capnjonny 

I have an old Trek 800 mtn bike that I ride sometimes and take to burning man every year. Last year , after riding it around all week the 14 mm nut that keeps the crank arm on the bottom bracket spindle came loose and the pedal an d arm fell of on the non drive side. Luckly the cap was in place so I didn't loose the nut. Back at camp I re installed the arm and tigtned the nut as much as I could with a socket wrench. The nut came loose again after another days riding and I tightened it again.

when I got home I again tightened the nut but this time also used blue lock tight.

Fast forward to this year and again after a few days use I had the same problem , tightened the nut, it came loose after about 1 mile riding , and tightened it again. Finally in desperation I found a repair camp in Black Rock City ( the temporary city built for 2 weeks in Nevada's Black Rock Desert) and had a guy spot weld the nut to the spindle in 2 places. I'm pretty sure the nut will never come loose again but I really need to give the bike a complete service if I want it to be reliable. I can grind off the welds and probably get the nut off at the shop but I can't figure out why it kept doing this.

If anyone has any idea why this kept happening I would like to know. I have access to any parts I need to fix the bike and could just replace the whole assembly when I tear it down but I would really like to understand why this happened. This pic is of the bike before Burning Man

trailangel

After it comes loose the first time and you ride it until it fall off, it's ruined and will never hold again. You need a new crank arm on the Non drives side.
Install new ... correct torgue value, ride it 20 miles and re-torque one time. You can put threadlocker on if you wish.

canklecat

For the sort of repairs you'd encounter at Burning Man, maybe a tube of Park SAC-2 grip compound might help.

But, yeah, what trailangel said. Supposedly once a crank arm has been enlarged, deformed or worn it won't hold securely.

dsbrantjr

Yes, the female taper on the aluminum crank arm gets distorted and will no longer fit properly on the steel bottom bracket male taper. Fortunately the non-drive crank is simpler to find and to replace, it doesn't need to be identical just the same length. Just make certain that the orientation of the taper, "square" or "diamond" is the same as the damaged one. Your crank almost certainly has a JIS Taper https://www.sheldonbrown.com/bbtaper.html If you decide to use threadlocker (I don't think it is needed/advisable if the torque is correct) just apply it to the threads not the taper.

Andrew R Stewart 

Now that the BB axle has been damaged by the spot welds, to try to keep the old arm on, it will likely need replacing too. If the nut can't be removed the assembly might need to be cut off. Andy

trailangel

If you welded it at Burning Man, how can you remember what actually happened?

phughes

Did you clean the spindle and apply a small amount of grease when you reinstalled it? Sometimes the crank arm will band a bit before it is properly seated. It will then loosen up as you ride. A clean spindle surface with a little grease can helps nd it will keep it from binding when you want to remove it, stopping the corrosion you get with two dissimilar metals. Try that, and actually use a torque wrench to apply the proper torque instead of going blind with a ratchet. Look up the torque values online and go by that. Generally the torque for square spindle crank arms is 25-32 ft-lbs.

capnjonny 

Thanks everyone for the input.

I will just grind off the spot welds and remove the nut then do a service on the bottom bracket and replace the spindle/axle if necessary. We have boxes of crank arms at the shop to replace this one with.

Burning man Eats bicycles. I have worked at a bike camp there before. No one should buy a "Burner Bike" that has not been completely serviced after spending a week at Black Rock City.