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Background

I've had my bike for several years and it's been fine with moderate use, but this summer, I ended up using it a lot (I rode almost 200km in less than three weeks).

Problem

It started squeaking pretty badly, so I sprayed some lubricant around all of the joints (pedal, crank-arm, etc.) then took it out for a spin. On that very first ride after the lube, the left crank-arm nearly fell off; I though the pedal broke, but when I stopped and checked, the nut had come loose. I managed to use my keys to tighten the nut and repeat every block or two until I got home.

Attempted fix

At home, I used a 14mm socket wrench to tighten the nut, but it keeps coming loose. I've resorted to carrying the socket-wrench with me when I ride and stopping to tighten it about ever kilometer. 😕

Research

I've read some stuff about the holes in the crank-arm wearing out and worn nuts and how Loctite isn't a solution and such, but none of them mentioned lubrication. My issue started immediately after I lubricated the area around the nut, so logic would dictate that it allowed the nut to slip. I've included a photo (fig.1) of the crank-arm hole and nut. To my eye, the hole doesn't look worn (I assume the bulging around the edges is intentional), but the catch-teeth on the nut do seem worn/flat. 🤔

Question

Anyway, I'm wondering if lubrication is indeed the problem and if cleaning the lubrication off (and maybe applying a mild adhesive like Loctite or just hairspray) would suffice.

Appendicies

Figure 1: Crank-arm hole and nut

Crank-arm hole and nut

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    That crank arm is badly worn where the nut contacts it. And the nut used is an odd one, with those serrations. I'm guessing the arm is worn to the point where the nut can no longer pull it all the way tight. Best replace the arm pronto, before the crank axle gets badly worn. In the near term some blue ("removable") Loctite should be modestly effective. – Daniel R Hicks Oct 4 '20 at 16:23
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    (Lubricating probably just made things wiggle around more, hastening the unscrewing action.) – Daniel R Hicks Oct 4 '20 at 16:24
  • It wasn't loose before the lube. That's my point, it was just fine until that point. 😕 Even if I replace it, should I not lubricate that part on the new one? – Synetech Oct 4 '20 at 20:52
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    If it wasn't loose it wouldn't have been squeaking. – whatsisname Oct 4 '20 at 21:03
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    Please confer to this question on square taper getting loose: bicycles.stackexchange.com/a/71561/30402 – gschenk Oct 4 '20 at 23:20
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Your crank arm is toast.

In my experience, if a square taper ever gets loose while riding, even once, it's done. Retightening the bolts buys you a few miles at best but it'll loosen up again before long.

I think the connection to lubrication is coincidental. You said the problem started after you applied lube, but that's not true. You did that because it was squeaking badly, meaning the problem was already underway. When it began to squeak, its demise was already imminent. Spraying lube did nothing to speed up or slow down the process. Additionally, the taper should be lubricated lightly when installing.

The problem was an insufficiently tight crank bolt, possibly from the day it was bought. When you replace the crank, make sure that bolt is tight. While others disagree, I find it cannot be overtighted as long as you use a normal sized driver, and that you don't have gorilla strength. Give it some muscle so it stays put.

Additionally, stop riding with that crank immediately, or you also risk wrecking your bottom bracket spindle too. BB spindles are made from hard steel, so they're not one-and-done like the crank arms are, but they can be worn down too if careless. If that happens, no cranks will fit snugly, and you'll wreck another crank arm, or more.

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    "The problem was an insufficiently tight crank bolt, possibly from the day it was bought. When you replace the crank, make sure that bolt is tight." — Be careful with this advise. That does not mean one ought to simply tighten the crank bolt on a bike. The crank will slightly move up the taper and the bolt will not be very tight after some riding. Tightening it again pushes the crank too far up the taper, widens it, and eventually leads to failure. Only tighten the bolts to the specified torque after removing and re-installing it. (cf. bicycles.stackexchange.com/a/71561/30402) – gschenk Oct 5 '20 at 12:42
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    I'd think debris/rust was the last thing that kept the crank arm on, so a creeping/penetrating component in the lube would have SOMEWHAT hastened the demise tho. – user3445853 Oct 5 '20 at 14:02
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The hole on the left picture should be perfectly square. It's so badly worn that the crank and possibly the bottom bracket as well should be replaced. It is not safe any longer.

This job requires special tools that are too expensive for a one-off job. Because of that it is a job for a professional mechanic at a bike shop. There are also many different standards of bottom bracket that each asks for its special set of tools.

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  • Why would replacing a crank arm require special tools? Usuallly you just put the crank arm onto the spindle and tighten the bolt. – sleske Oct 5 '20 at 11:44
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    Replacing the bottom bracket bearings will require special tools. – gschenk Oct 5 '20 at 12:37
  • Like @whatsisname said the BB axle is steel and may be OK. If it is, all he needs is a wrench to put a new left arm. I'd at least look at the BB spindle and consider trying it, to be honest all this stuff (left square taper arms and square taper BBs) is cheap enough even if he did wreck a new left crank arm it wouldn't be the end of the world. – Ivan McA Oct 5 '20 at 14:52
  • For that matter, while a crank puller and BB tool (not needed for putting ON a left arm) are technically "special tools" they are not exactly expensive. There are bike tools that are ludicrously expensive, hundreds of dollars, to the point you NEED to go to the shop. But crank puller / BB tool are things I got in my first $30 kit (and haven't used in 20 years as modern cranks don't need a puller). It's not necessarily bad advice to tell him to take it to a shop, but I'm not sure it's utterly essential either, if he's of a mind to learn his own maintenance. – Ivan McA Oct 5 '20 at 15:01
  • @IvanMcA: They may not exactly be expensive tools even a BB wrench may not be. But both can do quite expensive damage, if used incorrectly and of the wrong type and inexperienced about direction of rotation for the BB. Buying these tools for a possible one-off job is also over-kill. Your LBS is probably cheaper. And cheap tools become often more expensive than quality tools, too. – Carel Oct 5 '20 at 18:11

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