6 km test ride result: stopped 5 times to check the bolt of the crank (took an allen key for it with me), everytime it had loosened a bit (a quarter turn to retension).
So looks like something wrong.
Then another test ride. Same story, only that I waited a longer time to retension the bold, for what the usage duration difference is worth: the bolt doesn't seem to loosen more than in the shorter time, alike the bolt ceases loosening at some point.
Still, a losening bolt means some force, or at least a direction vector of it, pushes or pulls it away. The only thing I can think of is different slopes of the tapering, which in case could be explained by an ISO versus JIS standard.
The broken crank arm is from the Stronglight Track 2000 set which is specified on the sellers site as "Requires 107mm bottom bracket with JIS (Shimano type) taper"
The old replacing crank arm is from the original crankset that the dealer of the bike specified as "Crankset Sugino XD 170mm" which is specified as "4-TAPER(JIS)".
So it doesn't look like a JIS/ISO difference cause.
My focus now is the cause of this crank breaking at the pedal "eye" (as it is named here).
I cleaned the parts and took some pictures:
First 2 pictures are from my nonprofessional (lack of crank puller and last resort in order to cause no further damage) crank removal
Last 3 hopefully contain some clues about the cause.
Remember: the pics show cleaned parts. Right after the breakage, a part of the break section was black, the other silverish, indicating an existing partly breakage before the total come off. I inspect (putting it upside down) the bike every morning upon arrival at work, and I whip off dirt but I didn't notice it. I should have taken pics before cleaning too, just realize it now, but too late.
Some notes:
- The thread inside the broken Stronglight crank got somehow quite damaged - there were 3-4 loose or partly loose thread strip offs, thin and narrow burrs. I remember when it was installed to replace the Sugino that the dealer afterwards had to replace the axle with a longer one due to chainline got changed to a chainring-frame colliding position. Apparently the crank puller or whatever it was seriously damaged the thread. Worser: the old Sugino, now mounted as replacement, also had serious thread damage. Meaning that a future crank puller usage may ruin the thread remainders instead of pulling the crank off the tapered end, leaving a bottom bracket replacement as sole solution to remove the crank.
- A difference between Sugino and Stronglight: the latter had around the hexagon bolt a threaded (outside) ring with two small tool holes (aside of the big centered crank bolt hole of course) in it, near its circumference, 180 degrees apart, which may have served as a dust cap or so, since the Sugino had some plastic nonthreaded thick ring instead.
I just started to think that this may (also?) serve as a bolt locker (the bolt has a washer end in once piece) so I decided to remove the bolt and replace the plastic nonthreaded ring with the threaded alu ring from the Stronglight crank.
Then I tightened that ring using two small screwdrivers.
Next is to test if that threaded ring indeed prevents the crank bolts loosening.