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Old April 17th 17, 09:52 PM posted to rec.bicycles.tech
Doug Landau
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Posts: 1,359
Default Selecting An Appropriate Bolt

On Friday, April 14, 2017 at 4:15:34 AM UTC-7, John B Slocomb wrote:
On Thu, 13 Apr 2017 20:07:48 -0700, Art Shapiro
wrote:

I had a bit of an adventure when one of the two handlebar-fixing bolts
on my Deda Murex quilled stem decided to snap with a rather impressive
cracking noise. I somehow didn't crash and happened to be only about
seven miles from home. I got slowly home holding the stem with one hand
and one of the brake levers on the dangling handlebars with the other
hand. (This is not recommended to the reader.)

I see that the bolt is a M6x18 tapered cone head Allen cap screw with
pressed-on washer.

The stem is two months shy of 15 years old, but I don't want to have
this happen again. Looking on eBay, I see quite a few appropriate
bolts, but I'm not sure what is optimal. Can anyone help?

Everything seems to be Grade 5. Is that safe enough, or do I really
want to (somehow) find Grade 8?

Many are titanium. Is that a better choice than the more-common steel?
Or should I look for stainless steel? I am always happy to save a few
grams, but not if that's a significant risk.

Advice welcome!

Art


Grade 5 bolts should certainly be strong enough to hold the handle
bars on. But there are grades 8 or 9 that are stronger.


What makes you say this? Do you have some #s to back this statement up, or is it just your wild guess? Have you calculated the load on this part when when a rider of a given weight hits a pothole at a given speed, or ??? And more importantly, why skimp here?

As an aside your description is incorrect. It might be an U.S. size
which might be 8-32 or it might be metric in which case it would be
M8-1.25 or maybe M8-1.0. A U.S. #8 bolt is about half the thickness
of a 8mm bolt.


Huh?!? What are you on about? It is you who is incorrect not he. He said it was an M6x18. The x is pronounced "by". Put M6x18 in google and click images. You will see M6 bolts in an 18mm length. He chose to identify the bolt by it's diameter and length, just like the rest of the world does most of the time.

You are describing bolts using diameter and pitch. This is incomplete, as it does not specify the length. Further is is irrelevant since the JIS and ISO standards both specify 1.0 as the standard pitch for 6mm bolts.

It is not an 8-32, nor is it an 8mm. Both of these suggestions are ridiculous. It is an M6x18 and while fine m6 bolts do exist in 1.10 pitch they are not common and it is safe to say it is, in this application, almost undoubtedly a 1.0. You seem to have latched on the number 8 for some reason.


Generally speaking stainless is no stronger than mild steel and
Titanium is about the same strength as steel of the same hardness.
stainless, of course doesn't corrode much and titanium is about half
the weight of steel.


Steel has or can have greater strength for a given diameter. Why skimp here?

Again an aside. The bolt(s) broke once in 15 years.....


Looking at it the other way, it already broke once. Furthermore some possible reasons e.g. (e.g. over-tightening) might have also affected the threads that the bolt went into.

The stem is two months shy of 15 years old, but I don't want to have

this happen again.


Get a new stem. This one is a flawed design. There is built-in problem with the shape of the part, and that is a lack of remaining metal around the bolt hole. The stem has been made bigger around the front bolt hole to overcome this, but it still has the 2-bolt-1-failure problem. The traditional shape does not make this concession to ease-of-handlebar-change, and carefully places the single bolt in the rear where there is plenty of metal surrounding the threads.
The traditional design is both less likely to experience a bolt failure, and - in the wild guess dept., be more likely to hold on to the bars and remain usable in the event that one does.

Deda Murex 2-bolt:
https://www.google.com/search?q=deda...f3AGoQ_AUIBygC

3ttt traditional:
https://www.google.com/search?q=3ttt...w=1306&bih=724


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