Hard Heads, Soft Heads, Hot Heads

[Admin]

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>. (Robert Martin/)

From the beginning—and for many years after—motorcycle cylinders and heads were made from cast iron, cooling fins and all. This was a natural choice. After all, from 1840 on the railroad business had raised the casting of iron and steel to a high art, while aluminum was still a weird novelty metal. In addition, cast iron makes a durable wear surface against which pistons and rings work very well, and cast-iron heads are hard enough to reliably bear the pressure of intake and exhaust valves seating against them.

The accelerated evolutionary forces of World War I and the great need for aircraft engines made it essential to put aluminum into large-scale production, and to do it quickly. Aluminum is about one-third the weight of iron, but conducts heat roughly five times better.

More Power, Better Materials

Motorcycle engines with cast-iron heads and cylinders were entirely satisfactory until tuning practice pushed a 500cc cylinder’s output up to 20–25 hp. Adding more cooling-fin area worked to a point, but the ultimate solution was a material offering higher heat conductivity. Harry Ricardo, in building his Triumph-Ricardo special for engineer-rider Frank Halford, chose aluminum-bronze as head material, making the cylinder from a finned aluminum casting with an iron liner. Bronze brought the head temperature down by conducting heat from the combustion chamber to the fin tips faster, but it was heavy.

As processes for extracting and alloying aluminum improved, the light metal was the obvious next choice. But being soft, it was pounded out of dimension by valves seating against it. It would be necessary to somehow fix hard metal rings into the soft aluminum, against which the valves could seat.

Aluminum cylinder heads need a hard material for the valves to seat against—here gold-colored oversize beryllium copper valve-seat rings are employed for use with titanium valves. (Ducati/)

Inserted valve-seat rings worked acceptably in water-cooled auto and aircraft engines with aluminum heads, but early aluminum alloys lost strength rapidly when heated above 350 degrees Fahrenheit in air-cooled applications. Seat rings loosened, and valve motion only made that worse. People tried casting the seats in the aluminum, screwing them in, placing the seats into a close-fitting counterbore, and then staking them in place by swaging the aluminum over them. They heated the heads, froze the seats, and brought the two together to result in a satisfyingly tight fit. Sorry—even such well-executed shrink fits lost their grip once the aluminum head became hot and grew weak.

Y-Alloy Provides the Solution

Bristol Aeroplane Company decided to redesign its nine-cylinder Jupiter radial engine, and to use aluminum heads. Its solution was to adopt Y-alloy, a new material which had much higher hot strength. Alas, because Y-alloy was troublesome to cast, Bristol decided to machine each head—fins, ports and all—from solid forgings.

Y-alloy was aluminum, internally reinforced against yielding under stress by myriad tiny precipitated particles of very strong intermetallic compounds. At melt temperature, the intermetallic components were soluble in aluminum. But as the material cooled from the furnace, they became less soluble to the point that they began to precipitate out of the melt in tiny clusters. This process, now called precipitation hardening, is widely used in strengthening metals. By impeding the processes leading to planes of atoms sliding over one another, the presence of such hard particles effectively hardens the material as a whole.

Aluminum Alloy 242

During the 1920s, English engineer S.D. Heron was working for the US Army. They were having difficulties developing high-performance air-cooled cylinders, and he needed a reliable way to keep valve seat rings in place in his experimental cast heads. Remembering Y-alloy’s attractive properties, he turned to staff metallurgist E.H. Dix, Jr., asking if he could in some way improve its castability.

The resulting material, alloy 242, was used to produce the several million cylinder heads needed by US radial aircraft engines in WWII. It soon found its way into the heads of air-cooled motorcycle engines as well. Here on a drawing for Vincent cylinder heads from the 1940s is the material callout: “Y-alloy, heat-treated to B.S. L35.” The aircraft-engine material became the basis of Harley-Davidson’s aluminum cylinder heads as well, and those of many other makers too.

Materials research may not be as exciting as 90-mph texting in the passing lane, but it certainly can have far-reaching consequences.

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[fastbob]

Imagine getting stuck in a lift with Kevin and accidentally mentioning that you rode motorbikes . 

[Guest]

1 minute ago, fastbob said:

Imagine getting stuck in a lift with Kevin and accidentally mentioning that you rode motorbikes . 

Two went in but only one came out.

[husoi]

Now I understand when yesterday while watching the news I heard an elderly lady saying "I should have stabbed him a few more times..." 

Edited October 30, 2021 by husoi

[Mr Fro]

Imagine having that much time on your hands that you can trawl the internet for information and regurgitate it with your name on, calling it your work.