Mountain Biking - Chronic crank looseness

I have a Specialized Hardrock Sport, around a 2000, 2001 year. It doesn't have many miles on it, but recently the crank started coming loose and clicking when I pedaled. I took it to a bike shop and they tightened it (I'm not very mechanically inclined and just moved into an apartment and don't have many tools with me yet.) It was fine for about 20 minutes, and then it came loose again and started clicking. I took it back, they tightened it a second time. Rode for about 20 more minutes and it's loose yet again. Now, I'm not doing hard riding by any means. I'm just riding around my college campus; I haven't take it on any trails since I've moved. I'm not jumping or bunny hopping or doing anything else harsh either. What are some reasons why the crank would keep coming loose after so little riding? A bent crank? Bad bearings? Thanks.

Jason

I had a similar problem a few months ago and it turned out to be an old/worn bottom bracket. Next time they tighten the cranks, have them take a look at the bb.

but recently the crank started coming loose and clicking when I pedaled. I took it to a bike shop and they tightened it (I'm not very mechanically inclined and just moved into an apartment and don't have many tools with me yet.) It was fine for about 20 minutes, and then it came loose again and started clicking. I took it back, they tightened it a second time. Rode for about 20 more minutes and it's loose yet again.
What you're describing happens when a square taper crank is ridden for some time with a loose crank bolt. (Probably from missing it's first tuneup or even from being missed on assembly. There's no guarantee when it happened)
It's not noticeable at first but slowly the Bottom Bracket (BB) spindle (being made of hardened steel) begins to hog out the hole in the softer (Aluminum) crank. By the time you notice it it's too late.
Tightening the crank arm pushes it a small bit up the taper of the BB spindle which gives you only a small area (Like a thread or two's width0 of "new" contact on a higher (fatter) part of the spindle. The fact that the rest of the hole has been hogged out means that with in a short time your crank will start flopping again. Tightening it again will have the same reaction. Usually after about the third time tightening it you'll wail on the bolt causing it to shear. (Always fun)
Bottom line: Your crank on that side is hosed Time to replace either the arm or the crankset. There's likely NOTHING wrong with the BB itself

What you're describing happens when a square taper crank is ridden for some time with a loose crank bolt. (Probably from missing it's first tuneup or even from being missed on assembly. There's no guarantee when it happened)
It's not noticeable at first but slowly the Bottom Bracket (BB) spindle (being made of hardened steel) begins to hog out the hole in the softer (Aluminum) crank. By the time you notice it it's too late.
Tightening the crank arm pushes it a small bit up the taper of the BB spindle which gives you only a small area (Like a thread or two's width0 of "new" contact on a higher (fatter) part of the spindle. The fact that the rest of the hole has been hogged out means that with in a short time your crank will start flopping again. Tightening it again will have the same reaction. Usually after about the third time tightening it you'll wail on the bolt causing it to shear. (Always fun)
Bottom line: Your crank on that side is hosed Time to replace either the arm or the crankset. There's likely NOTHING wrong with the BB itself

Informative post Raiyn. Thanks.

Informative post Raiyn. Thanks.
I do try. Check your crank bolts folks!

I had that happen to me before and I used aluminum foil as a shim and it lasted over a year.

Steve

Raiyn is right. I have the wasted crank arms to prove it.

I wonder if JB Weld would work for a temporary (actually, permanent) fix? You would ultimately have to be willing to sacrifice your BB spindle to the sawzall, though. But that'd be okay since if it didn't work, you'd probably want to go ISIS anyway.

i had that happen to a trek 3700 I bought new. I didn't know anything about maintaining bikes, but i took it in once a month for its free tuneups from the LBS. Once I noticed the crank loosening, i started tightening it myself. After the crank was thoroughly ruined, i tried to cry foul on trek, and on the LBS. Neither took responsibility, so now I don't buy trek, and I found a better LBS.