some production fuel tanks might not make the 45-minute motos that were held back in the late 1980-early 90s, the factory teams wanted the rule changed so they could run bigger fuel tanks. The AMA initially refused to budge—evidenced by the fact that Pro Circuit’s Peak Honda team had to paint their stock plastic fuel tanks blue every weekend—because the paint would chip off during the races. Mitch Payton asked the AMA if he could build identical aluminum tanks (no smaller, no lighter and no narrower) and submit the gas tanks to the AMA for approval. Not because he wanted a performance advantage, but because he didn’t want to have to paint red gas tanks blue every Wednesday. The AMA said, “No.” At the time the AMA felt that the rule was there to help fix the flaws of the production bikes by forcing them to build bikes of the equivalent layout of the factory’s former works bikes.
Mechanics hold all sorts of trick to make the stock gas tanks hold more fuel. The most common was to pour boiling water into the stock plastic tank to get it to swell up, while Kawasaki added auxiliary aluminum tanks under the stock fuel tank. The most common sights were mechanics topping off the riders gas tanks when they came back from the sighting lap.
When the factory teams came asking for the right to run bigger aluminum tanks, the AMA came up with the rule above (which you will note says nothing about maintaining the same width or fuel position (in relation to the tank it replaces). When the right people squeak and the rules get changed. It is all moot, because the motos were shortened to 30-minutes plus two laps.