I had an issue with my headset coming loose every couple of weeks. Replaced the headset and to be sure I replaced the stock stem for a Truvativ Hussefelt stem which had bigger 5mm bolts, torqued them to spec at between 7-8Nm and that thing never twisted or moved again.

 

Your stem was tight enough. Sometimes, bars will just spin a little during an impact. Not a bad thing considering the next point of give could be your front wheel. Or worse, your forks crown. Better to simply adjust your bars than to true or replace a front wheel.

 

I think the bike industry mostly sets torque values based off of a range between tight enough to not move most of the time and loose enough to not snap bolts. I have doubts that they are actually specifying torques based on an engineering analysis of the component, as evidenced by the nearly blanket statement of 5Nm on so many parts. I mean geez, very few parts come with actual high strength bolts, instead using chintzy, decorative, or even rubbish stainless steel fasteners.

 

Stems twisting in a crash is very common. Most stems come with 4mm bolts and those can barely handle the stated 5nm, going any higher means the heads might snap off (usually when it comes time to loosen them).

Higher torque needs bigger 5mm bolts.

 

It quite common to twist the forks on a dirtbike during a crash, and I'm referring to a 250# machine with double triple clamps, with multiple bolts on each clamp. It's common practice to knock the front wheel against a tree to straighten them back out after going down.
That said, if I'm comfortable going 85mph on that setup, having a bicycle stem twist during a crash doesn't alarm me either. All it needs to do is be dependable during the roughest riding conditions, once you're on the ground it's a moot point to me.

 

Yea, things clamping to round surfaces can.. Spin. It's pretty much normal, actually better than the alternative.

 

FWIW, I always check my stem bolts and wheel skewers on any road trip requiring 2hrs+ drive time to the trail head. AND more importantly, I check the fasteners any time I have my bike serviced at the LBS. I really shouldn't have to check the fasteners after its been serviced but mistakes happen and I learned the hard way.

Also, blue Loctite the threads.

 

FWIW, I always check my stem bolts and wheel skewers on any road trip requiring 2hrs+ drive time to the trail head. AND more importantly, I check the fasteners any time I have my bike serviced at the LBS. I really shouldn't have to check the fasteners after its been serviced but mistakes happen and I learned the hard way.

Also, blue Loctite the threads.

I hate to say it but yeah, you should do a quick safety check after your LBS has worked on your bike. No way of knowing if the mechanic was interrupted with a phone call mid-job or they let the teenage intern work on your bike. It's not okay, but I've seen it happen. Just grabbing the bars and seeing if you can twist them in any axis is adequate.
When we assemble a new bike we leave the stem bolts loose on purpose till we put it on the ground and set the proper angles, then there's no chance of leaving them un-torqued but falsely appearing to be safe to ride.
I've gotten in the habit of using my pre-set torque tool on every bolt on the stem to double check before every bike goes out the door, in front of the customer. Both for their safety and in case they let their cousin 'who knows bikes' try to adjust their handlebars later and blames us when they crash.

 

Great replies. Thanks everyone. The stem spin in the crash seemed like it was supposed to happen but i have seen so little of this in 25 years of mtb i wanted to ask. I’ve probably just been over tightening my stem all these years. Guess ill back to the recommended 5nm.

 

I've been running .5 to 1 Nm higher torque values on stem at both steerer and handlebar clamp. Also using Motorex carbon assembly paste on both handlebar and steerer. That has kept everything nice and tight with no slippage and no damage 👍

 

Torque specs are to either protect the hardware or the part itself. Stems will twist in a crash. I think you'd want that. I'd rather have the stem move than to have potential damage to another bike part or even myself. I do not recommend the over tightening of any bike part. I've seen parts damaged by over tightening. Carry a small multitool with a torque bit...problem solved. I carry a Topeak Nano Torqbar. It's got a 5nm bit.

 

IMO, it's not going to matter much on an aluminum steerer; just don't go crazy. Flame away!

 

Choosing the right stem can help. My favorite at the moment is the RF Atlas, torque spec on the steerer bolts is 10.8-13.6 N-m, the clamp area is reasonably tall and it has no cutouts in that area.