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Loose bolt on left crank arm
#1
hi everyone,

just joined this site today it looks really useful. ive recently started cycling to work and got a 2nd hand bike. its a diamondback sorrentose.

the left crank arm keeps coming off, the bolt keeps coming loose. I have replaced it with a new one which had some grip on it but it came off this morning and had worn down quite a lot.

I cant really afford for an expensive fix and dont want to have to stop and tighten it everyday so can anyone tell me if there is supposed to be some kind of washer or something to stop the bolt from slipping?

many thanks in advance

Chris
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#2
Usually, there is a washer between the bolt head and the crank.

Make sure that the assembly is tight.

Typical replacement part, includes new washer:
http://www.amazon.com/Sunlite-Bottom-Bracket-Cartridge-Brackets/dp/B000AO5KGA/
Nigel
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#3
Sounds like your crank thread is stripped.

You can fix it by replacing it with a Shimano UN54 bottom bracket.

You an also try a fix till you have money.
Clean the threads very carefully with, nail polish remover (acetone, Rubbing alcohol, automotive brake or carburetor cleaner. Be careful around paint.

Put the arm on get it in tight with some light taps and than apply red locktite thread locker( automotive stores) to the bolt and tighten snug plus half turn or so. This is a stud and bearing mounting product and requires torch heat to remove. You can try locktite blue, but it sounds like a badly stripped crank. Be careful not to cross thread , use a new bolt with a washer.
Never Give Up!!!
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#4
I think you probably need a new crank arm. If the square hole has gotten flared out, the nut won't hold it regardless. If you did have stripped thread that could cause this. But it coming loose repeatedly is pretty classic sign of a trashed crank arm.
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#5
I'm with Dave on the worn crankarm taper.
Wheelies don't pop themselves. (from a QBP fortune cookie)
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#6
x3 on that, sounds like you need a new crank and or arm if you can find one. Take a pic of it we might have one
There are two kinds of people in the world, "Those who help themselves to people, and those who help people!"
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#7
Hmm... Its true that at this point you may have two problems. A stripped crank thread and a rounded out arm hole.

"The left crank arm keeps coming off, the bolt keeps coming loose. I have replaced it with a new one which had some grip on it but it came off this morning and had worn down quite a lot. Chris"


From your description of a worn down bolt it sounds like a loose bolt may have led to a rounded arm hole. Possibly cross threading or over tightening led to this.

Examine both carefully.

IMO a properly tightened low mileage crank arm would not wear itself and than loosen the bolt.
Never Give Up!!!
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#8
[attachment=2776][attachment=2777][attachment=2778][attachment=2779][attachment=2780]

Thank you all for your comments, I have taken some more pictures that may help clear this one up.

The first 2 pictures are of the crank and it looks ok (to my very untrained eye!) The 3rd Picture is the bolt that keeps coming loose.

when I brought the bolt, it was completely black in colour and had some grip to it, the silver colour in the picture is where it has worn away and now has very little grip and comes loose after about 5 miles cycling.

The last 2 pictures are of the right crank arm bolt which I am unable to undo to have a look.

The way I see it is I am either missing some kind of washer that grips the bolt in place or something is not quite right with the crank and I can either use Locktite or get a new crank?

Any further advice would be very much appreciated.

p.s I know my bike really needs a clean!!!

Many Thanks

Chris.
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#9
That kind of nut is not supposed to have a washer. And if you are able to tighten it down without it slipping, no big reason to think the threads on the spindle or nut are the problem. This is classic sign of a bad crank arm. Can't guarantee, but this is what bad cranks do. Since they don't sit solid on the spindle, they start to wobble, which works the nut loose. Thread lock, etc. won't fix this. Get a new crank arm (left only) and another new nut for good measure.
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#10
(09-14-2011, 09:18 PM)DaveM Wrote:  That kind of nut is not supposed to have a washer. And if you are able to tighten it down without it slipping, no big reason to think the threads on the spindle or nut are the problem. This is classic sign of a bad crank arm. Can't guarantee, but this is what bad cranks do. Since they don't sit solid on the spindle, they start to wobble, which works the nut loose. Thread lock, etc. won't fix this. Get a new crank arm (left only) and another new nut for good measure.

100% agree.

The areas of the nut that are no longer black are aluminum build up from the crank arm, not the black coating wearing away.

The tapered square hole in the crank has become too large, and the nut is bottoming out before clamping the arm in place.

You need a new crankset. This is the cheapest I have seen: http://www.amazon.com/Sunlite-Crankset-Cotterless-Steel--38-48/dp/B000AO9ONU/
Nigel
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#11
Thank you Dave and Nigel,

Are they pretty standing fittings? Basically what do I need to consider when buying a crankset?

