Internal combustion engines convert the chemical energy stored in fuel into the mechanical energy used to power machines when the fuel air mixture is compressed in the space between the piston and the cylinder head and ignited by the spark plug.
The design of the piston, cylinder and cylinder head plays an important role in determining the operating temperature of the engine. Finding ways to effectively dissipate heat has been a consistent challenge for makers of internal combustion engines since their invention, and the consequences of overheating can be serious, including piston seizures due to oil lubrication failure and damage to engine components caused by an issue called engine knock.
As explained by Kevin Cameron, the technical expert at Cycle World, normal combustion is a controlled process that occurs when fuel and air are mixed in the correct proportions and ignited by a spark. The resulting flame front propagates through the mixture at a steady rate, producing a progressive release of energy to propel the piston downwards, driving the crankshaft. Knock (or "detonation"), on the other hand, occurs when some part of the unburned fuel becomes overheated and ignites spontaneously before the mixture has fully combusted, creating a shockwave. It is this uncontrolled, rapid explosion that gives the phenomenon its common name, knocking (or pinking/pinging), so called because of the distinctive noise the engine makes when this happens. The violent force caused by knocking can damage engine parts.
One way Honda and other manufacturers of air-cooled engines maintained operating temperatures at a level that minimised the chance of detonation was by using aluminium alloys - metals which have better heat conducting properties than iron and therefore dissipate heat more rapidly - in the construction of their engines.
Iron skull cylinder heads
The cylinder heads used in the early Super Cubs were made entirely of iron but these were superseded by heads constructed using aluminium alloy.
Although Honda eventually adopted all-aluminium heads, the C90s of the 60s and 70s came with a bimetallic head: the combustion chamber is made from iron and the remainder of the head is created with aluminium that has been cast around this iron "skull".
The continued use of iron in the cylinder head will presumably have resulted in relatively higher engine temperatures, so it is not obvious why Honda chose to do this. A clue might be that the same type of head was used on many of the Honda twins of the same era and other manufacturers, including Laverda and Susuki, did the same, perhaps indicates there were challenges manufacturing all-aluminium heads at this time.
It seems that, whatever the difficulties were, they had been overcome by the mid 70s and Honda motorcycles started to be fitted with all-aluminium heads from this point. The 1974 CT90 model - which otherwise shared the same engine as the C90 - got an all-aluminium head, although this was not fitted to the 6v C90s in the UK.