The biggest bore that could be reliably used at the time was 94 mm (888). The 916 used the same crankcases so couldn`t be enlarged. Massimio Bordi decided to get more capacity by enlarging the stroke instead to 66mm. Hence 916.
The 851 was 92mm bore x 64mm stroke (850.89cc) and when they moved up to the 888 they simply bored it out to 94mm to get 888.29cc. The 916 was a(nother) natural evolution of this strategy that Ducati have used, and continue to use, by upping the stroke on the 888 motor to 66 to give 916.05cc. The 996 is an over-bored 916 motor at 98mm, giving 995.67cc - in fact the "in between" 955 was a 96mm bore.
The 1098 is a long-stroke 999 engine. Same bore at 104mm but 64.7mm stroke (to give 1099.24cc) compare to the 999's 58.8mm stroke. Natural progression again!
Yes, the original Testastretta engine was 100 x 63.5 giving 997.46cc and first used in the 996R. The 998R was the first engine with the short-stroke version, at 58.8mm with an increased bore at 104mm to make up the difference in CC, in an effort to get the revs up for racing. Then the 999 came along and they continued with the lazy long-stroke version of the engine in both base and S models but put the short-stroke into the 999R. So basically all the engines used in the 999 series had already been used in the 996 and 998! The short-stroke engine was the one that they went forward with to make the 1098.
