Securing the nut with a high-strength grade of Loctite will validate whether or not the tapered square bore can be used for any length of time.

If the nut holds and the crank doesn't move back and forth on the spindle noticeably, it may last a long time.

For harder usage, the arm should be replaced.

I would secure the crank nut to normal torque, then stress the assembly by having the rider stand on the pedals while stationary, rotating the cranks 180-degrees and repeating before torqueing the nut again, and before the Loctite in the nut threads hardens. Perhaps with the arm getting "worked" onto the spindle and re-torqued, the movement that loosens the nut can be eliminated.

Thread is right on.

Although it's not ideal, I have successfully tried the Locktite solution as was described to me in the past and used a driveside crankarm for several years after it would have otherwise been rendered useless: put blue Locktite on the spindle and crank the arm as far onto the spindle as you physically can. If it holds holds, you can keep using that crank for as long as your bottom bracket lasts.

Just get a used Sugino crankset and be done with it. Wouldn't you rather have peace of mind instead of having to worry that your crank is going to come loose 50 miles from home everytime you ride?

A minor point, one discussed by Sheldon Brown BTW. Under normal riding the crank arms are stressed only one way w.r.t. the spindle. With pedaling forces the left arm is torqued to the front (counterclockwise viewed from the NDS) w.r.t. the spindle. This torques the spindle to the front (clockwise viewed from the DS) w.r.t. the right arm. In contrast, pedaling forces on the right arm are countered by the chainring instead of the spindle, so this doesn't torque the spindle at all. What this means is if you level the pedals (such as for a bump) with left arm forward you are torquing the arm/spindle joints the way they normally work during riding. If you level the pedals with right arm forward you are reverse-torquing the arm/spindle joints. Reverse torquing them is more likely to cause a problem to develop if the bolts (or nuts) aren't correctly tightened. Once they are damaged you may never get them to stabilize fully but at least you don't have to keep making it worse.

:lol: That is definitely a quick fix. Very permanant, too.

I agree with others who say it should be replaced.

When it came loose earlier it likely has worn away the crank arm enough so that it wobbles no matter how much torque is applied. When it wobbles, then it works the bolt loose.

In this case I recommend to replace but you can try to use a hard epoxy filler to keep it from wobbling as a fix (usually temporary). Coat the spindle and threads "fully" with auto paste wax first so that if this does not work you can still get the epoxied crank off. Fill in the crank arm a bit, tighten it down, leave it for an hour, remove the crank arm then let set off the bike for another 24 hours or longer. Clean up the spindle so it can be used with a new arm if this does not work.

Bear in mind this is something of a mid term fix, and may not fix at all. I used this method once when I drove to an out of state MS150 ride only to find the day before my left crank arm was toast. It held up for the ride just fine, but of course was soon replaced. Not sure how long it would have lasted.

Here is the pictures I Have on the crank arm.

Is this the culprit & is there enough material still here to work with to try a cheap fix??

http://bikeforums.net/attachment.php...hmentid=393167http://bikeforums.net/attachment.php...hmentid=393168http://bikeforums.net/attachment.php...hmentid=393169http://bikeforums.net/attachment.php...hmentid=393170http://bikeforums.net/attachment.php...hmentid=393171http://bikeforums.net/attachment.php...hmentid=393172http://bikeforums.net/attachment.php...hmentid=393173http://bikeforums.net/attachment.php...hmentid=393174

That taper looks pretty toasted to me.

Have you tried an up-to-spec heavy torqueing using Loctite on the bolt threads?

Only way to find out for sure! I would apply forward and then reverse force/torque to both pedals (with the cranks horizontal) after the first torqueing, then immediately re-torque and allow the Loctite to fully set for a few hours before riding. I give it at least half a chance!

Coincidentally, my left-side Maxy crankarm came loose at the end of a 44-mile ride through the hills yesterday.
I had just done a couple of consecutive, uphill- maximal "chase-down" efforts of passing hard-charging roadies when, after the last hill leveled, I thought that my SPD pedal was perhaps falling apart, but it was the crank's taper connection that was clicking and wobbling. This came on very suddenly, and after one-legging in to the finish (free wine, baguettes, blueberries and brie, can you believe???), hydrating, and finally removing the cap and bolt by hand, I saw that the inside taper bore still looked fine!
I was offered a car ride home after a borrowed 15mm socket/ratchet turned out to have too-thick walls to fit within the crankarm's threaded extractor/cap bore.
I'll now also replace the stock bb before reinstalling the crankarm , since the original one seems to have excessive chainline offset for the 7-speed freewheel I've using.

^ yeah, that spindle looks terrible.

after seeing that, it's what i would try first -- either a new, replacement spindle or a cheap ($15) cartridge bb from shimano.

From looking at the arm it looks like the spindle is supposed to have flats as the right arm is the same way & it doesn't come loose?? Please compare?

^ the spindle should certainly have flats. awful, awful looking spindle.

Spindle appears in fine condition, the alloy arm can do it no harm.

Not a high-end spindle of course, but an accurate JIS taper I'm sure.

