My pedal just unscrew. It has a hexagon-headed screw with '10.9' screw label. Anybody knows what size of wrench I need for this screw?

  • 1
    Yeah, you use the one that fits. Just be sure you're using metric wrenches, and the one you choose fits snugly. Oct 17 '14 at 10:40

10.9 is the type of steel alloy used, not the size of the hex key (aka allen key) you need.

Get a set of hex keys (almost surely a metric set is needed, given that its on a bicycle) and then try them out to see which one fits. I'd guess its something in a 3mm, 4mm, 5mm or 6mm range.

  • A mechanic once told me this number is maximum torque in Nm and the higher it is, the higher quality the bolt is.
    – Vorac
    Oct 17 '14 at 11:32
  • 4
    The mechanic is wrong - 10.9 an ASTM F568M class number, and does not determine the torque in Nm you can use (this is obvious - you can use the same class of steel to fasteners of different sizes/threads and you shouldn't expect them to behave the same). However, higher class numbers do correspond to stronger fasteners (somewhat - there are subclasses and stuff, but lets ignore those). If you want to know the torque you should use, you have to look in the pedal manual this case (or maybe its written somewhere on the pedal as well).
    – Batman
    Oct 17 '14 at 12:04
  • Also I've never seen any bike torques specified to that sort of precision. Certainly my torque wrenches would not allow me to set anything so specific. 10Nm would be the right order of magnitude for pedal tightness but I think, as Batman says, this is coincidence
    – PeteH
    Oct 17 '14 at 12:58
  • This link might be useful but this is only a guideline. I'd guess it would be less than 10Nm (maybe 7Nm), but this should be in the pedal manual.
    – Batman
    Oct 17 '14 at 13:42
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    Yeah. Off the top of my head its 4Nm for a typical stem, 40Nm for a typical cassette, other numbers I'd need to look up. But we're not talking 0.1Nm, and we're not talking 100Nm, that's all I really meant when I said "order of magnitude"
    – PeteH
    Oct 17 '14 at 14:23

Common sizes are a 5 or 6 millimetre Allen key or Hex Driver on the back side of the crank. Or, you can also use a 15mm pedal spanner on the two flats which will be on the pedal-side of the thread. These tend to be a bit thinner than a normal adjustable spanner.

Do be aware your left foot pedal is a left hand thread so you turn it the opposite way to tighten than what you would expect. The rule is that tightening the pedal will tighten the chain if you don't stop the crank rotating.

Also consider why did it unscrew in the first place? Was it never tight?

I've recently found clipless pedals with either 8mm or 10mm hex sockets on the end. That's a hefty-sized hex key, too big to carry on the bike for regular use.

  • 1
    @dlu I feel you're getting a mite-petty with the edits there. I don't go correcting Americanised spellings (eg color/aluminim/meter) to International English (eg colour/aluminium/metre) on other people's posts. If you disagree with my statements, perhaps a comment would be better than an edit.
    – Criggie
    Sep 29 '15 at 4:35
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    Shoot, I didn't mean to correct the millimetre spelling. Was just trying to find a way to mention that 5 mm is also common (I've got pedals with both). I'm sorry about that.
    – dlu
    Sep 29 '15 at 5:03

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