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The aluminium stem on my road bike says the max torque for each of the four faceplate bolts is 6nm. So I set my calibrated torque wrench to 6nm... and one of the bolts stripped its thread.

I guess “max” torque is a true upper bound?

Anyway: with three remaining good, tight bolts, is my stem likely to be safe to use for a couple of days until a replacement stem arrives?

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    You might consider checking your torque wrench too, either against another known good one or with a lever arm and spring balance and mathematics. It could be maladjusted or mis-calibrated. Another possibility is the thread was already damaged by a previous over-tightening and your work was within spec. – Criggie Dec 11 '20 at 23:58
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    @Carel I'm not so sure about that. If it's rated for 6 Nm, I'd expect it not to fail until at least 10 or 12 Nm as most components are going to be engineered to have a significant safety margin, if only for liability reasons. I'm in agreement with Criggie here - some other factor induced this. And I've also tried that under-torquing stem face plate bolts for a larger safety margin - and then I hit a bump and my bars rotated about 40 degrees. Back to spec'd torque I went.. – Andrew Henle Dec 12 '20 at 13:47
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    It can strip like this from past over-tightening causing fatigue or from the threads being poorly formed on either the stem or the bolt. Note that a lot of stems (not all) have plenty of material to helicoil. – Nathan Knutson Dec 12 '20 at 18:45
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    I would say get a new stem. The existing one is probably OK for a few days if you don't over-stress it, but don't lollygag about getting and installing the new one. – Daniel R Hicks Dec 12 '20 at 21:23
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    I think I could probably do 6Nm with just my little finger and a screwdriver from a christmas cracker. Something else was wrong here, or your wrench calibration was way off – Caius Jard Dec 13 '20 at 17:20
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This really is an opinion more than a factual question. That you still have three bolts means its held together, but it was designed with 4.

So there's a non-zero chance another bolt will go because the load is increased. Look at the Arecibo cables just last week for an example of how quickly things can fail.

Nobody can categorically say "that is safe" OR "that is unsafe"

You're a functioning adult and can make a judgement for your own risk tolerance levels. You know how much pressure it took to strip one thread, now that there is one-fewer holding the load the remaining extra to strip will be less.

OPINION: if you have another bike as an option, ride that instead. If you can take a bus or walk, do that. If you were stranded somewhere, you might choose to ride very carefully, or walk, or phone for a pickup.

I would not ride that bike until it is repaired. You only have one face, and a stem failure can be catastrophic.

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    I'd probably find a longer, probably thinner, bolt with a nut as a temporary repair to be used with caution. That would work with most of my stems,but not every stem – Chris H Dec 13 '20 at 8:36
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Answer this question for yourself:

WHEN the stem fails, what's going to happen to me - AND EVERYONE AROUND ME?

And only ride in ways that you're willing to live with the answer to the above question. Because when that stem fails, you're probably going to lose control of your bike and have a really hard time stopping - or worse, a really easy time stopping, err, splatting.

Noodling around on the local flat, empty-because-it's-winter MUP at 10 kph? Probably fine.

Hard, fast training on pavé for spring classics with your teammates? Not so fine.

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Dont use the stem , its rated at 4bolts@ 6nm , by putting in the 3bolts you may need to over tighten one of the other bolts to compensate for the same level of clamping force which may distort the clamp and or strip the threads in the other bolts.

Food for thought ,why not consider to repair the thread. Just grab the next size tap/thread and put in a bigger bolt with a matching thread.

PSA : store your torque wrench at zero (nm), if its that spring click mechanism type.It will hold its calibration longer , also helps if you don't drop it too :).

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    +1 for repairing the thread; though if a bigger size bolt doesn't fit, one can also use a thread repair insert (helicoil or similar). – jpa Dec 12 '20 at 18:49
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I say no, the stem is not usable. I recommend considering the real potential for severe harm when the stem fails. The danger will likely not only involve you; I certainly wouldn't want to be riding, driving, or walking near you until the stem is replaced.

Attach a lightbulb to the stem and turn it into an artistic cycling-themed lamp. Don't sell the stem or give it away to the "next sucker".

Here's my personal experience, and a story of caution:

When I was young, I rode a bike with a minor stem tightening issue that I couldn't afford to fix. I thought it was okay until I landed a jump. The handlebars rotated and my chest plunged straight into the stem at full force. Back then, we didn't wear protection. I still remember the impact and the pain to this day. I consider it a miracle that I survived and am still alive.

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I have no personal experience or engineering background. Based on lay intuition, I would agree with the posters suggesting that leaving one bolt out is a bad idea.

In the interim, there’s a potential solution if you can get to a hardware store. Find a high quality steel metric bolt.

  1. You will need to match the thread pitch. I suspect it’s an M5 thread. Find an M5 bolt and hold one of the other stem bolts up to the candidate bolt. If the thread is finer or coarser, it should be visually apparent.
  2. You’ll need to match the length of the threaded section. If you have a caliper at home, go and measure the length of the threaded section from the head to the end of the bolt. Else, ask for help at the store.
  3. You’ll need to match the type of head. I suspect you are looking for a socket head bolt.
  4. This is the tricky bit: you’ll need a bolt that’s strong enough. This site suggests that there are at least three common strength classes for (non stainless) steel: 8.8, 10.9, and 12.9 in increasing order of strength. The site said to avoid 8.8. It gave a recommendation for a grade for stainless steel. If you are determined to get back on the road, then I’d say go for the non-stainless.

There is some risk involved here, and the hardware store personnel might not know the strength grade. Also, steel quality might vary even within a given rated strength grade. Whatever risk there is should be less than riding a stem with 3 bolts.

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    I read the question that the stripped thread is from the stem, so there's nothing left for a bolt to thread into. Tapping to the next larger size up might work, but buying the tool is probably about the same cost as a stem. – Criggie Dec 13 '20 at 11:19
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    @Criggie that makes sense. I will leave the answer as is just to provide info on general bolt sizing. – Weiwen Ng Dec 13 '20 at 13:31
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It depends on your riding style and how the bike will be ridden. With 3 bolts you can easily ride the bike to get you from point A to B, just don't ride the bike in a way that will put pressure on your handlebars. For example, you shouldn't do any tricks on the bike. If you can't avoid putting on the handlebars, then I wouldn't ride the bike.

I often ride my bike using only one hand because I'm carrying something in my other hand. This is one way to make sure you don't put a lot of pressure on your handlebars.

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  • Riding with one hand would put a more uneven pressure on the faceplate. – Dan K Dec 15 '20 at 20:38
  • It doesn't matter if you have uneven pressure on the faceplate, as long as it's not enough pressure to move the handlebars. Besides, would you prefer to walk? – Jeffrey Dec 22 '20 at 13:37

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