2014 also lacked the judder spring, which I just added, big improvement...the 821 vs. 939, both Duc twins, 821 is almost 1200 crank stroke with smaller bore, for a small engine, very good midrange, belt driven cams...939 is internal chain driven cams (no more belts to replace), shorter stroke/bigger bore, obviously little bigger engine, I'm sure people on this forum know more than I do about the deltas...I would consider the 939 lower maintenance...I do love my 821 though...