TECH ESSAY: Making Things

Italy: The land of manual forging in a modern world.

Giacomo Agostini vintage race action

When I spent a day going through the giant trove of classic MV Agusta race bike parts at Robert Iannuccis's Brooklyn shop, I found a number of raw connecting-rod forgings. To my surprise they were quite plump, as though they had been pressed from bread dough and then allowed to "rise," becoming puffy, containing at least two to three times more metal than a finished rod.

Of course, I was accustomed to the forgings that go into high-production engines: I have milk crates full of Yamaha's 278 and 3G2 conrods removed in the process of rebuilding TZ race engine cranks. Those rods were die-forged to pretty much net shape, after which their big- and small-ends were machine-finished. The rest of the rod—its beam and the ODs of both ends—retained an as-forged finish.

A video I saw a year or so ago showed me that the MV forgings I had seen were hammer-forged, a process which lends itself to producing smaller numbers of parts. Instead of placing a red-hot blank into a die set which then closes to squeeze the hot metal into the desired shape, the technician uses tongs to pull a glowing “hockey puck” of alloy steel from a furnace. Stepping to a small forging machine that cycles each time he steps on a pedal, he quickly manipulated the puck with successive blows from the machine to gradually achieve the desired shape. The result was very like the puffy-looking MV rod blanks I had seen.

The value of forging as opposed to casting or machining-from-solid is that it can align and elongate the crystalline grains of which the metal is made so that they develop directional strength.

Bristol Mercury RAFM

Bristol’s Jupiter 9-cylinder radial.

After this manual blanking procedure, each MV blank was presumably machined, copper-plated to prevent a surface hardening process from affecting anything but the two bearing bores, and finally, those bores ground to dimension. Outer surfaces of each rod were fully machined to a high finish, with all transitions smoothly radiused, to avoid forming stress-raisers.

The fatigue-resisting value of such organic shapes and polished surfaces was discovered in the 1920s by companies seeking to improve the reliability of aircraft engines. At the Bristol Aeroplane Co. in Britain, at McCook Field in the US, and surely in the shops of Italian, German, and Japanese engine makers, shape and surface finish became essential tools in building reliability.

At Bristol, the process began with detailed consultation with Professor Leslie Aitchison, a metallurgist with an appreciation of what was required. He went through every part of Bristol’s Jupiter 9-cylinder radial (photo above), showing how fatigue cracking could originate from scratches, sudden changes or cross-section, or from defects within the material. Soon this emerging science was embraced by the automakers and others, who added fatigue-testing to their design process, all but eliminating such early problems as broken axles and fasteners.

Italy remains full of small-volume producers of all kinds of things, keeping alive a long tradition of manual methods that remain valuable even in our era of push-button technologies.