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I'm looking for a mechanical petcock and found one that I hoped would fit but was leaking a fair bit. Does anyone have any good recommendations for a 82' 440 petcock?

What's the bolt hole spacing? If 34mm, then Yamaha has a number of manual fuel cocks that will work. Grizzly, Raptor, and I forget what others. made in Japan. some are kind of squat and fit tight under the tank (good for carb clearance in some cases). others are longer. 
Do not get one made in china >> make sure it's oem.   I'm assuming you mean manual, instead of vacuum actuated.