One thing though about nutted spindles is that because of the coarse threading and larger 10mm thread diameter, you get less press-fit force for a given amount of torque on the nut, as compared to any fine-threaded 8mm bolt.
That's partly why I recommended repeated torqueing after the crankarms had some torque applied. This should result in a deeper and heavier press-fit that will better help the assembly to resist pedaling forces by keeping the spindle more-fully inserted in the arm's square taper bore..
The arm is obviously damaged, but mostly only near the big end of the taper bore.

I wonder if this is a case of mismatched taper widths, JIS vs. ISO, or whatever. It could keep the arm from seating properly, with too much contact at the inboard flare and not enough on the taper itself.

@dddd, i was looking at the second pic in post 32 which appears rounded in the context of the op asking if the taper is "supposed to have flats." that led me to believe it was damaged.

I'm thinking (but aren't really sure) the OP was asking about the beveled corners on the spindle. They should be there. But part of that arm did look bunged up. However it is really hard to judge the fit of a crank taper just by looking at it.

I wonder too if maybe the arm is cracked. Check for a micro-crack!

Under the abusive conditions in which the crank arm has been used, the likelihood of a crack is much higher than normal.

Post a few pics of the arm, I can check my local CoOp, but that arm looks looser than.........(insert ex girlfriend slam)

Interestingly enough, I wondered if an ISO spindle might better fully engage the entire depth of the square hole, but since this is a case of trying to effect a quick-'n'-cheap repair, I thought that I would recommend the Loctited-threads repair that I would first try myself if a spare arm weren't handy.

Some square JIS spindles like Dura-Ace also have sharper corners (smaller bevels) that might better engage this damaged taper, but again I'm not recommending adding expensive parts to even an effective kluge repair.

In general, these "non-machined", nutted spindles have a somewhat crude appearance from their mass production, but the tapers are usually extremely fine in their measurable dimensions.
How did they do that? The metal is extremely hard, so all machining appears to have been done before heat treatment.

When my crankarm loosened yesterday, the first thing I checked for was if it had cracked. I loaned a 1990's Veloce-equipped bike to a strong rider/friend a few weeks back, and mid-ride he cracked the left arm open during a particularly steep climb. The arm quickly became extremely loose. He luckily knew of a nearby friend who was able to give him a ride home while I rode back on my own. Bike had been a rental so probably suffered from routine tightening of the crank bolts, which can eventually split open an alloy arm.

What's the story here... tightening was required due to service conditions, or superfluous due to ignorance on the part of the "mechanics"?

I can't tell from your picture but how are the threads on your spindle? The spindle and arm both look like they have some experience so perhaps you have some damaged threads. If it's not too bad maybe they could be cleaned up and you could get a better fit... I think I would try a new bb first if you have one around, though, personally. The arm can't get any better but it can certainly get worse.

It is common knowledge that once a square taper crank comes loose the chances are 99% it's toasted. The usual suspect is always the crank arm interface. The square taper interface usually has a steel spindle and a aluminum crank arm. When the two come together the spindle retains it's shape because it is harder than the aluminum crank arm. The crank arm interface then expands slightly when it is pressed onto the spindle with something called plastic deformation; this holds the crank on the spindle, but turning the crank puts lateral forces on the interface (rocking it side to side) so a crank bolt is needed to keep the interface strong enough that the rocking forces will not loosen the interface (if you ever have a crank arm stuck on a spindle, just ride it around the block without a crank bolt and it will come off). Because of this, if the crank bolt comes loose, the crank arm will start rocking side to side and unscrew the bolt. By the time you have realized what happened, the crank arm interface will have been permanently rounded it will never hold together tight enough to stop the rocking forces (this damage is rarely visible to the naked eye). The spindle interface however will not be damaged.

As far as a replacement, I assume your crankest looks like this (from velobase VeloBase.com - Component: Sugino No. 5 / Silstar almost any Japanese square taper JIS crank arm will work as a replacement as they are mostly the same. If you want an exact replacement you're probably looking at some money because the older styles, like yours are getting scarce but any of these will work... http://www.ebay.com/sch/i.html?_odkw...y%29+&_sacat=0

^ i've only had it happen once, and fortunately the interface was good enough to retighten and not loosen again (knock wood.) it was a campagnolo record arm and spindle on a new build.

Great thread. And timely for me. I have a Sugino GT 175mm left crank arm that seemed to make up loose last season. I bought new Sugino spindle nuts and put them on. It ran ok for a few hundred miles this year, then came loose again resulting in an hour's walk home. I tightened it, but hadn't ridden it in a while. The reason it's important is that it's about to go on a long trip--then along came this thread.

I just pulled it--and indeed, several of the tapers, rather than being sharp edged, were rounded. From the parts box I pulled a Deore XT 175mm crank arm with lots of pedal scratching but pristine tapers. Comparing the two, it was quite a difference. And it was quite a difference in how nice and stiff the tapers made up to the spindle.

So thanks to the OP and all who weighed in.

I'll never tell the '84 Miyata 610 that it's got a Shimano Deore XT left crank if anyone else doesn't. Until I can find a Sugino GT/AT or similar replacement to swap out the Deore XT for....

Anyone?

OP,
Did you say a bike shop told you that crank was fine? Go back and slap them that wallered out thing!