On the Tiara it seems no matter how hard a set the left axle nut it wants to eventually wiggle loose, so much so that I took a 14mm socket with me on the ride today (it loosened twice).

When I look at the nut I see the little "locking nubbins" (not knowing exactly what to call them) but they seem awfully smooth to me. Is this my problem? Should I try to get a new one at an LBS or try ar lock washer in between the nut and the crank arm?

Ideas? Common problem?

I wouldn't mess around if I suspected a faulty nut. That is cake to fix. The chance that this happened before and the crank arm is tweaked, ugh.

Or just succumb to the bolted cartridge bottom bracket

The arm is completely solid when the nut is cinched up...I'll try to locate a new nut tomorrow and see what happens. Thanks.

Locking nubbins=serrations, it is a serrated nut. It probably won't hurt to try a new nut, but I think the crank arm has already been damaged.

I don't understand where an axle nut has anything to do with crank arms, since you're talking about a "nut" vs. a "bolt."

I'll assume you mean the nut that secures the crank arm to the bolt that protrudes from a square taper BB spindle.

1-pull the crank arms (both sides).
2-inspect the threads on both sides for any damage.
3-inspect the nuts from both sides for damage.
4-swap sides with the nuts, tighten, see what happens.
,,,a) If the other side is now loose, it's the nut.
,,,b) If the same side keeps getting loose, it's the bolt.

5-If it's the nut, replace. I've found them a Lowe's before. I wouldn't recommend a lock washer, but you can try it.
6-If it's the bolt, you can chase the threads, or you can switch to a different bb.

Never re-tighten the nut or bolt. The correct procedure is to pull the arm and then torque the nut or bolt properly.

The crank arm is probably the culprit here. They get slightly rounded out and the square taper system no longer works properly resulting in the crank arm coming loose. Replace the crank arm to fix.

Well, I went to the local Ace hardware, didn't have a nut, LBS wasn't open yet. But I did compare the serrations on a new nut to the one I had and the new nuts serrations seemed to be a lot sharper and well defined. So I brought the old nut home and just touched the edge of each serration with the Dremel cutoff wheel just to freshen the edge a little. The I used some blue (read: removable) Loctite on the threads. We'll see how this goes. If not I'll replace the nut, and if necessary the crank arm. I gotta tell you though when the arm is installed there isn't the slightest amount of wiggle or movement in it at all. Solid as I could imagine it would be. Time will tell, thanks for the tips.

I think you did the right thing. If the arm is secure, the nut only serves to keep it from falling off should it ever work itself free, which is unlikely unless it's already been damaged. If the spindle threads or nut threads are stripped, you'll only encounter a problem if you remove and reinstall.

I took it out for a hard 15 this afternoon...pushed hard into the wind...nary a wiggle. I think it's good now.

Unfortunately I seem to have lost one of the dust caps (somewhere in the house I think)

Blue Loctite.

Thank God for chemical technology!

Gotcha covered

Seriously...