Yes, the Bell fuel system!!
Well it's not that bad ,to start with the 412 Classic is basically the same as the 205 and 212 fuel system with 5 main tanks giving approx 1450lbs fuel.
The 412 SP/HP/EP, has a ten tank system 6 floor tanks, with fwd, centre and aft floor tanks on each side, the centre and aft tanks take the same space as the 2 floor tanks in the 205 /212/ 412Classic with the fwd tanks fitting into the floor cavity behind the pilots seats.
3 of the 4 aft tanks are the same as the 205 /212/ 412 Classic with the extra tank befind situated in the rear hellhole area. With the fuel system extended fwd and rear in the airframe longitudinal CofG became a problem , so Bell came up with a series of stand pipes and valves to keep the CofG move to a minimum (a total move of approx 4 ". between 136.0 -140.0)
To do this Bell do it in a series of 6 "Burns"
Burn 1 is approx the top 1/4 of the 4 rear tank as the flow into the floor tanks
Burn 2 is the centre floor tank on each
Burn 3 the next 1/4 of the aft 4 tanks
Burn 4 is the fwd floor tank on each side (approx 800lbs left at end of burn 4)
Burn 5 last 1/2 (rest) of 4 aft tanks
Burn 6 remaining fuel in aft floor tanks.
Hope that helps and isn't too confusing, it had me stuffed to when i first did the course!!
Oh yeah and the main reason for the fuel system change?
More fuel Classic 1450lbs approx 225 nm
SP/HP/EP 2150lbs approx 350nm