Loose left crank arm
 

Hi there,

​​​​​I am a bicycle courier and have just run into a problem with my left crank arm. All shops are shut here in Spain at present with the lockdown so just want some advice on how to fix my bike with the tools I have. I first noticed a slight click on my pedal yesterday, which developed into a big wobble in the crank which quickly became unrideable. I came off the bike and realised that the crank arm was completely loose. I took the bike home and tightened the crank bolts with an Allen key (I don't have a torque wrench) and it did the job for the rest of the day. This morning the same problem has come back. I will probably be able to do the same quick fix, but I'm sure the problem will come back. Does anyone have any advice what the root cause of the problem would be? Would using a torque wrench be enough to fix it or do I need to buy some new components. Just to give a bit of background to the bike. I bought it last year in November and have done ALOT of cycling with it since then. I typically do around 2000km per month. I understand that it is hard to know exactly what the problem is without seeing the bike, but given that I have no experience with bike mechanics and all the shops are closed I just wondered if someone with more knowledge would perhaps know what could be going on? Many thanks for any help!

There different ways crank arms are secured, but guessing the type you most likely have.

Knock the crank arm on hard with a hammer. You may use a block of wood or something, so you don't damage the crank arm. You can lie the bike down, and put another block of wood under the other side to support it. Don't get so carried away that you bend the crank arm. Then tighten the bolt hard with an allen key.

It is possible that aluminum crank arms can become worn on the inside, and not fit properly, or stay tight. Most people then replace it. If it was me, I would try filling the gap with epoxy resin, knocking it on tight, and tightening the bolt. I have not done this, but it would probably be effective for a cheap bike.

There is a good chance the inside taper in the crank is now distorted. Look inside and see if you can see a step or mark where flopping about has damaged the smooth taper. You may be able to clean it up with a triangular file. Another temporary fix would be to get some Loctite 660 and use that to try and fill the gaps. https://www.henkel-adhesives.com/us/...ctite_660.html And do the allen bolt up real tight.
Ultimately though, unless you get lucky, it'll be new crank arm time.

Thanks for the replies. The problem has quickly come back again. Problem is I don't have any tools for taking apart the chainset. I only have an Allen key and it seems I need a special kind of key to put the top cap back on?

There are too many different crank designs for guesswork. Post the name and model number of the crankset to get meaningful replies.

Sorry, it is a Shimano Sora crankset. It is the same model as the one posted in the Amazon link but I just don't know what I would get with what is advertised on Amazon. Would you normally get the BB and crank arm or do you have to buy them separately? Sorry for the stupid questions but I am a complete novice when it comes to bike repair.

Does your crank bolt look like this?
https://cimg5.ibsrv.net/gimg/bikefor...06388b14ce.jpg
One key with pin spanner holes.
You do not need to remove the ring to tighten the crank bolt. It sounds like there was inadequate torque used for assembly. After checking for damage to the tapers as recommended in the previous posts, you'll want to grease your crank bolt threads and reassemble. Shimano one key release torque values with 8mm Allen bolts are up to 400 in/lbs, or as tight as axle nuts are supposed to be. You will need to get the crank bolt as tight as you can. You're unlikely to snap the bolt with an L shaped wrench alone.

No, the crank bolt is on the opposite side on crank arm and is a black bolt which needs a specific key to tighten. I have tightened the two bolts to the top and side of this main black bolt but I still have this wobble in the cranks and the crank arm eventually comes loose after a bit of riding.

You have a hollowtech crank. The black "bolt" is a preload bolt, meant to squeeze the crank arm on.
For any shimano product still common in the world, you'll be able to help yourself by going to their technical documents repository online.

Search for your model number, which should be on the inside of the crank arms, and start there. SI documents are the installation instructions and will walk you through the recommended installation procedure.

I used a needle nose pliers opened slightly and wedged into the lobes to preload the crank arm. Thankfully it doesn't need to be really tight to get the pinch bolts to engage.
The proper tool for the job is always preferred, though.

Once the bike has been ridden with the crank arm loose the inside taper has likely been damaged and the arm will need to be replaced. It is highly unlikely that you will be able to file it back into the proper shape, nor will the bolt hold it no matter how tight it is torqued.
Replacement arms are available from Amazon, you will need to know the length, which should be cast into the inner side of the crank, it will be a number near or equal to 170(mm). You will also need to determine if it is a "square" or "diamond" orientation. https://smile.amazon.com/s?k=bike+cr...f=nb_sb_noss_1 This listing has examples of diamond and hole orientation: https://smile.amazon.com/Epessa-Crank-Left-Diamond-170mm/dp/B07K7579HK
You will also need a wrench to swap your pedal, the left pedal is left-hand threadede. Since you are a bicycle messenger it would be wise to invest in a basic set of tools to use in maintaining your bike; your crank failure is a result of not having the securing bolt kept tight; pedals are also susceptibel to damage from being loose.
Edit: If you have a Hollowtech crank instead of square taper these will not work.

YOu need to look on the back of the crank arms and see which model of Sora it is so we know what type of spindle interface it uses.
Square taper, Octalink, HTII are some of the types it may be.

Should say something like FC-3300, FC-3303 etc.

If it's square taper you can shim the taper with some aluminium cut from a beverage can as a temporary solution until the real repair of replacing the crank arm can be done.

https://cimg0.ibsrv.net/gimg/bikefor...09d818450f.jpg

Fellas, OP is describing a HTII crankset
 

It says 175 FC-R300/3030. So I'm guessing that is the HTII crank set, it looks exactly like the one in the video. So do I need to replace the whole crankset or just the crank arm?

https://www.giantbikespares.com/Shim...detail/3-44459

As a temporary repair you could TRY wrapping some thick aluminium foil as a shim around the spindle and reinstalling the arm until a replacement arrives.

You should be able to pull the crank off and have a look at the splines on the axle and inside the crank. The axle is steel, so that should be fine, how do the splines look in the crank?. One thought... are the bearings OK? Pull the axle and chainring side crank out and check the bearings roll smoothly.