For mass manufacturing, simplifying the production process yields more products at a cheaper unit price. As Honda ramped up production from the early days of J-Benly, SA/SB-ME-MF singles, their goal was to build a reliable and punchy OHC twin, which was indeed inaugurated in 1957. Honda designers must have decided that making thousands of little valve seats for the cylinder heads could be replaced by a cast-iron cylinder head “skull” which incorporated the combustion chamber, including valve seats along with the spark plug hole. The cast-iron skull was then set into another mold in which was poured molten aluminum, fusing the two dis-similar metals together into a single unit. From there, valve seats could be easily cut and the combustion chamber either left in the 250cc-sized hemispherical shape or the edges were chamfered out to match the step up from the 54mm 250 piston size to 60mm enlargement for the 305cc displacement.
As brilliant as the design was for manufacturing, the heads did have a tendency to have the skulls work loose from the alloy castings, causing oil leaks around the edges of the spark plug holes.
This discussion will revolve around the “wet-sump” 250-305 engines which began production in 1960 and carried on until 1967. The earlier dry-sump engines are in a different class of design and application and are seldom seen in the US. When Honda came to the US in 1959, they offered the CE71 and some various 250-305cc versions of the dry-sump (separate oil tank supply) bikes, but swiftly moved into the newly-designed wet-sump types.
The 250-305 Dreams were introduced in 1960 in the new wet-sump format, followed by the CB72-77 Super Sports in 1961. The range was extended in 1962 with the newly designed CL72 250cc Scrambler.
For the first two years, the cylinder heads were offered with tiny 10mm spark plug holes. In early 1962, the heads were machined for the commonly seen 12mm (NGK D8HA spark plugs) and that continued throughout the rest of the production runs for all models.
Visually, the early heads were most easily identified by the U-shaped machine work done between the exhaust ports on the cylinder head fins. In 1965, there were numerous detail changes to the engines, including the elimination of the machining step for the fins, as they were now cast in a clean V-shaped design.
1965-on V-pattern head
People often question about the interchangeability of the cylinder heads across the three models. To manage the number of parts needed to build these engines, all the cylinder head castings are the same to allow for cross-building of the various engines. Apart from the single intake port Dream heads, all the dual port heads will interchange between Scramblers and Super Hawk models. Early Dreams had smaller exhaust valves, which were upgraded to the CB series exhausts in 1962. For 250-305 dual port heads, they all share the same valves, despite the difference in displacement sizes. There is a slight difference in the intake port size for the 250 heads, where the 22mm carburetor insulators match up to the intake ports.
Honda was able to use the same castings for the dual port heads, as far as the combustion chambers are concerned. The 54mm bore size of a 250, requires a tight hemispherical combustion chamber with a small squish area at the edge of the piston crowns. The same castings used for the 60mm 305 engines require a beveled chamfered machine cut to allow for the fitting of the larger pistons.
Combustion chamber for the 250cc engines
305cc combustion chamber
These cylinder heads do not use valve stem seals, rather there was a thin air passage drilled into the head that matched up with some cross-drilled intake valve guides. This system was thought to break up the oil film on the intake valve stems, reducing the tendency to suck engine oil past the stems and guides into the combustion chambers. Remarkably, this system seems to have worked efficiently for all the years, until 1967. Honda decided to eliminate the whole system just in the last year. The last generation engines don’t seem to be more prone to using oil than the previous types, however.
So, in conclusion, the main difference between 250 and 305 cylinder heads is the machine work on the edges of the combustion chambers. Heads are generally interchangeable between the various versions of the engines. Considering the engineering challenges faced in the late 1950s, to build motorcycle engines that could rev up to 9k plus in a world of plodding, low-rev OHV designs from European/British companies, the speed of manufacturing of components led to the dominance of Honda in the world of motorcycle manufacturing.
Bill Silver
aka MrHonda
10-2024
