I've got a Trek 7.3 FX. Recently the crank arm became loose while riding. I stopped and tightened it. Long story short-bike shop wound up replacing the whole crank. This was two months ago. While riding yesterday, the crank arm became loose again. Any ideas why this is happening? Thanks

Because it was likely not properly torqued to begin with.

If you look at the crank arm around the bolt there should be a torque setting suggested. Getting to this torque is best done with a (torque) wrench 8-12" long with a socket adapter.
Since the crank is only 7" long this almost requires a second person to hold the opposite side crank down to get to the suggested torque. I suspect cranks were tightened
"tight enough" but no where near suggested torque. You certainly can't do it with a portable tool kit or even an allen wrench which in 8mm size is ~5 inches long. Riding
a slightly loose crank will very rapidly tear up the splines and ruin the crank.

The same applies to a square taper crank interface, the crank arm, being the softer metal, is very quickly damaged by riding it loose.

IIRC the torque for my crankarm was 40nm which is a lot relatively speaking.

Sit on the bike, put all your weight on the pedal and tighten up the opposing crank arm bolt. Switch sides and repeat. Do 'er up good.

Shimano recommends 35 to 50 N-m (26 to 37 ft-lbs). I use an inexpensive 3/8" drive torque wrench set to 35 ft-lbs - and have never had this issue. At the end of 170mm crank arm, 35 ft-lbs is 63 lbs (about the same as 7½ gallons of water).

Some good advice, some not quite claims and some missing advice.

Yes, the crank arm needs to be properly torqued down in the beginning. No, it only takes one person with proper technique to do this (placing the arm and wrench close together so when tightening the wrench/bolt one's hands are pressing together, and a wrench with enough lever arm). What's missing is that the arm's retaining bolt should be checked for proper tightness after a few hundred miles. This is but one of the reasons that better shops offer a 30-90 day free bike check up/tuning after the sale. Failure to do so can result in the posted problem (as well as other "I was just riding along..." problems. FAILURE TO DO PROPER MAINTANANCE ISN"T THE SHOP"S FAULT. (not that the OP said it was).

I'll bet a good beer that it was the LH arm that came loose. It almost always is. All of the stress of pedaling (and half of the body weight) is going through the fairly tiny contact surface between the arm and the axle (which with a tapered square design is less then a square inch). The RH side loading is less because at least half the pedaling force is going directly through the crank arm and into the rings. Andy.

Yes, it was the the left crank arm. I probably rode it a hundred yards with it loose. At first I thought my cleat was coming loose from my shoe since I had just put on new cleats. I unclipped and immediately saw it was the crank arm. Frustrating to say the least. My bike shop, although good folks, don't know much more than I do. They said they "would make it right." Gonna be hard to ride the bike now and feel confident that it won't happen again.

Lots of crank arms are tightened with an 8mm allen bolt. If you are using a 6" allen key to tighten that bolt you're not getting 40nm.

According to park tools website it's actually 44nm.

I just used the allen pack I had with me to tighten it the first time. When I got home I tightened it with a bigger allen wrench. It kept loosening on me when I was riding, and like previously mentioned, it eventually wallowed out the splines, hence replacing the whole assembly. This last time it happened, (Tuesday) I left it as it was and took it straight to the shop (actually my wife dropped it off so I didn't get a chance to talk to them). I'm going by there today to hear their diagnosis.

Splines? Was this a Power Spline crank? If so that's some of the issue. For the number of them in service and for their claims to be stronger (splines Vs tapered square) they seem to have more loosening up problems. Maybe it's because they come on some bikes that are made to appeal to the up and coming but not yet savvy rider but don't cost enough to have a shop do a better then usual assembly job. I don't know why I just see more issues with them statisticly then other designs.

Power Splines are a Truvativ product and are not Octalink or ISIS. We don't see the same problems with them as we do with Power Splines. Andy.

If it's the stock crankset and bottom bracket for a Trek 7.3 fx, it should be square taper, not splined.

I'm sorry, I used to wrong terminology, I guess. The bottom bracket is square shaped. The bike shop employees today all blamed it on each other. One of them torqued both crank arms to the correct specs today and said it should not happen again.

If they didn't replaced the crank arm there's a good chance it will happen again because all the wiggling opens up the hole so it's no longer a precise taper fit on the spindle.

That's what I'm afraid will happen. I did, however, stop riding it when I noticed it was loose. We will see.