How Norton Twin Heads Dealt With Warmth

Kevin Cameron has been writing about motorcycles for nearly 50 years, first for <em>Cycle magazine</em> and, since 1992, for <em>Cycle World</em>.

Kevin Cameron has been writing about bikes for practically 50 years, first for <em>Cycle journal</em> and, since 1992, for <em>Cycle World</em>. (Robert Martin/)

The Norton Commando so revered by collectors is descended from a 500cc export twin designed by Bert Hopwood in 1947, simply after he had spent the warfare years at Triumph.

Edward Turner had in 1936 designed the primary mass-market British parallel twin—the Triumph Velocity Twin. Considered one of Hopwood’s wartime duties had been to provide a Velocity Twin by-product to energy a generator to be used in plane beginning or as in-flight energy for different functions. As Hopwood would study, a number of options of that engine made it straightforward for it to overheat, offering an schooling in “how to not do it.” This understanding contributed to design enhancements he would quickly incorporate in Norton’s postwar Mannequin 7 Dominator.

First of those issues was lack of free entry by cooling air to cooling fins situated straight above the engine’s two combustion chambers. Early overhead valve (OHV) bike engines had uncovered pushrods, rocker arms, and valve springs, which was messy as a result of it required fixed lubrication. Uncovered valve gear made it straightforward to find the rocker arms up above the top on pillars, permitting cooling air to cross beneath the mechanism by means of vertical fins from entrance to again.

When Edward Turner supplied full enclosure of the Velocity Twin’s valve gear, the presence of two transverse rocker packing containers on the cylinder head (the ahead one for the exhaust rockers and the rear for the consumption rockers) acted as fences, blocking direct entry to cooling air. In addition they significantly decreased the realm accessible for vertical cylinder head finning to the recessed area down between the 2 rocker packing containers.

This was in all probability passable at Velocity Twin’s preliminary energy ranking of 27 hp, particularly since no high-speed motorways then existed in England (sections of the primary such motorway, the M1, opened in November of 1959).

The second downside Hopwood encountered was extra combustion chamber floor space, which elevated the circulation of warmth from sizzling combustion gasoline into the cylinder head. By giving the Velocity Twin very deep and absolutely hemispheric combustion chambers, every with its two valves splayed aside by an included angle of 90 levels, Turner had doubled their floor space as compared with that of flat discs of bore diameter.

Hopwood rotated the Norton twin’s cylinders apart at the front of the engine, creating an opening through which cooling air could enter.

Hopwood rotated the Norton twin’s cylinders aside on the entrance of the engine, creating a gap by means of which cooling air may enter. (CW Archives/)

This had been extra tolerable in engines having very small bores and lengthy strokes, such because the 65 x 112mm of Fiat’s 1922 Grand Prix automotive engine, which had pioneered massive valve included angles (it was given 100 levels!) that may be extensively copied. However Turner, working 14 years later, selected a extra fashionable ratio of bore to stroke at 63 x 80mm. This significantly elevated warmth circulation into his cylinder head, inflicting Hopwood, after his design work with the generator engine, to say that it suffered “…extreme cylinder head overheating issues.”

Two of the advert hoc measures taken by Hopwood have been 1) to present the generator engine aluminum head and cylinder instead of the unique forged iron, and a couple of) to chill the engine with a blower, delivering air into sheet metallic baffling that compelled the air to cross by means of fin area. To higher match the baffling, the top and cylinder have been made square-cornered. Forged-in bosses, drilled and tapped, have been supplied on the edges of the cylinders by which to connect the baffling.

When introduced in 1947 with Norton’s clear sheet of paper, Hopwood “…decided to enhance the airflow circumstances.” Within the Triumph head, the vertical planes by means of every consumption/exhaust pair of valves have been parallel, however in his new design, Hopwood rotated them aside on the entrance, creating a gap by means of which cooling air may enter from the entrance, then circulation by means of diagonal vertical head fins atop every combustion chamber, discharging on the two sides. This supplied entry for a high-speed circulation of cooling air over the recent combustion chambers.

The following activity was to scale back warmth circulation into the top at its supply: the uncovered floor space of the combustion chambers. He decreased combustion chamber floor space by making them shallower, giving them the significantly decreased valve included angle of 58 levels.

Hopwood’s cylinder head design can be continued because the Norton twin’s displacement was elevated from 500 to 600cc in 1956, then to 650 in 1962, and to 750 as Commando in 1967, then lastly to 830cc in 1973.

Peter Egan's 1974 Norton Commando.

Peter Egan’s 1974 Norton Commando. (drew ruiz/)