The motorcycle has been around for over 100 years, yet it's possible to look at machines form the first generation and see aspects still present in one form or another on our current motorcycles. But that doesn't mean there hasn't been any progress. There has. Lots of it...
Aluminum, steel and cast iron have all been used, but it's the little things that make the difference. Aluminum cylinders lose heat faster and more effectively than cast iron predecessors, and cast aluminum cylinders with water-cooling and plated bores are even better. But Yamaha took most of the '60s to learn how to get chrome bores to stay stuck on their two-stroke racers, their road motorcycles using cast iron liners for three decades.
Crankshafts used to be bolted together with roller bearing big ends, and ball and roller mains. Advances in machining quality, oil technology and bearing materials means 99% of new motorcycles are all plain bearings and high pressure oil, only a few designs now persevere with the old ways. Plain bearings allowed crankcases to be split differently – horizontally instead of vertically – to reduce oil leaks. Less metal was needed to support the new, more rigid bearings so engine weight dropped.
Computer design allows ultra-thin crankcase walls and calculates exactly where extra metal is needed. Advanced vacuum casting techniques mean metal can be put only where it's needed, allowing engine weight to drop even further. Immediately after WWII, most designs were cheaper versions of late 1930s motorcycles, usually with lower quality materials. But things changed quickly. Exposed springs on valve gear were covered by the '50s, coils replacing hairsprings. In racing, double overhead camshafts quickly became common, but on the street the first move was single overhead camshafts, pushed by Japanese motorcycles in the early '60s.
Most engines of this period were two-valve (one inlet, one exhaust), and opposed at very high angles – this gave large combustion chambers that needed filling by equally large domes on the tops of pistons to achieve good compression. Four-valve engines had been around for a long time but the real benefits were not understood until the mid-'60s. Murray Walker's dad Graham had pointed the way back in 1931, winning a TT on a Rudge Ulster, a four-valve single-cylinder engine using a bronze head for all-important 'between valve' cooling.
As soon as water-cooling became common it was easier to siphon heat away from the area of head between the valves and four-valve heads and compact. Almost flat combustion chambers promptly took over.