The paint method is the way that the old guys used to determine the length of "drag pipes" on Harleys. Back then you couldnt buy aftermarket exhaust that were already "tuned" to the correct length. They would paint a light coat on .... fire up the engine....ride around till the engine and headers got hot...shut off engine....let cool.....and then trim the tubes to the length where the paint burnt off. (told to me by my grandfather - an Ol' Harley Head from way back in the Knucklehead days....
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Supposedly, this is the point at which the headers were their most effective at scavenging exhaust gases.