Sword in the Stone

Sword in the Stone

Cycle World Sep 1967 magazine cover

The magazine ‘Classic Racer’ recently ran a meaty article about George Beale’s reproduction of the Honda RC-174, which was the 297-cc air-cooled six-cylinder racer ridden by Mike Hailwood to two runaway 350 titles, in 1966 and ’67.

Although these machines were highly sophisticated for their time, present-day commentary can become so worshipful that those bikes and their technology are presented as magical–beyond the present moment. Shall we call this “sword-in-the-stone journalism?” Did Mr. Honda’s mastery of metallurgy enable him to o’erleap the known to create alloys unknown to 21st century engineering?

While it’s fun to imagine Mr. Honda in a conical purple wizard hat, we know a useful amount about the crankshafts in those sixes. To enable use of the roller rod and main bearings then considered essential, they were pressed together. To avoid the problem of transmitting power the full length of an inline six crank, cam and clutch drives were located at the center, not between cylinders three and four (which would add unacceptable width) but rather behind the cylinder, as Mercedes did in their center-drive straight-eight M196 GP car engine of 1953-4. This is a compact design.

To save weight and provide adequate press-fit torque capacity, the crankpins of numbers one and six cylinders were smallest, those of two and five a bit larger, and those of three and four larger yet. Main bearings were made in stepped sizes, as well. To prevent the usual problem of flywheel vibration in the “elephant ears flapping” mode, disc flywheels were not used, with much lighter narrow rectangular bars used in their place. To reduce as far as possible the number of slippage-vulnerable press-fits, each of these rectangular bars was made in unit with a crankpin or main shaft. Counterweighting was by tungsten slugs opposite each crankpin.

When Beale gave the French specialist producer JPX the task of functionally duplicating such a crank, they had to choose a suitable material. As a result of their examination, they selected a gear steel in the French alloy system, 16NCD13, as most similar to Honda’s material. A few minutes on the keyboard reveals that this is closely equivalent to AISI 9310 in the U.S. system–a familiar steel for gear manufacture because it can be surface hardened by carburizing, yet retain a tough, non-brittle core with very high tensile strength.

To further improve fatigue strength, this material can be vacuum-arc remitted to allow most retained foreign matter (which can act as crack nucleation sites) to evaporate.

This choice makes good sense because:

  1. Crankpin and main bearing surfaces must be hardened to act as inner races for the high contact forces of needle roller bearings.
  2. To provide maximum possible press-fit torque capacity, the finished material must maintain a very high tensile strength.
  3. If we imagine a crankshaft life of 1000 miles, or roughly ten hours, that implies something close to ten million stress cycles. For that we will want the maximum fatigue properties of vacuum remitted materials.

Gear steel, too, must bear the extreme contact pressure of tooth-to-tooth loadings, yet those teeth must resist the bending forces produced by those loadings through many cycles.

When I saw the similarity to 9310, I remembered a casual remark by a technologist friend; “Oh, yeah, 9310 is old hat now–we have materials way beyond that today.”

When I looked for them, I hit upon alloys such as Fermium C64, which has a hefty shot of cobalt in it, enough that it no longer deserves the description ‘low alloy.’ While this alloy offers slightly higher surface hardness than 9310, its real purpose is its ability to retain its properties at higher temperature in applications such as heavily loaded helicopter gears.

Those 297cc Honda sixes did the job they were designed to do–to deny those 350 titles to Agostini and MV–after which the company focused on its fast-expanding car business. The bikes entered history, where we continue to find them fascinating.

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