Pinging crankset noise and mixing and matching 9/10 drivechain components outcome

Apologies in advance for the long post....

So... cycling along on the trails, and I realize it sounds like I have a can with a ball bearing in being rattled around whenever I pedal. In fact, it sounded so much like this, I stopped, removed the seatpost, turned it upside to shake it, half expecting a bb or some piece of metal to drop out... none did. After some experimentation, I definitely narrowed down the sound to the bottom bracket area, and only when I pedaled.

At this point, my biggest fear was a crack in the frame, and after getting home I inspected the frame closely, but I couldn't see any crack (thankfully!). I took out the bottom bracket, everything seemed smooth so I cleaned the bb threads, put new antiseize on, tightened the chainring bolts and put it all back together... to hear exactly the same problem. I tried putting different pedals on to rule out the pedals, still the same pinging noise. Again, at this point, I was again worried the frame had a crack. I removed the bottom bracket, and actually put a spare one along with a spare crankset I had on the bike, and success! The noise was gone -so it is the bottom bracket or crankset. But was it the bottom bracket spindle or the crankset? Which one?

I put the offending bb in a vice, and then put the non drive side crank arm on and gave it a pull... no noise.... put on the driveside crankarm -and ta da! There was a pinging noise when I pulled on it. I repeated this to make sure the bb wasn't somehow slipping and got the same result. I also really tightened the chainrings again just to make sure, but again I got a pinging noise. The crankset is toast; no way would I continue riding that with the suspicion there's a nasty metal fatigue crack in it!

At this point, I have to point out my drivechain is 9 speed and the crankset is an early 2000's Shimano LX octalink -and while I've done many miles on this crank (3rd set of chainrings and second bb) I couldn't find an equivalent replacement after googling. In the end I got a Shimano M610 with external bearings for around 40 quid*, including bb. Since the octalink bottom bracket still seemed very smooth, I donated it to a mechanic who supports a charity ride, so I was quite happy with that.

This was also my first foray into mixing and matching 9 speed drivechain components with a 10 speed specific crankset; I'd read some people need 0.6mm spacers for this, and some people just found it worked. I just went with it without spacers (though the spacing is definitely slightly tighter) and can confirm the shifting is fine (Shimano 9 speed RapidFire shifters) and I had no chain rub! Of course, YMMV.

*yeah, about that forty quid. I also replaced the cassette and chain as well as they were pretty old. So I did end up spending quite a bit more. :sad:
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Loch side.
It would be interesting to find out whether it was a crack in the crank or failure of the Octalink splines. Octalink was a spectacular failure and even a redesign of the first version didn't solve the problem. Octalink (1) had splines which were too short and Octalink (2) attempted to fix the failures with longer splines but Shimano's engineers under estimated the amount of lash in the splines which were enough to loosen the bolt and allow the splines to slip.
This problem was exacerbated by how easy it was to botch the installation. The entry into the splines was blind (you couldn't see the right way to put it in and had to guess) as well as press-fit. This means a poor start of the fitment could not be detected early on and one would just start cranking up the bolt, eating away the soft aluminium crank.


I didn't investigate further whether it was the crank splines or the crankarm part -regardless, I wasn't going to cycle on a part like that.....

Concerning octalink, I think it was just another marketing gimmick -yet another interface that questionably offered some weight savings over a traditional square taper (at the time I bought it, I think it was the Shimano LX offering, which was the level I was happy with). However, I've never had problems with square taper or octalink interfaces, they've been pretty reliable, and since this crankset was from the early 2000s I can't really quibble about it. Perhaps it wasn't a sales success (don't know), but from my personal experiences I certainly can't describe it functionally as a "spectacular failure".

I can't imagine how you can botch the installation though, and I'm not exactly a master mechanic. Easy peasy. I'm thinking how many times I removed and reinstalled this crank and I came up with about 5-6 times as the crank was migrated to a couple of other bikes as well as bb replacements -in all fairness to Shimano, not once was it an issue and it was easy to feel when you had a positive insertion of the crankarm on the splines. I think you had to be fairly cack handed to cock this up!

I think if I had easy access to a reasonable replacement crank that was octalink, and the cost was less than I paid for the M610 (which included the bb), then I would have gone for it given I already had a reasonable condition octalink bb.
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