fcoy2

I have a Fuji Saratoga road bike from around 1990, and my left crank is starting to come loose. I tightened up the nut and added some threadlock about 3 months ago and it has just come loose again. Would replacing the crankset with something like a Shimano M361 Hybrid Crankset be the solution or would this require replacing the bottom bracket as well? Thanks!

dsbrantjr

If you have ridden the bike with the arm loose, there is a good chance that the tapered eye of the arm has gotten deformed (assuming you have the common square taper system). It is possible to replace just the arm; you need to match the length (generally cast into the inside of the arm) and the orientation of the taper, so-called diamond or square.

fcoy2

I have stopped my ride (on a cycling trainer) each time it has come loose, so unless it causes damage before you can feel the "wobble" I wouldn't say I've ridden on it loose. It doesn't seem right that it has come loose twice in the past 3-4 months, which is why I am considering replacing the crankset. Thanks for your reply!

Homebrew01

Is it a square taper crank ?
There could be a crack in the crank arm.

fcoy2

I believe it is (I'm not the most knowledgable about the specifics of the bike hardware). I didn't notice a crack, but I wasn't looking for one either. I'll check it out. Thx!

sovende

I would start looking for a replacement crank arm ! If you're just using the bike on a trainer, I wouldn't be terribly concerned about a "brand" match, as long as the arm length and the mode of fit to the bottom bracket spindle are the same. I'd pay attention to the torque specifications when mounting the replacement crank arm. I can't recall them at the moment but just ask the "Google"! Too loose is obviously an issue but too tight can deform the square taper on the crank arm and cause continued issues.

trailangel

Keeps coming loose. Time for a new one. Comes loose one time and it's toast.

sch

One other thing to look at: mount the crank, tighten the fixing bolt/nut to 'prescribed' torque if known, remove the
bolt/nut and see if the end of the sq taper spindle is at least 1/16" inside the crank. If the spindle end is flush
with the bearing surface of the crank which is the part the bolt/nut push on then loosening of the crank will occur.
Some have shimmed the sq taper on the spindle to push the crank out eg small squares of soda can metal in
this scenario.

Miele Man

I have shimmed many a square taper crank by gluing soda can shims to each flat of the spindle. That has given the crank years more use. Be aware that it might move the crank outboard a bit.

Cheers

Matthew913

I have an ebike with the same issue, only it's almost brand new - only 125 miles so far. I hand tightened it and it still comes loose. Should I try threadlock or something similar first? I reported it to the mfg to see what they say.

mdarnton

Thanks for this. I have wondered for a long time if wrapping aluminum foil around the male part would work.

dedhed

You can typically buy just a left crank arm to replace it.

mack_turtle

depends on the type of crank arm/ spindle interface. this is common with conventional square taper cranksets. most likely, the fixing bolt was never properly torqued in the first place (should be in the neighborhood of 30 foot-pounds). if it came loose and was ridden like that at all, the alumuminum crank arm will wallow out on the harder steel spindle. replace the crank arm only for the easiest fix, and use a torque wrench to ensure that the fixing bolt is done up right the first time.

Miele Man

NO! It won't work as the aluminium foil is far to delicate. I use a gel cyanoacrylic glue to hold each of 4 soda can shims on each of the 4 spindle flats. I make each shim JUST big enough to cover the spindle flat. I let the glue cure well before putting on the crankarm.

Which crankset do you have and how long is the crankarm?

Cheers

cxwrench

Normally when a square taper crank arm is damaged like this from riding it while loose the tapers in the arm get damaged 2 ways. Straight up enlarged, and also they get a bit twisted. I've been doing this for a living (being a mechanic, not killing crank arms) for a long time, over 25 years, and I've never once heard anyone talk about aluminum can shims or claim they work well/for a long time. That's one of the lamest fixes I've ever heard.

Matthew913

Everyone here was right, the nut coming loose indicated that the threads inside the crank were stripped. Lectric gave me a new one right away and the problem is solved. Thanks to everyone's input on this thread

Geepig

Logically at least one of those statements cannot be true -> if the second is true it nullifies the first

As an R&D engineer I would say that most of my work has involved things no one ever talk about before, and I have seen crazy sounding things work. I now rather hope I will one day get to test the idea.

capnjonny 

My solution when it happened to my bike at burning man - have an itinerant welder (there were a couple there) - tack weld the nut to the bottom bracket spindle. It worked till I got home then I replaced the spindle and left crank arm