I would guess that your 81 has most of the updates put in to make it a Series 3(1982 or later) machine. The pre-82 N6 was prone to engine problems. They were turning too many rpms and spun bearings and throwing rods, this can be fixed by having the fuel system updated to the Series3_R series 670I specs. Your's should be the 670HI engine of the 7080,7580,Early N6, changing the sheave on the main drive belt and slowing the engine down will make a big difference in engine life, and reliability. The 670HI engine is VERY CRITICAl that the rod bolts and bearings are changed every 1500 hr or less until it gets updated. Easiest way to update is find a high serial n machine in salvage with the American Bosch fuel system and replace the Roosamaster_AC fuel system and sheaves . You will slow the engine down and get better fuel economy with less smoke and extend the engine life. My 82 N6 went 3100 hrs before I did the "Underhaul" and it could have gone longer but the bolts and bearings at 3100 was cheap insurance. You might want to go into "TIPS" and do the "hyperizing" of the P1 processor, it will make a running fool out of the machine and doesn't pull as much engine power as the factory set up. My 20 year old N6 hyperized out performs any of the current factory set machines(any brand) with the same size heads. I love the capacity and handling of the machine. Cousin has a 9510 Deere and uncle has a 2388 Case which I have run and the old N6 makes them look sick. Neighbor has a 9650STS that he can't give away, Not even JD dealers will trade for that piece of JUNK, his 2388 Case has done 75% of the harvests since he got the STS.