Fuel taps different too, the 500/4 has 2 outlets and the 550 k3 ones I have have only 1 outlet on the tank, taps are not interchangable so it's inprovisation time with the pipes if swapping tanks or carbs around, not fun as it's a tight place to route pipes in neatly and kinks or tight clearences = slit fuel lines, rubbed thru fuel lines or fuel starvation, in short a pain in the bum (ive been told that appart from the cost and smell issues of the contents of your tank spillling down your leg that there are safety issues to do with fires but at that stage getting home is normally higher up the list then that)