This is off the top of my head but a 261 head could be found in a 86 cc version or a 91 or 92 cc version (I forget whether it is 91 or 92). The 54-55 5913 head is 86 cc. For the 1956 model year, the 848 head was introduced with a chamber of 79.1 cc. This is why everyone heads for the 848 head.
