Geez, Net nannys all over the place! that missing word is P as in O as in R as in N or "pr0n" as the kids say <g>
Hindsight is 20-20, of course, but if they had built all the 226 engines from the beginning as they did the 54-55 supercharged engines, with the valve rotators as well as a finned aluminum head to help get rid of heat in critical areas, it would likely have been a longer lived engine.
As well, never overlook really simple stuff. I remember many years ago riding in three different Mercedes at different times, a 180D, a 220 and a 250 gas, and seeing the marks on the speedo showing the 'maximum' speeds for each gear.