The "Fatty" was the pipe that FMF was selling for the KDX until sometime in the last few years, they decided to offer only the "Gnarly" pipe for the perimeter frame KDX. Mine has a fatty. It comes in torque (K-35) and rev (K-30) flavors. From what I've been able to see, the main difference between the fatty and the gnarly, is that the gnarly is constructed out of heavier gauge metal and hence is tougher ( and weighs more...)
$2600 sounds pretty steep for a '96 unless everything is fresh and the bike has some goodies (pipe, suspension, o-ring chain, etc...) that you'd otherwise spend $$$ to get.
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