I have found one that I think looks ok that is sold in the UK:

http://www.amazon.co.uk/Shimano-FCM151-48-38-28-Triple-Chainset/dp/B003LRAEIC/

Does this look ok to you guys?

Thanks again

Chris
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#12
I'm with you dave on this too. Someone has put on a washer to compensate for extreme wear. New unit!
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#13
(09-15-2011, 08:19 AM)9734Chris Wrote:  ....
http://www.amazon.co.uk/Shimano-FCM151-48-38-28-Triple-Chainset/dp/B003LRAEIC/
.....
Hi Chris;

That will work fine.

We Americans are certainly spoiled with low prices !!
Nigel
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#14
Yep , 10-4 to all the above. Its a nut not a bolt. Proper terminology and photos would have saved some guessing. (some cranks have an inner thread so a bolt goes into them, that's what your description sounded like)

Another photo of the hole in the crank arm off the spindle would tell the story. The spindle looks beat up too. Turn it to see if it feels smooth. You still may need a BB. At least regrease it. Soak everything in a penetrating oil before attempting disassembly.

The threads look worn but probably still OK . The nut has a build in non slip washer, so none are needed. Get a new nut. Put it on carefully and see if it threads on all the way before putting the crank on do not force. Careful not to overtighten and strip it when installing crank. That's where torque wrench and experience from school of hard knocks comes in.

Ye we certainly get good prices here, maybe because we buy more.
Never Give Up!!!
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#15
Note that most better shops will have a left crank arm by itself that you can buy. As long as it is the same length (usually stamped on the back) it should work without a problem. Might be a lower cost option if the drive side is in ok condition.
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#16
This is a common problem and as others have said, you need a new / replacement left hand crank arm, not a new crankset, unless you have other problems.
The thread and the axle are fine, as probably is the nut.
Look at the shape of the square hole on inside of the arm, if this is at all bell mouthed, no amount of tightening will succeed.

A new arm should not cost much, here in the UK they start from about £8 for steel, and £10 for alloy, some LBS sell s/h ones for even less, but do check for wear on the taper and the thread for the pedal, and also make sure you get the same length as your ex one. Yours is an alloy Shimano, probably 170mm. long.
BUT, BUT it is very important to get the new fitting VERY tight from the start, or you will repeat the same exercise several times, if you use your LBS they will fit it for you for a modest fee.
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#17
I have the same problem. I guess I did not put the bolt in tight enough the first time, but, I think it's more likely that the crank arm (which I got new) was just not made well enough from the start?

I wonder if I can regrind the taper? I've got at least 1/4" "extra" space before the place where the bolt meets the arm would become flush with the end of the spindle.

My crank arm fell off in the middle of the road today - I had been on my way to the local bike co-op to replace the bottom bracket, which, although is relatively new, seems to be missing a ball bearing or two as it slips with a clunk and feels a little rough sometimes.
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#18
A crank arm that falls off can be a sign of damaged tapers on the BB axle or the crank arm. Regrinding them could be done with high precision tools I guess since the angles have to be absolutely correct. I guess getting a new one is a) cheaper and b) a lot less work.
The slipping in the BB may be excessive chain / sprocket / chain ring wear.
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#19
In my case, the clunking/slipping BB has got to be either some missing bearing balls, or, something inside there has a chunk of the metal missing, or, it somehow came loose inside without the caps being loose (which seems pretty unlikely) because there is no visible wear on the chain or chainring (all less than a year old and not a lot of milage, all the parts were either new "pulls" that I bought cheap from bike stores where the customers "upgraded" [at the time that they bought a new bike] and didn't ask for the old parts back, or, were just extremely good deals new (got my Fuji aluminum MTB frame for about 85% off at Bike Nashbar)...

I guess you're right, trying to grind the taper on my crank arm back to square would most likely be an exercise in frustration, best just to buy a new/used one - this time, I'll try to make sure and put it on tight, maybe with some Loctite too? Too bad I can't just use a cotter pin through the bolt Tongue
(06-25-2012, 01:50 PM)Joe_W Wrote:  A crank arm that falls off can be a sign of damaged tapers on the BB axle or the crank arm. Regrinding them could be done with high precision tools I guess since the angles have to be absolutely correct. I guess getting a new one is a) cheaper and b) a lot less work.
The slipping in the BB may be excessive chain / sprocket / chain ring wear.
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#20
Hi everybody.

I had the same problem mentioned here- that the crank arm kept falling off. I tried something today and maybe it can help you guys as well: I made sure that the square shaft's stem does not touch anything on the crank arm, so I hit the crank arm with a hammer in place. In doing that I tried to achieve a snug fit between the square shaft and the crank arm. After doing so I closed the nut tightly.
I will report if the crank arm holds in place.

Later,

Adar

(Attached is a model I made in Solidworks, to explain what I did)
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