PunctualAlex

Hey all, I have been having an infuriating issue lately with my daily rider. Every few days (probably 10-20 miles total, it's a commuter bike) the non-drive-side crank will start to feel loose- I start to notice it when the pedal spindle shifts at the top of my pedal stroke, then when I pull over and check the crank i can wiggle it in and out on the bb spindle- not forward/backward, but in and out from the center of the bike. It's been happening so often that I've been carrying a 14mm thin-wall sprocket wrench in my bag to tighten the bolt, but it always comes loose again. Used to happen every several days, not it's happening every day. The cranks are Sugino and the BB is Shimano... are they the same taper? I feel like it's not a taper problem because the drive-side crank is solid as a rock and always has been.

So here's the question: is it just a ****ty crank bolt that's rattling loose? Could I stick them back in with some blue loctite to solve the problem, or could something be screwed up on the threads on the inside of the bb spindle? How much damage could this do to the BB, would it be worth replacing the BB/cranks? This is seriously driving me crazy, thanks in advance for the help!

Retro Grouch 

You need a new crankarm.

The first time that it came loose riding it that way hosed out the square hole in the crankarm. Now it's never going to stay tight for very long.

If you aren't picky about having an exact match, replacement left crankarms are readily available. They usually cost around $20.00. Be sure to get the same length and be sure that the orientation of the square hole matches what you have.

When you install your new crankarm, torque it to 30 lb/ft. If you're using a 6" long allen key, you aren't going to get to 30 lb/ft.

BCRider

I have to agree. If it wasn't you that bruised the taper out of spec it was someone before.

On my square taper bikes I find that the bolts are always less tight than I leave them but the arms are still jammed on. It's like on a proper fit taper the arms actually walk themselves ONTO the taper rather than off. Only a deformed taper socket in the crank would walk itself off like this.

And yes the taper angles are all universal.

DasProfezzional

This is not true. I'm not up on what standard they may or may not be using, and my guess is that most are the same nowadays, but I can tell you from very unpleasant experience that they are not universal.

BCRider

DOH! Just when I though they were. That must be some pretty specific types. I've had no issues using Sugino, Shimano, old Campy or a few other cranks on various old style and new style BB's over the years.

Retro Grouch 

Basically there's Campy (ISO) and otherwise (JSA). The taper angle is the same, but the cross section size is a little different. Since I can't imagine a Campy guy putting a no-name non-matching crankarm on his bike it's really a moot issue.

DasProfezzional

Well if you're talking about taper that's one thing, and that's pretty easy to figure out. The only wrench in the works is that there's JIS standard and JIS short taper, which isn't even that much of a wrench. But the square/diamond thing is something entirely different, and you just have to pay attention. Luckily it's only something to consider if you're mixing and matching crankarms.

ebrady

I had the exact same problem on a mountain bike. I replaced the crank arm, and it fixed the problem temporarily. After about a week, I noticed the new crank arm was damaged like the original one. Taking a closer look at the bottom bracket axle, I noticed the taper on it looked odd. I put in a new bottom bracket, (plus a second crank arm to replace the newly damaged one), problem solved.

PunctualAlex

Well, lucky for me I have a bike co-op in town with tons of partner-less left crankarms, I'll grab one and see if it solves the problem- the matching thing isn't really a problem, this is a no-budget beater commuter bike. If that doesn't solve the problem, I'll invest in a new BB and chuck the junk crankarms and no one will care or remember them.
Thanks for the advice, all!