On the very tip of the Shimano FC-M770 crank arms there are some markings. They are to be found on both crank arms on both of my sets, so they are definitely no random scratches. I suspect these are leftovers from the hollow forging process, but I am not entirely sure. Furthermore, why has Shimano decided to not machine the tip of the arm in order to remove these markings? I mean It's an XT crank, cheaping out shouldn't be the main reason here.

  • It could stem from the manufacturing process, I'd bet it slipped through quality control. Contact Shimano and add the pictures.
    – Carel
    Jan 25 at 16:36
  • 1
    This is intentional, you can even see these markings on some of shimanos promotional photos @Carel
    – user430
    Jan 25 at 16:59

The only people that can answer this with 100% genuine authority would be Shimano's R&D team.

Clearly those marking are not there by chance so lets presume what is a reasonable reason :

  1. Weight Saving (it's going to be a fraction of a gram at best) but some people are picky
  2. Reduced Drag (it could be a fractional difference while spinning at 90rpm as opposed to 70rpm)
  3. Intentionally built in to make them harder to clone in the manufacturing process

Your best bet is to get in touch with Shimano and ask them, they will either tell you or they wont.

  • 4. Racing stripes
    – Armand
    Jan 27 at 0:17
  • #3 Seems reasonable. The grooves in the ones I've had on hand had grooves that need a special tool to make, as they have a particular profile, something like this: _/ and the ends are perfectly squared. So, If I see cranks with other groove profile or rounded ends, I'd suspect they are counterfeit. (Not all Shimano cranks have those grooves though)
    – Jahaziel
    Feb 25 at 15:12

I'm guessing, but these grooves could be a cunning way of reducing pedal cross-threading.

When you fit a pedal axle into the crank, it has to be at right angles to the ground and to the centerline of the bike. There are few visual clues on the bike, with most tubes being at some angle.

The cranks themselves have a subtle curve to them as well, so your "eye-cromiter" can be deceived resulting in mangled crank threads.

By having some clear lines, it helps the pedal axle to be threaded in orthogonally to those lines, saving on warranty claims and bike-downtime.

  • 2
    Maybe other people do it differently, but I never rely on eyesight when inserting a screw into threads. Instead, I place the screw on the thread and turn the screw backwards under light pressure until it falls into the thread. When this happens, the threads of the screw and hole are in touch all the way around the hole, and the screw naturally aligns to the thread in the hole. Then I start screwing in the screw, knowing that I cannot have misaligned the screw. Jan 26 at 10:00
  • 1
    @cmaster-reinstatemonica I have a lazy method if the pedal has a hex socket on the inside - I feed the driver through the frame and the crank eye, then hold the pedal on the end and slide it into place. A longer "lineup" is far easier to keep level in both planes. If the internal thread is munched, I have sometimes fitted a pedal into the crank from the back-side, which looks weird but works.
    – Criggie
    Jan 26 at 10:28

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