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Old 10-03-11, 09:13 AM   #1
mikehattan
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Handlebar slip - how much torque on quill stem pinch bolt?

I just changed out drop bars to bullhorns (Origin-8) on a road bike. I'm experiencing a bit of slip when I pull up on the bars hard while sprinting or climbing.

It's a Nitto quill stem circa 1989 (bike catalog specs list it as a "Young" model). Both stem and bars are 26.0. The handlebar pinch bolt has 6mm hex on top and a nut underneath (clamp is not tapped). There is still a gap on the stem clamp.

I don't have a torque wrench, but I have the handlebar pinch bolt wrist-tight using the Park 3-way hex tool (link).

Can I go tighter? Is there anything I can do to the bars or stem to minimize slippage?
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Old 10-03-11, 09:28 AM   #2
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Need a torque wrench.. an answer to this, properly,here, can only be expressed
in Numbers, with any accuracy..
Words... adverbs, adjectives, metaphors and analogies are not useful.

short of that .. if it slips, tighten incrementally till it stops. the steel nut
inset in the stem should not strip out

a wee bit of grease on the threads of that bolt will help tighten the screw smoothly.

adding a little thread lock like LocTite, into the surface that the bars and stem meet
may cure to be non slip.

Last edited by fietsbob; 10-03-11 at 09:37 AM.
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Old 10-03-11, 11:32 AM   #3
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Ok thanks, if I can borrow a torque wrench, what the max for that bolt?

What is likely to be the part to fail first with over tightening?
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Old 10-03-11, 12:07 PM   #4
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Nitto's specs for M6 pinch bolts is around 5 N-m (around 4 ft lbs = 48 in-lbs). But for M8 pinch botls, it's 13 N-m or around 10 ft-lbs. If you have a 6mm allen wrench bolt, you will need to check if the driver hole is 6mm allen but the threaded bolt might be bigger. That is sometimes true with single clamp quill stems.

To give you an idea of the "feel" of tightness, with nominal 2 inches of leverage of an allen key wrench, you will need around 24 lbs of force to tighten to achieve 4 ft-lbs. If you have a longer reach allen with 3 inches, then 16 lbs of force, etc. For 10 ft-lbs (i.e. 120 in-lbs) you can see how tight that can be.

Note: Before you go over torquing something to compensate for slipping, I'd recommend cleaning the clamp surface first of any thing that might act as a lubricant, and then carefully greasing the threads on the clamp bolt but not getting that grease into the clamping surface.

Most mechanical bolt designs for metal parts have at least a 2-to-1 safety factor. But you shouldn't have to go past spec to have it hold for you. If you over torque, the most likely damage would be to strip the bolt, shear off the head of the bolt, or strip the nutted threads. In some cases, you could flex and crack the clamp.
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Old 10-03-11, 12:21 PM   #5
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Thanks all. I'm going to try re-cleaning the clamp and bars, then see if that helps.
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Old 10-03-11, 03:49 PM   #6
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The only catch with that is that most of us like to smear on a thin coat of grease to avoid corrosion issues over the long term. I use a thin smear of grease all the time and I've never had anything slip as a result. So I tend to think you're either under torqueing the fastener or the new bars are not as good a fit.

All in all sometimes tolerances can pile up to confound you. Nitto stems are pretty stiff at the clamping split. If the bar is a bit undersize and the clamp a little over you may be "wasting" much of your tightening torque just getting the clamp to close down onto the bars even before you begin achieving any clamping pressure. Instead try backing off the clamp bolt and see if you can fit in a full wrap of pop can metal as a shim. If it fits then use it. That way all your clamping pressure is going towards actually clamping the bars. As a bonus you won't be tending to deform the clamp opening into more of an egg shape by forcing it closed more than it should be. That will lead to more even pressure on the bars and less likelyhood of it slipping.
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Old 10-03-11, 07:41 PM   #7
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+1, 25.4mm bars in a 26mm stem causes problems..
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Old 10-03-11, 08:33 PM   #8
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So here's what I know now: Even though the bar is listed on package as 26.0, it is a little undersized after putting the calipers to 'em. May be 25.4, but I could have compressed the diameter so it difficult to say with certainty. Second, there was some nasty stuff on the inside of the stem clamp. Appeared to be old sticky grease mixed oxidized aluminum powder. This could have contributed to the slipping. I cleaned as well as I could with mineral spirits. Reinstalled the bar with no shims. Seem to be better, but too late for a ride tonight. I'll take it out tomorrow for a test ride. If still slips I'll try shims.
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