How Norton Twin Heads Dealt With Heat
The Norton Commando so revered by collectors is descended from a 500cc export twin designed by Bert Hopwood in 1947, just after he had spent the war years at Triumph.
Edward Turner had in 1936 designed the first mass-market British parallel twin—the Triumph Speed Twin. One of Hopwood’s wartime tasks had been to produce a Speed Twin derivative to power a generator for use in aircraft starting or as in-flight power for other purposes. As Hopwood would learn, several features of that engine made it easy for it to overheat, providing an education in “how not to do it.” This understanding contributed to design improvements he would soon incorporate in Norton’s postwar Model 7 Dominator.
First of these problems was lack of free access by cooling air to cooling fins located directly above the engine’s two combustion chambers. Early overhead valve (OHV) motorcycle engines had exposed pushrods, rocker arms, and valve springs, which was messy because it required constant lubrication. Exposed valve gear made it easy to locate the rocker arms up above the head on pillars, allowing cooling air to pass under the mechanism through vertical fins from front to back.
When Edward Turner provided full enclosure of the Speed Twin’s valve gear, the presence of two transverse rocker boxes on the cylinder head (the forward one for the exhaust rockers and the rear for the intake rockers) acted as fences, blocking direct access to cooling air. They also greatly reduced the area available for vertical cylinder head finning to the recessed space down between the two rocker boxes.
This was probably satisfactory at Speed Twin’s initial power rating of 27 hp, especially since no high-speed motorways then existed in England (sections of the first such motorway, the M1, opened in November of 1959).
The second problem Hopwood encountered was excess combustion chamber surface area, which increased the flow of heat from hot combustion gas into the cylinder head. By giving the Speed Twin very deep and fully hemispheric combustion chambers, each with its two valves splayed apart by an included angle of 90 degrees, Turner had doubled their surface area in comparison with that of flat discs of bore diameter.
This had been more tolerable in engines having very small bores and long strokes, such as the 65 x 112mm of Fiat’s 1922 Grand Prix car engine, which had pioneered large valve included angles (it was given 100 degrees!) that would be widely copied. But Turner, working 14 years later, chose a more modern ratio of bore to stroke at 63 x 80mm. This considerably increased heat flow into his cylinder head, causing Hopwood, after his design work with the generator engine, to say that it suffered “…excessive cylinder head overheating problems.”
Two of the ad hoc measures taken by Hopwood were 1) to give the generator engine aluminum head and cylinder in place of the original cast iron, and 2) to cool the engine with a blower, delivering air into sheet metal baffling that forced the air to pass through fin space. To better fit the baffling, the head and cylinder were made square-cornered. Cast-in bosses, drilled and tapped, were provided on the sides of the cylinders by which to attach the baffling.
When presented in 1947 with Norton’s clean sheet of paper, Hopwood “…determined to improve the airflow conditions.” In the Triumph head, the vertical planes through each intake/exhaust pair of valves were parallel, but in his new design, Hopwood rotated them apart at the front, creating an opening through which cooling air could enter from the front, then flow through diagonal vertical head fins atop each combustion chamber, discharging at the two sides. This provided access for a high-speed flow of cooling air over the hot combustion chambers.
The next task was to reduce heat flow into the head at its source: the exposed surface area of the combustion chambers. He reduced combustion chamber surface area by making them shallower, giving them the greatly reduced valve included angle of 58 degrees.
Hopwood’s cylinder head design would be continued as the Norton twin’s displacement was increased from 500 to 600cc in 1956, then to 650 in 1962, and to 750 as Commando in 1967, then finally to 830cc in 1973.