Our eyes loved the Death Star because its intricate and fascinating surface features led our eyes on a journey. In similar fashion, years ago I loved the look of engine crankcases covered with stiffening ribs. I looked under the crowns of pistons to find similar features.

Under my desk, I have a mighty 6-1/8-inch-diameter piston from a postwar R-3350 radial aircraft engine. Its 18 cylinders gave 3,700 hp for takeoff. Reducing that to horsepower per square inch of piston area gives a number of 7 hp/square inch.

Pratt & Whitney made similar power from its four-row, 28-cylinder R-4360, but at a lower-duty 5.2 hp/square inch of piston area. Turning a 4360 piston upside down reveals an array of ribs and fins machined into its interior surfaces.

Now turn over the 3350 piston and find…nothing of the kind. No ribs. No machined skirt cooling fins. Just the smooth organic contours of a forging.

Interesting. The harder-working piston—producing 7 hp from each square inch of piston area—has only smoothness in its interior. And the less-hard-worked P&W piston, at only 75 percent as much power-per-square-inch, is internally covered in fins and ribs.

For a given weight of metal, pistons run cooler in a plain thick head than they do with a thinner section and internal ribbing.

Getting back to motorcycling: Norton's racing manager in the 1930s, ex-TT rider Joe Craig, had hundreds of hours on racing Nortons himself, plus running dyno race simulations and examining parts at the track, practice by practice (it took only minutes to have the head off a Manx for inspection, so they were off every time there was a question). In a caption for his June 26, 1941, article in Motor Cycling, Craig said, "For a given weight of metal, pistons run cooler in a plain thick head than they do with a thinner section and internal ribbing."

Piston-crown thickness is the "heat pipe" that carries combustion heat away from the hot crown to the cooled cylinder wall. In 1992, when Yamaha had to deal with the more knock-prone fuel resulting from the FIM's low-lead rule, TZ250 piston crowns were made 11mm thick (0.433 inch) to reduce dome temperature. No fins, no ribs.

Craig also noted, “Some manufacturers have tried internally ribbing the crown of the piston to improve heat dissipation. Many years ago, Professor Lea of Sheffield University showed that, weight for weight, no benefit was derived from such finning, and my practical experiments confirm his early tests.”

In C.F. Taylor's magisterial The Internal Combustion Engine in Theory and Practice, three beam sections are compared in terms of maximum stress. The first is a plain beam, the second is that beam but with ribs added, and the third is the original beam but with the volume of material used previously to make ribs simply added to it as some extra thickness. Taking stress in the first beam as 1.0, adding the ribs to it raises maximum stress by 60 percent (that is, makes it more likely to fail). By adding the material from the ribs to the original beam as extra thickness, maximum stress falls to just 64 percent of that in the first no-ribs beam.

Why? Adding ribs introduces sudden changes of cross-section, which are well-known through experience to be stress raisers.

Taylor presented an illustration showing the interiors of two crankcase halves—the original design with prominent ribs and lightening holes, the other, as a result of formal stress analysis, with a smoother, organic, and rib-free shape. He comments that by eliminating the stress-raising ribs and holes it proved possible to reduce weight by 27 percent.

Our intuition whispers to us that complex shapes braced by stiffening ribs ought to be strong. It turns out that our intuition is wrong.