This weekend I was assembling another French bike from the frame on up. I had a set of Stronglight 105 cranks that I wanted to use on it. While cleaning the left arm I noticed a small spot of dirt that wouldn't clean up.
On closer examination it looked like a crack was starting so I examined the arm under my 10x-60x-200x digital microscope. The crack was easy to see at 10x and 60x it looked like the Nile River Delta!
The crack is at the pedal end of the forged in flute.
There's one picture that I drew in a red line to show that the crack is not only across the arm at 90° but there is some laminar failure under the surface to.
What I suspect is that the billet may not have been hot enough when the flute was hammered in and/or there was a lap seam flaw in the aluminum billet.
(LAP \u2014 A surface irregularity appearing as a fissure or opening\u201a caused by the folding over of hot metal\u201a fins or sharp corners and by subsequent rolling or forging (but not welding) of these into the surface.)
This makes 2 orphan Stronglight 105 RH crankarms... ;-(
Stronglight produced the 105 crank arms with the forged in logo from around 1875 until about 1980, After that they switched to a shallower flute (see last picture).
That's around the same area I've seen a number of Campy cranks crack. In the early 80s most crank makers switched to flat top or slightly curved top crank arms. I've not seen a pedal end crack like this one on any of the latter style cranks.
So, another suggestion for those long winter nights, closely examine your crankarms for any signs of cracks developing.
Good riding...
Chas. Colerich
Oakland, CA USA