1. Probably already know but, shoe sieves are much longer in 66 series, the squirrel cage fan was needed to supply enough air in comparison to larger cleaning surface...longer sieves give much more capacity, very noticeable on hillsides. 2. Also probably already know that they had trouble with the extra weight the longer cleaning shoe as they didn't change the machine sides to carry the weight...Marvin has a kit to stiffen the machine sides where the bellcrank is...We mentioned that to IH service manager in 1999 and he replyed, 'Where did you hear about the sheet metal problemIJ'...Harvester came out with a stiffener also in later models. 3. Specialty rotor is also a move up from the 60 series, especially if you tweak the spec rotor with Marvin's bars...Almost a necessity if running beans...Concaves have extention on the left side, different than 60 and 80 series, and more open area in the rotor cage...Flip a lever to swing unloading auger, better than having to hold a lever. I think the biggest change the CaseIH combine came when they went from the 60 to 66, and from 80 to 88 series, as far as practical use of a combine is concerned. From what I have seen 'here' the 16 series is a more working man's type machine that can be kept going by us fix it yourself type farmers, very important to me to keep things as simple as possible and still be able to get a good job, harvest with little loss and in a timely manner...That is why we went to a 1460 in the first place, fewer moving parts, hardly any roller chains, easy to set combine so it would harvest the crop with little damage. I found in 1999 that 1688s were much more plentiful than 66s due to the rollover program the larger BTOs used, 66 we felt was plenty of combine but we went with 88 because it was same price and had more machines to pick from...I have never regretted that decision...more room between the front tires and side of machine makes it much easier to service...We have Gorden's bars on three front rows of the spec rotor, air jet chaffer which is the most improvement I have ever seen in tweaking a combine = more than well worth the cost, no green stripe on the hillsides any more. so I prefer the 16 series over the 21 or 23 unless a guy likes fancy chrome and push buttons...The 16 series made the few faults that the 60 and 80 series had, disappear, but is still a machine without what some would say uneccessary frills. I prefer the 88 series because of the added width of the rotor and cleaning shoe, more power, more combines to pick from as most 66s find their permanent home by when they are sold the second time...Price for all practical purposes is the same for either 1688 or 1666.